Update

python/ql/src/experimental/Security/CWE-611/XXE.py

@@ -1,13 +0,0 @@
-from flask import request, Flask
-import lxml.etree
-import xml.etree.ElementTree
-
-
-@app.route("/example")
-def example():
-    xml_content = request.args['xml_content']
-
-    parser = lxml.etree.XMLParser()
-    parsed_xml = xml.etree.ElementTree.fromstring(xml_content, parser=parser)
-
-    return parsed_xml.text

python/ql/src/experimental/Security/CWE-611/XXE.ql

@@ -1,22 +0,0 @@
-/**
- * @name XML External Entity abuse
- * @description User input should not be parsed by XML parsers without security options enabled.
- * @kind path-problem
- * @problem.severity error
- * @id py/xxe
- * @tags security
- *       external/cwe/cwe-611
- *       external/cwe/cwe-776
- *       external/cwe/cwe-827
- */
-
-// determine precision above
-import python
-import experimental.semmle.python.security.XXE
-import DataFlow::PathGraph
-
-from XXEFlowConfig config, DataFlow::PathNode source, DataFlow::PathNode sink
-where config.hasFlowPath(source, sink)
-select sink.getNode(), source, sink,
-  "$@ XML input is constructed from a $@ and isn't secured against XML External Entities abuse",
-  sink.getNode(), "This", source.getNode(), "user-provided value"

python/ql/src/experimental/Security/CWE-611/XXE.xml

@@ -0,0 +1,4 @@
+<?xml version="1.0"?>
+<!DOCTYPE dt [
+<!ENTITY xxe SYSTEM "file:///etc/passwd">]>
+<test>&xxe;</test>

python/ql/src/experimental/Security/CWE-611/XmlInjection.py

@@ -0,0 +1,25 @@
+from flask import request, Flask
+import lxml.etree
+import xml.etree.ElementTree
+
+app = Flask(__name__)
+
+# BAD
+@app.route("/bad")
+def bad():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser()
+    parsed_xml = xml.etree.ElementTree.fromstring(xml_content, parser=parser)
+
+    return parsed_xml.text
+
+# GOOD
+@app.route("/good")
+def good():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser(resolve_entities=False)
+    parsed_xml = xml.etree.ElementTree.fromstring(xml_content, parser=parser)
+
+    return parsed_xml.text

python/ql/src/experimental/Security/CWE-611/XmlInjection.qhelp

@@ -5,31 +5,18 @@
 
 <overview>
 <p>
-Parsing untrusted XML files with a weakly configured XML parser may lead to an XML External Entity (XXE) attack.
+Parsing untrusted XML files with a weakly configured XML parser may lead to attacks such as XML External Entity (XXE),
+Billion Laughs, Quadratic Blowup and DTD retrieval.
 This type of attack uses external entity references to access arbitrary files on a system, carry out denial of
 service, or server side request forgery. Even when the result of parsing is not returned to the user, out-of-band
 data retrieval techniques may allow attackers to steal sensitive data. Denial of services can also be carried out
 in this situation.
 </p>
-<p>
-Refer to the following links to check the details regarding how and which libraries are vulnerable:
-</p>
-
-<ul>
-<li><a href="https://docs.python.org/3/library/xml.html#xml-vulnerabilities">Python 3</a>.</li>
-<li><a href="https://docs.python.org/2/library/xml.html#xml-vulnerabilities">Python 2</a>.</li>
-</ul>
-
-<p>
-This query currently identifies vulnerable XML parsing from the following parsers:
-<code>xml.etree.ElementTree.XMLParser</code>, <code>lxml.etree.XMLParser</code>, <code>lxml.etree.get_default_parser</code>,
-<code>xml.sax.make_parser</code>.
-</p>
 </overview>
 
 <recommendation>
 <p>
-Use <a href="https://docs.python.org/3/library/xml.html#the-defusedxml-package">defusedxml</a>, a Python package aimed
+Use <a href="https://pypi.org/project/defusedxml/">defusedxml</a>, a Python package aimed
 to prevent any potentially malicious operation.
 </p>
 </recommendation>
@@ -39,10 +26,17 @@ to prevent any potentially malicious operation.
 The following example calls <code>xml.etree.ElementTree.fromstring</code> using a parser (<code>lxml.etree.XMLParser</code>)
 that is not safely configured on untrusted data, and is therefore inherently unsafe.
 </p>
-<sample src="XXE.py"/>
+<sample src="XmlInjection.py"/>
+<p>
+Providing an input (<code>xml_content</code>) like the following XML content against /bad, the request response would contain the contents of
+<code>/etc/passwd</code>.
+</p>
+<sample src="XXE.xml"/>
 </example>
 
 <references>
+<li>Python 3 <a href="https://docs.python.org/3/library/xml.html#xml-vulnerabilities">XML Vulnerabilities</a>.</li>
+<li>Python 2 <a href="https://docs.python.org/2/library/xml.html#xml-vulnerabilities">XML Vulnerabilities</a>.</li>
 <li>Python <a href="https://www.edureka.co/blog/python-xml-parser-tutorial/">XML Parsing</a>.</li>
 <li>OWASP vulnerability description: <a href="https://www.owasp.org/index.php/XML_External_Entity_(XXE)_Processing">XML External Entity (XXE) Processing</a>.</li>
 <li>OWASP guidance on parsing xml files: <a href="https://cheatsheetseries.owasp.org/cheatsheets/XML_External_Entity_Prevention_Cheat_Sheet.html#python">XXE Prevention Cheat Sheet</a>.</li>

python/ql/src/experimental/Security/CWE-611/XmlInjection.ql

@@ -0,0 +1,22 @@
+/**
+ * @name XML injection
+ * @description User input should not be parsed without security options enabled.
+ * @kind path-problem
+ * @problem.severity error
+ * @id py/xml-injection
+ * @tags security
+ *       external/cwe/cwe-611
+ *       external/cwe/cwe-776
+ *       external/cwe/cwe-827
+ */
+
+// determine precision above
+import python
+import experimental.semmle.python.security.dataflow.XmlInjection
+import DataFlow::PathGraph
+
+from DataFlow::PathNode source, DataFlow::PathNode sink, string kind
+where XmlInjection::xmlInjectionVulnerable(source, sink, kind)
+select sink.getNode(), source, sink,
+  "$@ XML input is constructed from a $@ and is vulnerable to " + kind + ".", sink.getNode(),
+  "This", source.getNode(), "user-provided value"

python/ql/src/experimental/semmle/python/Concepts.qll

@@ -44,6 +44,25 @@ class LogOutput extends DataFlow::Node {
   DataFlow::Node getAnInput() { result = range.getAnInput() }
 }
 
+module XML {
+  /**
+   * A data-flow node that collects functions parsing XML.
+   *
+   * Extend this class to model new APIs. If you want to refine existing API models,
+   * extend `XMLParsing` instead.
+   */
+  class XMLParsing extends DataFlow::Node instanceof XMLParsing::Range {
+    /**
+     * Gets the argument containing the content to parse.
+     */
+    DataFlow::Node getAnInput() { result = super.getAnInput() }
+
+    /**
+     * Holds if the parsing method or the parser holding it is vulnerable to `kind`.
+     */
+    predicate vulnerable(string kind) { super.vulnerable(kind) }
+  }
+
   /** Provides classes for modeling XML parsing APIs. */
   module XMLParsing {
     /**
@@ -59,34 +78,28 @@ module XMLParsing {
       abstract DataFlow::Node getAnInput();
 
       /**
-     * Holds if the parser may be parsing the input dangerously.
-     *
-     * Specifically, this predicate holds whether the XML parsing parses/extends external
-     * entities in the parsed XML stream.
+       * Holds if the parsing method or the parser holding it is vulnerable to `kind`.
        */
-    abstract predicate mayBeDangerous();
+      abstract predicate vulnerable(string kind);
     }
   }
 
   /**
- * A data-flow node that collects functions parsing XML.
+   * A data-flow node that collects XML parsers.
    *
    * Extend this class to model new APIs. If you want to refine existing API models,
- * extend `XMLParsing` instead.
+   * extend `XMLParser` instead.
    */
-class XMLParsing extends DataFlow::Node instanceof XMLParsing::Range {
+  class XMLParser extends DataFlow::Node instanceof XMLParser::Range {
     /**
      * Gets the argument containing the content to parse.
-   *
-   * Specifically, this predicate holds whether the XML parsing parses/extends external
-   * entities in the parsed XML stream.
      */
     DataFlow::Node getAnInput() { result = super.getAnInput() }
 
     /**
-   * Holds if the parser may be parsing the input dangerously.
+     * Holds if the parser is vulnerable to `kind`.
      */
-  predicate mayBeDangerous() { super.mayBeDangerous() }
+    predicate vulnerable(string kind) { super.vulnerable(kind) }
   }
 
   /** Provides classes for modeling XML parsers. */
@@ -104,34 +117,11 @@ module XMLParser {
       abstract DataFlow::Node getAnInput();
 
       /**
-     * Holds if the parser may be dangerously configured.
-     *
-     * Specifically, this predicate holds whether the XML parser parses/extends external
-     * entities in the parsed XML stream.
+       * Holds if the parser is vulnerable to `kind`.
        */
-    abstract predicate mayBeDangerous();
+      abstract predicate vulnerable(string kind);
     }
   }
-
-/**
- * A data-flow node that collects XML parsers.
- *
- * Extend this class to model new APIs. If you want to refine existing API models,
- * extend `XMLParser` instead.
- */
-class XMLParser extends DataFlow::Node instanceof XMLParser::Range {
-  /**
-   * Gets the argument containing the content to parse.
-   */
-  DataFlow::Node getAnInput() { result = super.getAnInput() }
-
-  /**
-   * Holds if the parser may be dangerously configured.
-   *
-   * Specifically, this predicate holds whether the XML parser parses/extends external
-   * entities in the parsed XML stream.
-   */
-  predicate mayBeDangerous() { super.mayBeDangerous() }
 }
 
 /** Provides classes for modeling LDAP query execution-related APIs. */

python/ql/src/experimental/semmle/python/Frameworks.qll

@@ -3,7 +3,7 @@
  */
 
 private import experimental.semmle.python.frameworks.Stdlib
-private import experimental.semmle.python.frameworks.XML
+private import experimental.semmle.python.frameworks.Xml
 private import experimental.semmle.python.frameworks.Flask
 private import experimental.semmle.python.frameworks.Django
 private import experimental.semmle.python.frameworks.Werkzeug

python/ql/src/experimental/semmle/python/frameworks/XML.qll

@@ -1,196 +0,0 @@
-/**
- * Provides class and predicates to track external data that
- * may represent malicious XML objects.
- */
-
-private import python
-private import semmle.python.dataflow.new.DataFlow
-private import experimental.semmle.python.Concepts
-private import semmle.python.ApiGraphs
-
-private module XML {
-  /** Gets a reference to `xml.etree.ElementTree`. */
-  private API::Node xmlEtree() {
-    result = API::moduleImport("xml").getMember("etree").getMember("ElementTree")
-  }
-
-  /** Gets a call to `xml.etree.ElementTree.XMLParser`. */
-  private class XMLEtreeParser extends DataFlow::CallCfgNode, XMLParser::Range {
-    XMLEtreeParser() { this = xmlEtree().getMember("XMLParser").getACall() }
-
-    override DataFlow::Node getAnInput() { none() }
-
-    override predicate mayBeDangerous() { any() }
-  }
-
-  /**
-   * Gets a call to `xml.etree.ElementTree.fromstring`, `xml.etree.ElementTree.fromstringlist`,
-   * `xml.etree.ElementTree.XML` or `xml.etree.ElementTree.parse`.
-   *
-   * Given the following example:
-   *
-   * ```py
-   * parser = lxml.etree.XMLParser()
-   * parsed_xml = xml.etree.ElementTree.fromstring(xml_content, parser=parser).text
-   * ```
-   *
-   * `this` would be `xml.etree.ElementTree.fromstring(xml_content, parser=parser)`
-   * and `xml_content` would be the result of `getAnInput()`.
-   */
-  private class XMLEtreeParsing extends DataFlow::CallCfgNode, XMLParsing::Range {
-    XMLEtreeParsing() {
-      this = xmlEtree().getMember(["fromstring", "fromstringlist", "XML", "parse"]).getACall()
-    }
-
-    override DataFlow::Node getAnInput() { result = this.getArg(0) }
-
-    override predicate mayBeDangerous() {
-      exists(XMLParser xmlParser |
-        xmlParser.mayBeDangerous() and this.getArgByName("parser").getALocalSource() = xmlParser
-      )
-    }
-  }
-
-  /** Gets a reference to `xml.sax`. */
-  private API::Node xmlSax() { result = API::moduleImport("xml").getMember("sax") }
-
-  /**
-   * Gets a call to `xml.sax.make_parser` and following calls.
-   *
-   * Given the following example:
-   *
-   * ```py
-   * BadHandler = MainHandler()
-   * parser = xml.sax.make_parser()
-   * parser.setContentHandler(BadHandler)
-   * parser.setFeature(xml.sax.handler.feature_external_ges, False)
-   * parser.parse(StringIO(xml_content))
-   * parsed_xml = BadHandler._result
-   * ```
-   *
-   * `this` would be `xml.sax.make_parser()`, `getAnInput()` would return `StringIO(xml_content)`
-   * and `mayBeDangerous()` would not hold since `xml.sax.handler.feature_external_ges` is set to
-   * `False` and so is not vulnerable.
-   * see https://docs.python.org/3/library/xml.sax.handler.html#xml.sax.handler.feature_external_ges
-   */
-  private class XMLSaxParser extends DataFlow::CallCfgNode, XMLParser::Range {
-    DataFlow::CallCfgNode attrCall;
-
-    XMLSaxParser() {
-      this = xmlSax().getMember("make_parser").getACall() and
-      attrCall.getFunction().(DataFlow::AttrRead).getObject().getALocalSource() = this
-    }
-
-    override DataFlow::Node getAnInput() {
-      attrCall.getFunction().(DataFlow::AttrRead).getAttributeName() = "parse" and
-      result = attrCall.getArg(0)
-    }
-
-    override predicate mayBeDangerous() {
-      attrCall.getFunction().(DataFlow::AttrRead).getAttributeName() = "setFeature" and
-      attrCall.getArg(0) = xmlSax().getMember("handler").getMember("feature_external_ges").getAUse() and
-      DataFlow::localFlow(DataFlow::exprNode(any(True trueName)), attrCall.getArg(1))
-    }
-  }
-
-  /** Gets a reference to `lxml.etree`. */
-  private API::Node lxmlEtree() { result = API::moduleImport("lxml").getMember("etree") }
-
-  /**
-   * Gets a call to `lxml.etree.XMLParser` or `lxml.etree.get_default_parser` and `mayBeDangerous()`
-   * identifies whether the argument `no_network` is set to `False` or the arguments `huge_tree`
-   * or `resolve_entities` are set to True. Since `resolve_entities` default value is `True`,
-   * the predicate will also succeed if the argument is not set.
-   */
-  private class LXMLParser extends DataFlow::CallCfgNode, XMLParser::Range {
-    LXMLParser() { this = lxmlEtree().getMember(["XMLParser", "get_default_parser"]).getACall() }
-
-    override DataFlow::Node getAnInput() { none() }
-
-    override predicate mayBeDangerous() {
-      DataFlow::localFlow(DataFlow::exprNode(any(False falseName)), this.getArgByName("no_network")) or
-      DataFlow::localFlow(DataFlow::exprNode(any(True trueName)),
-        this.getArgByName(["huge_tree", "resolve_entities"])) or
-      not exists(this.getArgByName("resolve_entities"))
-    }
-  }
-
-  /**
-   * Gets a call to `lxml.etree.fromstring`, `xml.etree.fromstringlist`,
-   * `xml.etree.XML` or `xml.etree.parse`.
-   *
-   * Given the following example:
-   *
-   * ```py
-   * parser = lxml.etree.XMLParser()
-   * parsed_xml = lxml.etree.fromstring(xml_content, parser=parser).text
-   * ```
-   *
-   * `this` would be `lxml.etree.fromstring(xml_content, parser=parser)`
-   * and `xml_content` would be the result of `getAnInput()`.
-   */
-  private class LXMLParsing extends DataFlow::CallCfgNode, XMLParsing::Range {
-    LXMLParsing() {
-      this = lxmlEtree().getMember(["fromstring", "fromstringlist", "XML", "parse"]).getACall()
-    }
-
-    override DataFlow::Node getAnInput() { result = this.getArg(0) }
-
-    override predicate mayBeDangerous() {
-      exists(XMLParser xmlParser |
-        xmlParser.mayBeDangerous() and this.getArgByName("parser").getALocalSource() = xmlParser
-      )
-      or
-      not exists(this.getArgByName("parser"))
-    }
-  }
-
-  /** Gets a reference to the `xmltodict` module. */
-  private API::Node xmltodict() { result = API::moduleImport("xmltodict") }
-
-  /**
-   * Gets a call to `xmltodict.parse` and `mayBeDangerous()` identifies
-   * whether the argument `disable_entities` is set to `False`.
-   */
-  private class XMLtoDictParsing extends DataFlow::CallCfgNode, XMLParsing::Range {
-    XMLtoDictParsing() { this = xmltodict().getMember("parse").getACall() }
-
-    override DataFlow::Node getAnInput() { result = this.getArg(0) }
-
-    override predicate mayBeDangerous() {
-      DataFlow::localFlow(DataFlow::exprNode(any(False falseName)),
-        this.getArgByName("disable_entities"))
-    }
-  }
-
-  /** Gets a reference to `xml.dom.minidom` or `xml.dom.pulldom`. */
-  private API::Node xmlDom() {
-    result = API::moduleImport("xml").getMember("dom").getMember(["minidom", "pulldom"])
-  }
-
-  /**
-   * Gets a call to `xml.dom.minidom.parse` or `xml.dom.pulldom.parse`.
-   *
-   * Given the following example:
-   *
-   * ```py
-   * parser = xml.sax.make_parser()
-   * parser.setFeature(xml.sax.handler.feature_external_ges, True)
-   * parsed_xml = xml.dom.minidom.parse(StringIO(xml_content), parser=parser).documentElement.childNod
-   * ```
-   *
-   * `this` would be `xml.dom.minidom.parse(StringIO(xml_content), parser=parser)`
-   * and `StringIO(xml_content)` would be the result of `getAnInput()`.
-   */
-  private class XMLDomParsing extends DataFlow::CallCfgNode, XMLParsing::Range {
-    XMLDomParsing() { this = xmlDom().getMember("parse").getACall() }
-
-    override DataFlow::Node getAnInput() { result = this.getArg(0) }
-
-    override predicate mayBeDangerous() {
-      exists(XMLParser xmlParser |
-        xmlParser.mayBeDangerous() and this.getArgByName("parser").getALocalSource() = xmlParser
-      )
-    }
-  }
-}

python/ql/src/experimental/semmle/python/frameworks/Xml.qll

@@ -0,0 +1,301 @@
+/**
+ * Provides class and predicates to track external data that
+ * may represent malicious XML objects.
+ */
+
+private import python
+private import semmle.python.dataflow.new.DataFlow
+private import experimental.semmle.python.Concepts
+private import semmle.python.ApiGraphs
+
+private module Xml {
+  /**
+   * Gets a call to `xml.etree.ElementTree.XMLParser`.
+   */
+  private class XMLEtreeParser extends DataFlow::CallCfgNode, XML::XMLParser::Range {
+    XMLEtreeParser() {
+      this =
+        API::moduleImport("xml")
+            .getMember("etree")
+            .getMember("ElementTree")
+            .getMember("XMLParser")
+            .getACall()
+    }
+
+    override DataFlow::Node getAnInput() { none() }
+
+    override predicate vulnerable(string kind) { none() }
+  }
+
+  /**
+   * Gets a call to:
+   * * `xml.etree.ElementTree.fromstring`
+   * * `xml.etree.ElementTree.fromstringlist`
+   * * `xml.etree.ElementTree.XML`
+   * * `xml.etree.ElementTree.parse`
+   *
+   * Given the following example:
+   *
+   * ```py
+   * parser = lxml.etree.XMLParser()
+   * xml.etree.ElementTree.fromstring(xml_content, parser=parser).text
+   * ```
+   *
+   * * `this` would be `xml.etree.ElementTree.fromstring(xml_content, parser=parser)`.
+   * * `getAnInput()`'s result would be `xml_content`.
+   * * `vulnerable(kind)`'s `kind` would be `XXE`.
+   */
+  private class XMLEtreeParsing extends DataFlow::CallCfgNode, XML::XMLParsing::Range {
+    XMLEtreeParsing() {
+      this =
+        API::moduleImport("xml")
+            .getMember("etree")
+            .getMember("ElementTree")
+            .getMember(["fromstring", "fromstringlist", "XML", "parse"])
+            .getACall()
+    }
+
+    override DataFlow::Node getAnInput() { result = this.getArg(0) }
+
+    override predicate vulnerable(string kind) {
+      exists(XML::XMLParser xmlParser |
+        xmlParser = this.getArgByName("parser").getALocalSource() and xmlParser.vulnerable(kind)
+      )
+    }
+  }
+
+  /** Gets a reference to a `parser` that has been set a `feature`. */
+  private DataFlow::Node trackSaxFeature(
+    DataFlow::TypeTracker t, DataFlow::CallCfgNode parser, API::Node feature
+  ) {
+    t.start() and
+    exists(DataFlow::MethodCallNode featureCall |
+      featureCall = parser.getAMethodCall("setFeature") and
+      featureCall.getArg(0).getALocalSource() = feature.getAUse() and
+      featureCall.getArg(1).getALocalSource() = DataFlow::exprNode(any(True t_)) and
+      result = featureCall.getObject()
+    )
+    or
+    exists(DataFlow::TypeTracker t2 |
+      t = t2.smallstep(trackSaxFeature(t2, parser, feature), result)
+    )
+  }
+
+  /** Gets a reference to a `parser` that has been set a `feature`. */
+  DataFlow::Node trackSaxFeature(DataFlow::CallCfgNode parser, API::Node feature) {
+    result = trackSaxFeature(DataFlow::TypeTracker::end(), parser, feature)
+  }
+
+  /**
+   * Gets a call to `xml.sax.make_parser`.
+   *
+   * Given the following example:
+   *
+   * ```py
+   * BadHandler = MainHandler()
+   * parser = xml.sax.make_parser()
+   * parser.setContentHandler(BadHandler)
+   * parser.setFeature(xml.sax.handler.feature_external_ges, False)
+   * parser.parse(StringIO(xml_content))
+   * parsed_xml = BadHandler._result
+   * ```
+   *
+   * * `this` would be `xml.sax.make_parser()`.
+   * * `getAnInput()`'s result would be `StringIO(xml_content)`.
+   * * `vulnerable(kind)`'s `kind` would be `Billion Laughs` and `Quadratic Blowup`.
+   */
+  private class XMLSaxParser extends DataFlow::CallCfgNode, XML::XMLParser::Range {
+    XMLSaxParser() {
+      this = API::moduleImport("xml").getMember("sax").getMember("make_parser").getACall()
+    }
+
+    override DataFlow::Node getAnInput() { result = this.getAMethodCall("parse").getArg(0) }
+
+    override predicate vulnerable(string kind) {
+      exists(DataFlow::MethodCallNode parse, API::Node handler, API::Node feature |
+        handler = API::moduleImport("xml").getMember("sax").getMember("handler") and
+        parse.calls(trackSaxFeature(this, feature), "parse") and
+        parse.getArg(0) = this.getAnInput() // enough to avoid FPs?
+      |
+        kind = ["XXE", "DTD retrieval"] and
+        feature = handler.getMember("feature_external_ges")
+        or
+        kind = ["Billion Laughs", "Quadratic Blowup"]
+      )
+    }
+
+    predicate vulnerable(DataFlow::Node n, string kind) {
+      exists(API::Node handler, API::Node feature |
+        handler = API::moduleImport("xml").getMember("sax").getMember("handler") and
+        DataFlow::exprNode(trackSaxFeature(this, feature).asExpr())
+            .(DataFlow::LocalSourceNode)
+            .flowsTo(n)
+      |
+        kind = ["XXE", "DTD retrieval"] and
+        feature = handler.getMember("feature_external_ges")
+      )
+    }
+  }
+
+  /**
+   * Gets a call to:
+   * * `lxml.etree.XMLParser`
+   * * `lxml.etree.get_default_parser`
+   *
+   * Given the following example:
+   *
+   * ```py
+   * lxml.etree.XMLParser()
+   * ```
+   *
+   * * `this` would be `lxml.etree.XMLParser(resolve_entities=False)`.
+   * * `vulnerable(kind)`'s `kind` would be `XXE`
+   */
+  private class LXMLParser extends DataFlow::CallCfgNode, XML::XMLParser::Range {
+    LXMLParser() {
+      this =
+        API::moduleImport("lxml")
+            .getMember("etree")
+            .getMember(["XMLParser", "get_default_parser"])
+            .getACall()
+    }
+
+    override DataFlow::Node getAnInput() { none() }
+
+    override predicate vulnerable(string kind) {
+      kind = "XXE" and not this.getArgByName("resolve_entities").asExpr() = any(False f)
+      or
+      kind = ["Billion Laughs", "Quadratic Blowup"] and
+      (
+        this.getArgByName("huge_tree").asExpr() = any(True t) and
+        not this.getArgByName("resolve_entities").asExpr() = any(False f)
+      )
+    }
+  }
+
+  /**
+   * Gets a call to:
+   * * `lxml.etree.fromstring`
+   * * `xml.etree.fromstringlist`
+   * * `xml.etree.XML`
+   * * `xml.etree.parse`
+   *
+   * Given the following example:
+   *
+   * ```py
+   * parser = lxml.etree.XMLParser()
+   * lxml.etree.fromstring(xml_content, parser=parser).text
+   * ```
+   *
+   * * `this` would be `lxml.etree.fromstring(xml_content, parser=parser)`.
+   * * `getAnInput()`'s result would be `xml_content`.
+   * * `vulnerable(kind)`'s `kind` would be `XXE`.
+   */
+  private class LXMLParsing extends DataFlow::CallCfgNode, XML::XMLParsing::Range {
+    LXMLParsing() {
+      this =
+        API::moduleImport("lxml")
+            .getMember("etree")
+            .getMember(["fromstring", "fromstringlist", "XML", "parse"])
+            .getACall()
+    }
+
+    override DataFlow::Node getAnInput() { result = this.getArg(0) }
+
+    override predicate vulnerable(string kind) {
+      exists(XML::XMLParser xmlParser |
+        xmlParser = this.getArgByName("parser").getALocalSource() and xmlParser.vulnerable(kind)
+      )
+      or
+      kind = "XXE" and not exists(this.getArgByName("parser"))
+    }
+  }
+
+  /**
+   * Gets a call to `xmltodict.parse`.
+   *
+   * Given the following example:
+   *
+   * ```py
+   * xmltodict.parse(xml_content, disable_entities=False)
+   * ```
+   *
+   * * `this` would be `xmltodict.parse(xml_content, disable_entities=False)`.
+   * * `getAnInput()`'s result would be `xml_content`.
+   * * `vulnerable(kind)`'s `kind` would be `Billion Laughs` and `Quadratic Blowup`.
+   */
+  private class XMLtoDictParsing extends DataFlow::CallCfgNode, XML::XMLParsing::Range {
+    XMLtoDictParsing() { this = API::moduleImport("xmltodict").getMember("parse").getACall() }
+
+    override DataFlow::Node getAnInput() { result = this.getArg(0) }
+
+    override predicate vulnerable(string kind) {
+      kind = ["Billion Laughs", "Quadratic Blowup"] and
+      this.getAMethodCall("disable_entities").asExpr() = any(False f)
+    }
+  }
+
+  /**
+   * Gets a call to:
+   * * `xml.dom.minidom.parse`
+   * * `xml.dom.pulldom.parse`
+   *
+   * Given the following example:
+   *
+   * ```py
+   * xml.dom.minidom.parse(StringIO(xml_content)).documentElement.childNode
+   * ```
+   *
+   * * `this` would be `xml.dom.minidom.parse(StringIO(xml_content), parser=parser)`.
+   * * `getAnInput()`'s result would be `StringIO(xml_content)`.
+   * * `vulnerable(kind)`'s `kind` would be `Billion Laughs` and `Quadratic Blowup`.
+   */
+  private class XMLDomParsing extends DataFlow::CallCfgNode, XML::XMLParsing::Range {
+    XMLDomParsing() {
+      this =
+        API::moduleImport("xml")
+            .getMember("dom")
+            .getMember(["minidom", "pulldom"])
+            .getMember(["parse", "parseString"])
+            .getACall()
+    }
+
+    override DataFlow::Node getAnInput() { result = this.getArg(0) }
+
+    override predicate vulnerable(string kind) {
+      exists(XML::XMLParser xmlParser |
+        xmlParser = this.getArgByName("parser").getALocalSource() and xmlParser.vulnerable(kind)
+      )
+      or
+      kind = ["Billion Laughs", "Quadratic Blowup"] and not exists(this.getArgByName("parser"))
+    }
+  }
+
+  /**
+   * Gets a call to `xmlrpc.server.SimpleXMLRPCServer`.
+   *
+   * Given the following example:
+   *
+   * ```py
+   * server = SimpleXMLRPCServer(("127.0.0.1", 8000))
+   * server.register_function(foo, "foo")
+   * server.serve_forever()
+   * ```
+   *
+   * * `this` would be `SimpleXMLRPCServer(("127.0.0.1", 8000))`.
+   * * `getAnInput()`'s result would be `foo`.
+   * * `vulnerable(kind)`'s `kind` would be `Billion Laughs` and `Quadratic Blowup`.
+   */
+  private class XMLRPCServer extends DataFlow::CallCfgNode, XML::XMLParser::Range {
+    XMLRPCServer() {
+      this =
+        API::moduleImport("xmlrpc").getMember("server").getMember("SimpleXMLRPCServer").getACall()
+    }
+
+    override DataFlow::Node getAnInput() {
+      result = this.getAMethodCall("register_function").getArg(0)
+    }
+
+    override predicate vulnerable(string kind) { kind = ["Billion Laughs", "Quadratic Blowup"] }
+  }
+}

python/ql/src/experimental/semmle/python/security/XXE.qll

@@ -1,37 +0,0 @@
-import python
-import experimental.semmle.python.Concepts
-import semmle.python.dataflow.new.DataFlow
-import semmle.python.dataflow.new.TaintTracking
-import semmle.python.dataflow.new.RemoteFlowSources
-import semmle.python.dataflow.new.BarrierGuards
-import semmle.python.ApiGraphs
-
-/**
- * A taint-tracking configuration for detecting XML External entities abuse.
- *
- * This configuration uses `RemoteFlowSource` as a source because there's no
- * risk at parsing not user-supplied input without security options enabled.
- */
-class XXEFlowConfig extends TaintTracking::Configuration {
-  XXEFlowConfig() { this = "XXEFlowConfig" }
-
-  override predicate isSource(DataFlow::Node source) { source instanceof RemoteFlowSource }
-
-  override predicate isSink(DataFlow::Node sink) {
-    exists(XMLParsing xmlParsing | xmlParsing.mayBeDangerous() and sink = xmlParsing.getAnInput())
-    or
-    exists(XMLParser xmlParser | sink = xmlParser.getAnInput() and xmlParser.mayBeDangerous())
-  }
-
-  override predicate isSanitizerGuard(DataFlow::BarrierGuard guard) {
-    guard instanceof StringConstCompare
-  }
-
-  override predicate isAdditionalTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
-    exists(DataFlow::CallCfgNode ioCalls |
-      ioCalls = API::moduleImport("io").getMember(["StringIO", "BytesIO"]).getACall() and
-      nodeFrom = ioCalls.getArg(0) and
-      nodeTo = ioCalls
-    )
-  }
-}

python/ql/src/experimental/semmle/python/security/dataflow/XmlInjection.qll

@@ -0,0 +1,44 @@
+import python
+import experimental.semmle.python.Concepts
+import semmle.python.dataflow.new.DataFlow
+import semmle.python.dataflow.new.TaintTracking
+import semmle.python.dataflow.new.RemoteFlowSources
+import semmle.python.dataflow.new.BarrierGuards
+
+module XmlInjection {
+  import XmlInjectionCustomizations::XmlInjection
+
+  class XMLInjectionConfiguration extends TaintTracking::Configuration {
+    XMLInjectionConfiguration() { this = "XMLInjectionConfiguration" }
+
+    override predicate isSource(DataFlow::Node source) {
+      source instanceof RemoteFlowSourceAsSource
+    }
+
+    override predicate isSink(DataFlow::Node sink) { sink instanceof Sink }
+
+    override predicate isSanitizerGuard(DataFlow::BarrierGuard guard) {
+      guard instanceof SanitizerGuard
+    }
+
+    override predicate isAdditionalTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
+      ioAdditionalTaintStep(nodeFrom, nodeTo)
+    }
+  }
+
+  private import DataFlow::PathGraph
+
+  /** Holds if there is an XML injection from `source` to `sink` */
+  predicate xmlInjection(DataFlow::PathNode source, DataFlow::PathNode sink) {
+    any(XMLInjectionConfiguration xmlInjectionConfig).hasFlowPath(source, sink)
+  }
+
+  /** Holds if there is an XML injection from `source` to `sink` vulnerable to `kind` */
+  predicate xmlInjectionVulnerable(DataFlow::PathNode source, DataFlow::PathNode sink, string kind) {
+    xmlInjection(source, sink) and
+    (
+      xmlParsingInputAsVulnerableSink(sink.getNode(), kind) or
+      xmlParserInputAsVulnerableSink(sink.getNode(), kind)
+    )
+  }
+}

python/ql/src/experimental/semmle/python/security/dataflow/XmlInjectionCustomizations.qll

@@ -0,0 +1,95 @@
+/**
+ * Provides default sources, sinks and sanitizers for detecting
+ * "ldap injection"
+ * vulnerabilities, as well as extension points for adding your own.
+ */
+
+private import python
+private import semmle.python.dataflow.new.DataFlow
+private import experimental.semmle.python.Concepts
+private import semmle.python.dataflow.new.RemoteFlowSources
+private import semmle.python.dataflow.new.BarrierGuards
+private import semmle.python.ApiGraphs
+
+/**
+ * Provides default sources, sinks and sanitizers for detecting "xml injection"
+ * vulnerabilities, as well as extension points for adding your own.
+ */
+module XmlInjection {
+  /**
+   * A data flow source for "xml injection" vulnerabilities.
+   */
+  abstract class Source extends DataFlow::Node { }
+
+  /**
+   * A data flow sink for "xml injection" vulnerabilities.
+   */
+  abstract class Sink extends DataFlow::Node { }
+
+  /**
+   * A sanitizer guard for "xml injection" vulnerabilities.
+   */
+  abstract class SanitizerGuard extends DataFlow::BarrierGuard { }
+
+  /**
+   * A data flow sink for XML parsing libraries.
+   *
+   * See `XML::XMLParsing`.
+   */
+  abstract class XMLParsingSink extends Sink { }
+
+  /**
+   * A data flow sink for XML parsers.
+   *
+   * See `XML::XMLParser`
+   */
+  abstract class XMLParserSink extends Sink { }
+
+  /**
+   * A source of remote user input, considered as a flow source.
+   */
+  class RemoteFlowSourceAsSource extends Source, RemoteFlowSource { }
+
+  /**
+   * An xml parsing operation, considered as a flow sink.
+   */
+  class XMLParsingInputAsSink extends XMLParsingSink {
+    XMLParsingInputAsSink() { this = any(XML::XMLParsing xmlParsing).getAnInput() }
+  }
+
+  /**
+   * An xml parsing operation vulnerable to `kind`.
+   */
+  predicate xmlParsingInputAsVulnerableSink(DataFlow::Node sink, string kind) {
+    exists(XML::XMLParsing xmlParsing |
+      sink = xmlParsing.getAnInput() and xmlParsing.vulnerable(kind)
+    )
+  }
+
+  /**
+   * An xml parser operation, considered as a flow sink.
+   */
+  class XMLParserInputAsSink extends XMLParserSink {
+    XMLParserInputAsSink() { this = any(XML::XMLParser xmlParser).getAnInput() }
+  }
+
+  /**
+   * An xml parser operation vulnerable to `kind`.
+   */
+  predicate xmlParserInputAsVulnerableSink(DataFlow::Node sink, string kind) {
+    exists(XML::XMLParser xmlParser | sink = xmlParser.getAnInput() and xmlParser.vulnerable(kind))
+  }
+
+  /**
+   * A comparison with a constant string, considered as a sanitizer-guard.
+   */
+  class StringConstCompareAsSanitizerGuard extends SanitizerGuard, StringConstCompare { }
+
+  predicate ioAdditionalTaintStep(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
+    exists(DataFlow::CallCfgNode ioCalls |
+      ioCalls = API::moduleImport("io").getMember(["StringIO", "BytesIO"]).getACall() and
+      nodeFrom = ioCalls.getArg(0) and
+      nodeTo = ioCalls
+    )
+  }
+}

python/ql/test/experimental/query-tests/Security/CWE-611/XXE.qlref

@@ -1 +0,0 @@
-experimental/Security/CWE-611/XXE.ql

python/ql/test/experimental/query-tests/Security/CWE-611/XmlInjection.qlref

@@ -0,0 +1 @@
+experimental/Security/CWE-611/XmlInjection.ql

python/ql/test/experimental/query-tests/Security/CWE-611/general.py

@@ -1,73 +0,0 @@
-from flask import request, Flask
-from io import StringIO, BytesIO
-import xml.etree
-import xml.etree.ElementTree
-import lxml.etree
-import xml.dom.minidom
-import xml.dom.pulldom
-import xmltodict
-
-
-app = Flask(__name__)
-
-# xml_content = '<?xml version="1.0"?><!DOCTYPE dt [<!ENTITY xxe SYSTEM "file:///etc/passwd">]><test>&xxe;</test>'
-
-
-@app.route("/lxml.etree.fromstring")
-def lxml_fromstring():
-    xml_content = request.args['xml_content']
-
-    return lxml.etree.fromstring(xml_content).text
-
-
-@app.route("/lxml.etree.XML")
-def lxml_XML():
-    xml_content = request.args['xml_content']
-
-    return lxml.etree.XML(xml_content).text
-
-
-@app.route("/lxml.etree.parse")
-def lxml_parse():
-    xml_content = request.args['xml_content']
-
-    return lxml.etree.parse(StringIO(xml_content)).text
-
-
-@app.route("/xmltodict.parse")
-def xmltodict_parse():
-    xml_content = request.args['xml_content']
-
-    return xmltodict.parse(xml_content, disable_entities=False)
-
-
-@app.route("/lxml.etree.XMLParser+lxml.etree.fromstring")
-def lxml_XMLParser_fromstring():
-    xml_content = request.args['xml_content']
-
-    parser = lxml.etree.XMLParser()
-    return lxml.etree.fromstring(xml_content, parser=parser).text
-
-
-@app.route("/lxml.etree.get_default_parser+lxml.etree.fromstring")
-def lxml_defaultParser_fromstring():
-    xml_content = request.args['xml_content']
-
-    parser = lxml.etree.get_default_parser()
-    return lxml.etree.fromstring(xml_content, parser=parser).text
-
-
-@app.route("/lxml.etree.XMLParser+xml.etree.ElementTree.fromstring")
-def lxml_XMLParser_xml_fromstring():
-    xml_content = request.args['xml_content']
-
-    parser = lxml.etree.XMLParser()
-    return xml.etree.ElementTree.fromstring(xml_content, parser=parser).text
-
-
-@app.route("/lxml.etree.XMLParser+xml.etree.ElementTree.parse")
-def lxml_XMLParser_xml_parse():
-    xml_content = request.args['xml_content']
-
-    parser = lxml.etree.XMLParser()
-    return xml.etree.ElementTree.parse(StringIO(xml_content), parser=parser).getroot().text

python/ql/test/experimental/query-tests/Security/CWE-611/lxml_etree.py

@@ -0,0 +1,76 @@
+from flask import request, Flask
+from io import StringIO, BytesIO
+import lxml.etree
+
+app = Flask(__name__)
+
+# Parsing
+
+@app.route("/lxml_etree_fromstring")
+def lxml_etree_fromstring():
+    xml_content = request.args['xml_content']
+
+    return lxml.etree.fromstring(xml_content).text
+
+@app.route("/lxml_etree_fromstringlist")
+def lxml_etree_fromstringlist():
+    xml_content = request.args['xml_content']
+
+    return lxml.etree.fromstringlist([xml_content]).text
+
+@app.route("/lxml_etree_XML")
+def lxml_etree_XML():
+    xml_content = request.args['xml_content']
+
+    return lxml.etree.XML(xml_content).text
+
+@app.route("/lxml_etree_parse")
+def lxml_etree_parse():
+    xml_content = request.args['xml_content']
+
+    return lxml.etree.parse(StringIO(xml_content)).getroot().text
+
+# With parsers - Default
+
+@app.route("/lxml_etree_fromstring-lxml.etree.XMLParser")
+def lxml_parser():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser()
+    return lxml.etree.fromstring(xml_content, parser=parser).text
+
+@app.route("/lxml_etree_fromstring-lxml.etree.get_default_parser")
+def lxml_parser():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.get_default_parser()
+    return lxml.etree.fromstring(xml_content, parser=parser).text
+
+# With parsers - With options
+
+# XXE-safe
+@app.route("/lxml_etree_fromstring-lxml.etree.XMLParser+")
+def lxml_parser():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser(resolve_entities=False)
+    return lxml.etree.fromstring(xml_content, parser=parser).text
+
+# Billion laughs and quadratic blowup (huge_tree)
+
+## Good (huge_tree=True but resolve_entities=False)
+
+@app.route("/lxml_etree_fromstring-lxml.etree.XMLParser+")
+def lxml_parser():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser(resolve_entities=False, huge_tree=True)
+    return lxml.etree.fromstring(xml_content, parser=parser).text
+
+## Bad
+@app.route("/lxml_etree_fromstring-lxml.etree.XMLParser+")
+def lxml_parser():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser(huge_tree=True)
+    return lxml.etree.fromstring(xml_content, parser=parser).text

python/ql/test/experimental/query-tests/Security/CWE-611/xml_dom.py

@@ -0,0 +1,44 @@
+from flask import request, Flask
+from io import StringIO, BytesIO
+import xml.dom.minidom
+import xml.dom.pulldom
+import xml.sax
+
+app = Flask(__name__)
+
+# Parsing
+
+@app.route("/xml_minidom_parse")
+def xml_minidom_parse():
+    xml_content = request.args['xml_content']
+
+    return xml.dom.minidom.parse(StringIO(xml_content)).documentElement.childNodes
+
+@app.route("/xml_minidom_parseString")
+def xml_minidom_parseString():
+    xml_content = request.args['xml_content']
+
+    return xml.dom.minidom.parseString(xml_content).documentElement.childNodes
+
+@app.route("/xml_pulldom_parse")
+def xml_pulldom_parse():
+    xml_content = request.args['xml_content']
+
+    return xml.dom.pulldom.parse(StringIO(xml_content))['START_DOCUMENT'][1].documentElement.childNodes
+
+@app.route("/xml_pulldom_parseString")
+def xml_pulldom_parseString():
+    xml_content = request.args['xml_content']
+
+    return xml.dom.pulldom.parseString(xml_content)['START_DOCUMENT'][1].documentElement.childNodes
+
+# With parsers
+
+@app.route("/xml_minidom_parse_xml_sax_make_parser")
+def xml_minidom_parse_xml_sax_make_parser():
+    xml_content = request.args['xml_content']
+
+    parser = xml.sax.make_parser()
+    parser.setFeature(xml.sax.handler.feature_external_ges, True)
+    return xml.dom.minidom.parse(StringIO(xml_content), parser=parser).documentElement.childNodes
+

python/ql/test/experimental/query-tests/Security/CWE-611/xml_etree.py

@@ -0,0 +1,66 @@
+from flask import request, Flask
+from io import StringIO, BytesIO
+import xml.etree
+import xml.etree.ElementTree
+import lxml.etree
+
+app = Flask(__name__)
+
+# xxe = '<?xml version="1.0"?><!DOCTYPE dt [<!ENTITY xxe SYSTEM "file:///etc/passwd">]><test>&xxe;</test>'
+
+# Parsing
+
+@app.route("/xml_etree_fromstring")
+def xml_etree_fromstring():
+    xml_content = request.args['xml_content']
+
+    return xml.etree.ElementTree.fromstring(xml_content).text
+
+@app.route("/xml_etree_fromstringlist")
+def xml_etree_fromstringlist():
+    xml_content = request.args['xml_content']
+
+    return xml.etree.ElementTree.fromstringlist(xml_content).text
+
+@app.route("/xml_etree_XML")
+def xml_etree_XML():
+    xml_content = request.args['xml_content']
+
+    return xml.etree.ElementTree.XML(xml_content).text
+
+@app.route("/xml_etree_parse")
+def xml_etree_parse():
+    xml_content = request.args['xml_content']
+
+    return xml.etree.ElementTree.parse(StringIO(xml_content)).getroot().text
+
+# With parsers
+
+@app.route("/xml_etree_fromstring-xml_etree_XMLParser")
+def xml_parser_1():
+    xml_content = request.args['xml_content']
+
+    parser = xml.etree.ElementTree.XMLParser()
+    return xml.etree.ElementTree.fromstring(xml_content, parser=parser).text
+
+@app.route("/xml_etree_fromstring-lxml_etree_XMLParser")
+def xml_parser_2():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.XMLParser()
+    return xml.etree.ElementTree.fromstring(xml_content, parser=parser).text
+
+@app.route("/xml_etree_fromstring-lxml_get_default_parser")
+def xml_parser_3():
+    xml_content = request.args['xml_content']
+
+    parser = lxml.etree.get_default_parser()
+    return xml.etree.ElementTree.fromstring(xml_content, parser=parser).text
+
+@app.route("/xml_etree_fromstring-lxml_get_default_parser")
+def xml_parser_4():
+    xml_content = request.args['xml_content']
+
+    parser = xml.sax.make_parser()
+    parser.setFeature(xml.sax.handler.feature_external_ges, True)
+    return xml.etree.ElementTree.fromstring(xml_content, parser=parser).text

python/ql/test/experimental/query-tests/Security/CWE-611/xml_sax_make_parser.py

@@ -2,7 +2,7 @@ from flask import request, Flask
 from io import StringIO
 import xml.sax
 
-# xml_content = '<?xml version="1.0"?><!DOCTYPE dt [<!ENTITY xxe SYSTEM "file:///etc/passwd">]><test>&xxe;</test>'
+# xxe = '<?xml version="1.0"?><!DOCTYPE dt [<!ENTITY xxe SYSTEM "file:///etc/passwd">]><test>&xxe;</test>'
 
 app = Flask(__name__)
 
@@ -74,12 +74,28 @@ def xml_makeparser_minidom_entitiesTrue():
     parser.setFeature(xml.sax.handler.feature_external_ges, True)
     return xml.dom.minidom.parse(StringIO(xml_content), parser=parser).documentElement.childNodes
 
-# Forward Type Tracker test
+# Forward Type Tracking test
+
+@app.route("forward_tracking1")
+def forward_tracking1(action):
+    xml_content = request.args['xml_content']
 
-def contrived_example(user_input, action):
     parser = xml.sax.make_parser()
     if action == 'load-config':
         parser.setFeature(xml.sax.handler.feature_external_ges, False)
         parser.parse("/not-user-controlled/default_config.xml")
     else:
-        parser.parse(StringIO(user_input))
+        parser.parse(StringIO(xml_content))
+    return 
+
+@app.route("forward_tracking2")
+def forward_tracking2(action):
+    xml_content = request.args['xml_content']
+
+    parser = xml.sax.make_parser()
+    if action == 'load-config':
+        parser.setFeature(xml.sax.handler.feature_external_ges, True)
+        parser.parse("/not-user-controlled/default_config.xml")
+    else:
+        parser.parse(StringIO(xml_content))
+    return 

python/ql/test/experimental/query-tests/Security/CWE-611/xml_to_dict.py

@@ -0,0 +1,17 @@
+from flask import request, Flask
+from io import StringIO, BytesIO
+import xmltodict
+
+app = Flask(__name__)
+
+@app.route("/xmltodict.parse")
+def xmltodict_parse():
+    xml_content = request.args['xml_content']
+
+    return xmltodict.parse(xml_content)
+
+@app.route("/xmltodict.parse2")
+def xmltodict_parse2():
+    xml_content = request.args['xml_content']
+
+    return xmltodict.parse(xml_content, disable_entities=False)

python/ql/test/experimental/query-tests/Security/CWE-611/xmlrpc_server.py

@@ -0,0 +1,10 @@
+from xmlrpc.server import SimpleXMLRPCServer
+
+def foo(n):
+    return n
+
+server = SimpleXMLRPCServer(("127.0.0.1", 8000))
+server.register_function(foo, "foo")
+server.serve_forever()
+
+# billion_laughs -> curl 127.0.0.1:8000 --data-raw '<?xml version="1.0"?><!DOCTYPE lolz [<!ENTITY lol "lol"><!ELEMENT lolz (#PCDATA)><!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;"><!ENTITY lol2 "&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;&lol1;"><!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;"><!ENTITY lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;"><!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;"><!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;"><!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;"><!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;"><!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">]><methodCall><methodName>foo</methodName><params><param><value>&lol9;</value></param></params></methodCall>'