JavaScript: Add query IncompleteUrlSchemeCheck.ql.

javascript/ql/src/Security/CWE-020/IncompleteUrlSchemeCheck.qhelp

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+<!DOCTYPE qhelp PUBLIC
+"-//Semmle//qhelp//EN"
+"qhelp.dtd">
+<qhelp>
+<overview>
+<p>
+URLs starting with <code>javascript:</code> can be used to encode JavaScript code to be executed
+when the URL is visited. While this is a powerful mechanism for creating feature-rich and responsive
+web applications, it is also a potential security risk: if the URL comes from an untrusted source,
+it might contain harmful JavaScript code. For this reason, many frameworks and libraries first check
+the URL scheme of any untrusted URL, and reject URLs with the <code>javascript:</code> scheme.
+</p>
+<p>
+However, the <code>data:</code> and <code>vbscript:</code> schemes can be used to represent
+executable code in a very similar way, so any validation logic that checks against
+<code>javascript:</code> but not against <code>data:</code> and <code>vbscript:</code> is likely to
+be insufficient.
+</p>
+</overview>
+<recommendation>
+<p>
+Add checks covering both <code>data:</code> and <code>vbscript:</code>.
+</p>
+</recommendation>
+<example>
+<p>
+The following function validates a (presumably untrusted) URL <code>url</code>. If it starts with
+<code>javascript:</code> (case-insensitive and potentially preceded by whitespace), the harmless
+placeholder URL <code>about:blank</code> is returned to prevent code injection; otherwise
+<code>url</code> itself is returned.
+</p>
+<sample src="examples/IncompleteUrlSchemeCheck.js"/>
+<p>
+While this check provides partial projection, it should be extended to cover <code>data:</code>
+and <code>vbscript:</code> as well:
+</p>
+<sample src="examples/IncompleteUrlSchemeCheckGood.js"/>
+</example>
+<references>
+<li>WHATWG: <a href="https://wiki.whatwg.org/wiki/URL_schemes">URL schemes</a>.</li>
+</references>
+</qhelp>

javascript/ql/src/Security/CWE-020/IncompleteUrlSchemeCheck.ql

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+/**
+ * @name Incomplete URL scheme check
+ * @description Checking for the "javascript:" URL scheme without also checking for "vbscript:"
+ *              and "data:" suggests a logic error or even a security vulnerability.
+ * @kind problem
+ * @problem.severity warning
+ * @precision high
+ * @id js/incomplete-url-scheme-check
+ * @tags security
+ *       correctness
+ *       external/cwe/cwe-020
+ */
+
+import javascript
+import semmle.javascript.dataflow.internal.AccessPaths
+
+/** A URL scheme that can be used to represent executable code. */
+class DangerousScheme extends string {
+  DangerousScheme() { this = "data:" or this = "javascript:" or this = "vbscript:" }
+}
+
+/** Gets a data-flow node that checks `nd` against the given `scheme`. */
+DataFlow::Node schemeCheck(
+  DataFlow::Node nd, DangerousScheme scheme
+) {
+  // check of the form `nd.startsWith(scheme)`
+  exists(StringOps::StartsWith sw | sw = result |
+    sw.getBaseString() = nd and
+    sw.getSubstring().mayHaveStringValue(scheme)
+  )
+  or
+  // propagate through trimming, case conversion, and regexp replace
+  exists(DataFlow::MethodCallNode stringop |
+    stringop.getMethodName().matches("trim%") or
+    stringop.getMethodName().matches("to%Case") or
+    stringop.getMethodName() = "replace"
+  |
+    result = schemeCheck(stringop, scheme) and
+    nd = stringop.getReceiver()
+  )
+  or
+  // propagate througb local data flow
+  result = schemeCheck(nd.getASuccessor(), scheme)
+}
+
+/** Gets a data-flow node that checks an instance of `ap` against the given `scheme`. */
+DataFlow::Node schemeCheckOn(AccessPath ap, DangerousScheme scheme) {
+  result = schemeCheck(ap.getAnInstance().flow(), scheme)
+}
+
+from AccessPath ap, int n
+where
+  n = strictcount(DangerousScheme s) and
+  strictcount(DangerousScheme s | exists(schemeCheckOn(ap, s))) < n
+select schemeCheckOn(ap, "javascript:"),
+  "This check does not consider " +
+    strictconcat(DangerousScheme s | not exists(schemeCheckOn(ap, s)) | s, " and ") + "."

javascript/ql/src/Security/CWE-020/examples/IncompleteUrlSchemeCheck.js

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+function sanitizeUrl(url) {
+    let u = decodeURI(url).trim().toLowerCase();
+    if (u.startsWith("javascript:"))
+        return "about:blank";
+    return url;
+}

javascript/ql/src/Security/CWE-020/examples/IncompleteUrlSchemeCheckGood.js

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+function sanitizeUrl(url) {
+    let u = decodeURI(url).trim().toLowerCase();
+    if (u.startsWith("javascript:") || u.startsWith("data:") || u.startsWith("vbscript:"))
+        return "about:blank";
+    return url;
+}