Add documentation

python/ql/src/Security/CWE-113/HeaderInjection.qhelp

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+<!DOCTYPE qhelp PUBLIC
+  "-//Semmle//qhelp//EN"
+  "qhelp.dtd">
+<qhelp>
+
+<overview>
+<p>Directly writing user input (for example, an HTTP request parameter) to an HTTP header
+can lead to an HTTP response-splitting vulnerability.</p>
+
+<p>If user-controlled input is used in an HTTP header that allows line break characters, an attacker can 
+inject additional headers or control the response body, leading to vulnerabilities such as XSS or cache poisoning.
+</p>
+
+</overview>
+
+<recommendation>
+Ensure that user input containing line break characters is not written to an HTTP header.
+</recommendation>
+
+<example>
+In the following example, the case marked BAD writes user input to the header name. 
+In the GOOD case, input is first escaped to not contain any line break characters.
+<sample src="examples/header_injection.py" />
+</example>
+
+<references>
+<li>
+SecLists.org: <a href="https://seclists.org/bugtraq/2005/Apr/187">HTTP response splitting</a>.
+</li>
+<li>
+OWASP:
+<a href="https://www.owasp.org/index.php/HTTP_Response_Splitting">HTTP Response Splitting</a>.
+</li>
+<li>
+Wikipedia: <a href="http://en.wikipedia.org/wiki/HTTP_response_splitting">HTTP response splitting</a>.
+</li>
+<li>
+CAPEC: <a href="https://capec.mitre.org/data/definitions/105.html">CAPEC-105: HTTP Request Splitting</a>
+</li>
+</references>
+</qhelp>

python/ql/src/Security/CWE-113/examples/header_injection.py

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+@app.route("/example_bad")
+def example_bad():
+    rfs_header = request.args["rfs_header"]
+    response = Response()
+    custom_header = "X-MyHeader-" + rfs_header
+    # BAD: User input is used as part of the header name.
+    response.headers[custom_header] = "HeaderValue" 
+    return response
+
+@app.route("/example_good")
+def example_bad():
+    rfs_header = request.args["rfs_header"]
+    response = Response()
+    custom_header = "X-MyHeader-" + rfs_header.replace("\n", "").replace("\r","").replace(":","")
+    # GOOD: Line break characters are removed from the input.
+    response.headers[custom_header] = "HeaderValue" 
+    return response