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python: model send_header from http.server #19432

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May 1, 2025
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python: model send_header from http.server #19432

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python: model `send_header` from `http.server`
yoff Apr 30, 2025
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python: remove copied comment
yoff Apr 30, 2025
e63b38c
python: add change note
yoff Apr 30, 2025
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4 changes: 4 additions & 0 deletions python/ql/lib/change-notes/2025-04-30-model-send-header.md
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@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* Added header write model for `send_header` in `http.server`.
15 changes: 15 additions & 0 deletions python/ql/lib/semmle/python/frameworks/Stdlib.qll
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Expand Up @@ -1963,6 +1963,21 @@ module StdlibPrivate {
/** Gets a reference to an instance of the `BaseHttpRequestHandler` class or any subclass. */
DataFlow::Node instance() { instance(DataFlow::TypeTracker::end()).flowsTo(result) }

/** A call to a method that writes to a response header. */
private class HeaderWriteCall extends Http::Server::ResponseHeaderWrite::Range,
DataFlow::MethodCallNode
{
HeaderWriteCall() { this.calls(instance(), "send_header") }

override DataFlow::Node getNameArg() { result = this.getArg(0) }

override DataFlow::Node getValueArg() { result = this.getArg(1) }

override predicate nameAllowsNewline() { any() }
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Copilot AI Apr 30, 2025

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[nitpick] Consider refining the predicate for nameAllowsNewline if possible, rather than using a blanket 'any()' which may reduce the precision of the taint tracking.

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override predicate nameAllowsNewline() { any() }
override predicate nameAllowsNewline() { not exists(this.getNameArg().toString().regexpMatch(".*[\n\r].*")) }

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It is not about the argument itself, but rather wether the server configuration would allow a newline to appear here. http.server does not have any protection against this, so any is correct.


override predicate valueAllowsNewline() { any() }
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[nitpick] Consider refining the predicate for valueAllowsNewline if feasible, to improve the accuracy of tracking header values in the taint analysis.

Suggested change
override predicate valueAllowsNewline() { any() }
override predicate valueAllowsNewline() {
not exists(DataFlow::Node value | value = this.getValueArg() |
value.toString().matches("%0A") or value.toString().matches("%0D")
)
}

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Same as for the name argument above.

}

private class AdditionalTaintStep extends TaintTracking::AdditionalTaintStep {
override predicate step(DataFlow::Node nodeFrom, DataFlow::Node nodeTo) {
nodeFrom = instance() and
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2 changes: 1 addition & 1 deletion python/ql/test/library-tests/frameworks/stdlib/http_server.py
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Expand Up @@ -83,7 +83,7 @@ def taint_sources(self):
def do_GET(self): # $ requestHandler
# send_response will log a line to stderr
self.send_response(200)
self.send_header("Content-type", "text/plain; charset=utf-8")
self.send_header("Content-type", "text/plain; charset=utf-8") # $ headerWriteNameUnsanitized="Content-type" headerWriteValueUnsanitized="text/plain; charset=utf-8"
self.end_headers()
self.wfile.write(b"Hello BaseHTTPRequestHandler\n")
self.wfile.writelines([b"1\n", b"2\n", b"3\n"])
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