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2.7.18.16: SimpleHTTPServer // open redirect (CVE-2021-28861) - #97
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CVE-2021-28861) Backport of python/cpython pythongh-87389 (pythonGH-93879, 4dc2cae) to 2.7.18.x. A request path starting with "//" that resolves to a directory made SimpleHTTPRequestHandler.send_head() answer with a 301 whose Location header also started with "//". Browsers treat "//host/path" as a scheme-relative URL, so a crafted link to the server redirected the user to an attacker-chosen host. BaseHTTPRequestHandler.parse_request() now reduces a leading run of slashes in the path to a single "/". Verified on 2.7.18.15: the "//python.org/..." request answers "Location: //python.org/..."; with this change it is "/python.org/...". 2.7.18.x: the fix lands in BaseHTTPServer.parse_request(); the test uses string concatenation and the literal 301 status instead of f-strings and HTTPStatus. Co-Authored-By: Claude Opus 5.5 <[email protected]>
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Oct 28, 2026
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No blockers.
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Fix matches upstream. The leading-slash collapse in
parse_request()is upstream pythongh-87389 (4abab6b). No later upstream fix changes it. -
Repro (
SimpleHTTPRequestHandler, serving a directory withexisting_directory/):request path base Locationthis branch Location//python.org/..%2f..%2f..%2f..%2f..%2f../%0a%0d/../existing_directory//python.org/…/python.org/…////evil.com/..%2f../existing_directory//evil.com/…/evil.com/…/existing_directory/existing_directory/unchanged -
Tests:
test_httpserverspasses. -
FYI, not for this PR:
/\evil.com/..%2f../existing_directorystill redirects toLocation: /\evil.com/..%2f../existing_directory/, which browsers read as//evil.com/…. Upstream 3.14 does exactly the same, so this isn't a gap in the backport.
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Backport of python/cpython pythongh-87389 (CVE-2021-28861, PSF-2022-5) to 2.7.18.x. This fix was released after Python 2.7's end of life.
Problem: if a request path starts with
//and resolves to a directory,SimpleHTTPRequestHandleranswers with a 301 whoseLocationalso starts with//. Browsers treat//host/pathas a URL to another host, so a crafted link to the server can redirect users anywhere.Fix:
BaseHTTPRequestHandler.parse_request()now collapses any leading run of slashes in the path to a single/.Test:
test_get_dir_redirect_location_domain_injection_bug, adapted from upstream with string formatting and the literal 301.Location: //python.org/..%2f…/existing_directory/.test_httpserverspasses 41/41.Part of 2.7.18.16. It targets the
2.7.18.16integration branch.