PYSEC-2026-3943

Summary

This is a third follow-up to CVE-2024-42353 / GHSA-mg3v-6m49-jhp3 and CVE-2026-44889 / GHSA-fh3h-vg37-cc95.

WebOb makes the Location header absolute when it serves a redirect. To stop a relative or protocol-relative target from redirecting users off-host, it checks the value for a URI scheme and for a leading //, then joins it against the request URI with urllib.parse.urljoin(). The previous fix additionally stripped ASCII tab/CR/LF from the value before those checks.

However, on Python 3.10+ urllib.parse.urljoin() (via urlsplit()) does more than remove tab/CR/LF: it also strips leading and trailing C0 control characters (U+0000–U+001F) and spaces from the URL before parsing it. Because WebOb's guard checks (SCHEME_RE and startswith("//")) run against the un-stripped value, a single leading space or control byte slips past them, and urljoin() then silently removes that byte and parses what remains as a protocol-relative — or even absolute — URL. The result is an open redirect to an attacker-controlled host.

Details

Response._make_location_absolute() (in src/webob/response.py) performed, prior to the fix:

value = value.replace("\t", "").replace("\r", "").replace("\n", "")

if SCHEME_RE.search(value):          # ^[a-z]+:   -> already absolute, return as-is
    return value

if value.startswith("//"):           # neutralize protocol-relative URLs
    value = f"/%2f{value[2:]}"

new_location = urlparse.urljoin(_request_uri(environ), value)

Consider the Location value " //www.example.com/test" (a single leading space):

1. The explicit strip only removes \t, \r, \n — the leading space survives.
2. SCHEME_RE (^[a-z]+:) does not match — the value starts with a space.
3. value.startswith("//") is False — the value starts with a space, not /. The // → /%2f neutralization is skipped.
4. urllib.parse.urljoin(_request_uri(environ), " //www.example.com/test") then strips the leading space before parsing, sees //www.example.com/test, treats it as protocol-relative, and returns http://www.example.com/test.

The same bypass works with a value such as " https://www.example.com/test" (leading space + a full scheme): SCHEME_RE does not match the space-prefixed string, but urljoin() strips the space and returns the fully absolute attacker URL https://www.example.com/test.

Any C0 control character works equally well in place of the space, e.g. "\x00//www.example.com/test" or "\x1f//www.example.com/test", because urlsplit() strips the whole leading C0-control-and-space run.

Affected entry points

• Response.location — any application that sets a relative/attacker-influenced Location and serves the response (the classic redirect path).
• Request.relative_url() — used urllib.parse.urljoin() directly and was subject to the same character stripping.
• webob.exc._HTTPMove subclasses (HTTPMovedPermanently, HTTPFound, HTTPSeeOther, HTTPTemporaryRedirect, HTTPPermanentRedirect, etc.) — these built their absolute Location with urlparse.urljoin(req.path_url, self.location) without going through _make_location_absolute() at all, so they bypassed even the tab/CR/LF strip and the // → /%2f neutralization. A protocol-relative location passed to e.g. HTTPFound(location="//evil.example") redirected off-host.

Proof of Concept

from webob import Response
from webob.request import Request

res = Response()
res.status = "301"
res.location = " //www.example.com/test"   # note the single leading space

req = Request.blank("/")                    # request host is "localhost"
print(req.get_response(res).location)
# Vulnerable (<= 1.8.10):  http://www.example.com/test   <-- open redirect
# Fixed:                   http://localhost/ //www.example.com/test

Absolute-URL variant:

res.location = " https://www.example.com/test"
# Vulnerable: https://www.example.com/test   <-- off-host
# Fixed:      http://localhost/ https://www.example.com/test

Via the HTTP exceptions:

from webob import exc

environ = {
    "wsgi.url_scheme": "http", "SERVER_NAME": "localhost",
    "SERVER_PORT": "80", "REQUEST_METHOD": "HEAD", "PATH_INFO": "/",
}
m = exc.HTTPFound(location="//www.example.com/test")
m(environ, lambda *a, **k: None)
print(m.location)
# Vulnerable: //www.example.com/test          <-- open redirect
# Fixed:      http://localhost/%2fwww.example.com/test

Impact

An unauthenticated remote attacker who controls (in whole or part) the redirect target of an application built on WebOb can redirect a user from a trusted host to an attacker-controlled host. This enables phishing and credential-theft campaigns that abuse the trusted origin, and can be chained with OAuth/SSO redirect_uri flows to leak tokens. Exploitation requires user interaction (following the redirect). Confidentiality and integrity impact are limited (L); the scope is changed (C) because the trust boundary of the originating site is crossed.

Patches

Fixed by replacing the use of urllib.parse.urljoin() with WebOb's own RFC 3986 reference-resolution implementation, webob.util.urljoin(), which resolves the reference exactly as given, character for character, with no whitespace or control-character removal.

• Response._make_location_absolute() now uses webob.util.urljoin().
• Request.relative_url() now uses webob.util.urljoin().
• webob.exc._HTTPMove now normalizes its Location through the same _make_location_absolute() code path as Response, so protocol-relative and whitespace-smuggled locations are neutralized there too.

Users should upgrade to the patched release. There are no API changes.

Workarounds

• Only ever set the Location header / redirect target to a fully-qualified URI whose host you control, or strictly allowlist redirect destinations before handing them to WebOb.
• Reject any redirect target that does not begin with https://yourhost/ (or a validated relative path with no leading whitespace/control bytes).

References

• This advisory: GHSA-6hx8-3wjj-gr8g
• GHSA-fh3h-vg37-cc95 (CVE-2026-44889) — second incomplete fix (tab/CR/LF)
• GHSA-mg3v-6m49-jhp3 (CVE-2024-42353) — original open redirect fix
• RFC 3986, Section 5 — Reference Resolution: https://www.rfc-editor.org/rfc/rfc3986#section-5
• Python urllib.parse URL stripping behavior (CPython 3.10+, removal of leading and trailing C0 control and space characters): https://docs.python.org/3/library/urllib.parse.html

To report a vulnerability to the Pylons Project please take a look at:

• Pylons Project security policy and reporting process: https://github.com/Pylons/.github/blob/main/SECURITY.md
• Security contact (private, coordinated disclosure): pylons-project-security@googlegroups.com (the Pylons Project requests a 90-day disclosure embargo)

Credit

Reported via the Pylons Project security mailing list by:

• tonghuaroot — for the residual open redirect in Response._make_location_absolute(): the 1.8.10 fix stripped only ASCII tab/CR/LF, but urllib.parse.urljoin() also strips leading C0 control and space characters, so values such as " //attacker.example/path" (and " https://attacker.example/path") still escaped off-host.
• Matheus Polkorny — for identifying that the webob.exc._HTTPMove redirect exceptions (HTTPFound and friends) performed their own urllib.parse.urljoin() normalization and never went through _make_location_absolute(), so a protocol-relative location such as //evil.example/path/ redirected off-host through that separate code path.