Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

5.4 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N

EPSS

• CVE-2026-115877: 0.26% estimated exploitation activity in the next 30 days; percentile 17.39 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary An incomplete header-value validation (CWE-444) in the Twisted HTTP server lets a bare carriage return or line feed in a header value hide a following header from the parser. twisted.web.http.HTTPChannel.headerReceived rejects a NUL byte in a header value but not a bare CR or LF, even though RFC 9110 section 5.5 requires a recipient to reject all three. Because the channel splits requests only on CRLF, a value carrying a bare LF and a second header arrives as one line, so the second header is folded into the value and the request is framed with no body. A front-end that treats the bare LF as a line terminator frames the request differently, and the disagreement lets an attacker smuggle a request past it. This is an incomplete fix of the earlier Twisted request-smuggling advisories, which guarded the NUL sibling of this exact check. Details headerReceived splits a header line on the first colon, strips space and tab from the value, and validates it with a single test that…

Published by the source

23 Oct 2026, 20:44 UTC

Last modified by the source

23 Oct 2026, 21:00 UTC

Source last checked

26 Oct 2026, 13:55 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-q7qq-6mhp-9rc4

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GHSA-q7qq-6mhp-9rc4

3. Revision 1 · created · 23 Oct 2026

   Published from ghsa record GHSA-q7qq-6mhp-9rc4

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# Patch Next advisory brief: CVE-2026-115877

Advisory: Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value
IDs: CVE-2026-115877, GHSA-q7qq-6mhp-9rc4
Record: https://patchnext.com/advisories/cve-2026-115877 (revision 3, source checked 26 Oct 2026, 13:55 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.4)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: twisted 26.10.0rc1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI twisted: affected <= 26.4.0; fixed in 26.10.0rc1

## Suggested inspection (read-only)
- `python -m pip show twisted`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on twisted.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-q7qq-6mhp-9rc4 (GitHub-reviewed advisory GHSA-q7qq-6mhp-9rc4 lists affected ranges: PyPI twisted >=0 <=26.4.0)
- Verified: https://osv.dev/vulnerability/GHSA-q7qq-6mhp-9rc4 (OSV record GHSA-q7qq-6mhp-9rc4 lists affected ranges: PyPI twisted >=0 <26.10.0rc1 (fixed))

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