# Autobahn Python permessage-deflate bypasses maxMessagePayloadSize after inflation

Advisory record · `GHSA-hxp9-w8x3-p566`

- **Identifiers:** `GHSA-hxp9-w8x3-p566` · `CVE-2026-77528` · `PYSEC-2026-4027` · `PYSEC-2026-4031`
- **State:** Confirmed
- **Severity:** CVSS 5.3 Medium
- **Critical:** No
- **Weakness (CWE):** [CWE-409](https://cwe.mitre.org/data/definitions/409.html) · [CWE-770](https://cwe.mitre.org/data/definitions/770.html)
- **Tags:** [Denial of service](https://agentprime.io/advisories/tag/dos.md) · [CWE-409](https://agentprime.io/advisories/tag/cwe-409.md)
- **Published:** 22 Sept 2026
- **Updated:** 1 Oct 2026
- **Reviewed by a person:** No

Published by the GitHub Advisory Database at 22 Sept 2026, 20:37 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `autobahn` (PyPI): affected `<26.7.1`; fixed in `26.7.1`
- `crossbar` (PyPI): affected `<26.7.1`; fixed in `26.7.1`

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> Autobahn and Crossbar versions before 26.7.1 are affected. The message size limit (maxMessagePayloadSize) is checked against compressed WebSocket frame length, but not after permessage-deflate inflation, so a small compressed frame can expand into a larger message that reaches application handlers. A remote client that can connect to an endpoint accepting permessage-deflate can exploit this to exceed the configured limit, enabling resource exhaustion. Upgrade autobahn and crossbar to 26.7.1 or later.

Written 1 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 5.3 Medium, CVSS 3.1: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L`, from CNA (GitHub_M)

- **CISA KEV:** Not listed.
- **EPSS:** 0.0052, the 44th percentile, on 5 Nov 2026: FIRST’s estimate of the chance of exploitation in the next 30 days.

## Timeline

- 31 Oct 2026, 14:09 UTC: First seen by agentprime
- 31 Oct 2026, 14:09 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/GHSA-hxp9-w8x3-p566)

## Sources

- [GitHub Advisory Database: GHSA-hxp9-w8x3-p566](https://github.com/advisories/GHSA-hxp9-w8x3-p566)
- [OSV: GHSA-hxp9-w8x3-p566](https://osv.dev/vulnerability/GHSA-hxp9-w8x3-p566)
- <https://github.com/crossbario/autobahn-python/security/advisories/GHSA-hxp9-w8x3-p566>
- <https://nvd.nist.gov/vuln/detail/CVE-2026-77528>
- <https://github.com/crossbario/autobahn-python/pull/1916>
- <https://github.com/crossbario/autobahn-python/commit/77d323a30b09b1828ad8be2ce6344e056970e613>
- <https://github.com/crossbario/autobahn-python>
- <https://github.com/crossbario/autobahn-python/releases/tag/v26_7_1>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/GHSA-hxp9-w8x3-p566>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/GHSA-hxp9-w8x3-p566/>. Everything for agents: <https://agentprime.io/llms.txt>.