Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of GHSA-hxp9-w8x3-p566)

Exploitation

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

CVSS

5.9 · CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-116579: 0.43% estimated exploitation activity in the next 30 days; percentile 36.83 (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 permessage-brotli inflates each frame fully before checking maxMessagePayloadSize, so one 421-byte frame makes the server allocate about 545 MiB even with a 1 MiB message limit (brotli expands about 1,000,000:1). Any unauthenticated client can exhaust server memory this way on servers that enable permessage-brotli. This bypasses the GHSA-hxp9-w8x3-p566 fix.

Details In 26.7.1 the receive path passes the remaining `maxMessagePayloadSize` budget to `decompress_message_data(..., max_output_len=...)`. deflate and bzip2 stop at that budget. `PerMessageBrotli.decompress_message_data` (`compress_brotli.py`) instead calls `self._decompressor.process(data)` with no limit and compares the length afterwards, so the whole frame is inflated first. The code comment and changelog say this is still "bounded on the wire by `maxFramePayloadSize`", but brotli reaches about 1,000,000:1 (841 bytes inflate to 1 GiB), so that bound means little. A 421-byte frame against a 1 MiB budget peaks…

Published by the source

28 Oct 2026, 21:10 UTC

Last modified by the source

28 Oct 2026, 21:30 UTC

Source last checked

29 Oct 2026, 06:10 UTC

1. Revision 2 · amended · 29 Oct 2026

   Amended from osv record GHSA-pjh6-c8mg-3x7m

2. Revision 1 · created · 28 Oct 2026

   Published from ghsa record GHSA-pjh6-c8mg-3x7m

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# Patch Next advisory brief: CVE-2026-116579

Advisory: Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of GHSA-hxp9-w8x3-p566)
IDs: CVE-2026-116579, GHSA-pjh6-c8mg-3x7m
Record: https://patchnext.com/advisories/cve-2026-116579 (revision 2, source checked 29 Oct 2026, 06:10 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.9)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: autobahn 26.10.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI autobahn: affected >= 26.7.1, < 26.10.1; fixed in 26.10.1

## Suggested inspection (read-only)
- `python -m pip show autobahn`: 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 autobahn.
- 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://osv.dev/vulnerability/GHSA-pjh6-c8mg-3x7m (OSV record GHSA-pjh6-c8mg-3x7m lists affected ranges: PyPI autobahn >=26.7.1 <26.10.1 (fixed))
- Verified: https://github.com/advisories/GHSA-pjh6-c8mg-3x7m (GitHub-reviewed advisory GHSA-pjh6-c8mg-3x7m lists affected ranges: PyPI autobahn >=26.7.1 <26.10.1)

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