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Response snapshot Oct 29, 2026, 02:31 UTC

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VulnerabilityCVE-2026-116579

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

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).

At a glance

Exploited
Not known
Severity
5.9 Medium (CVSS 3.1)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: autobahn: 26.10.1. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-pjh6-c8mg-3x7m (opens a new tab)

What happened

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.

Continue reading

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 at 545 MiB before the 1009 rejection.

An autobahn asyncio server on 127.0.0.1 accepting permessage-brotli, with maxFramePayloadSize=64 KiB and maxMessagePayloadSize=1 MiB, received one small brotli frame from an autobahn client and peaked at 545 MiB (baseline 28 MiB). A few concurrent connections can exhaust memory.

brotli ≥ 1.2.0 has Decompressor.process(data, output_buffer_limit=N), so the backend can stop at max_output_len like deflate and bzip2 (and the extra's floor can be raised to brotli>=1.2.0).

Tested on autobahn 26.7.1 (latest) with brotli 1.2.0; compress_brotli.py on master is unchanged. This affects servers that enable permessage-brotli (opt-in, needs the compress extra).

Proof of concept

How to run the PoC

python3 -m venv venv && . venv/bin/activate
pip install autobahn==26.7.1 brotli==1.2.0
python3 poc_autobahn.py
# PoC: autobahn 26.7.1 permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked.
import brotli, resource
from autobahn.websocket.compress import PerMessageBrotli

c = brotli.Compressor() # build a 256 MiB-of-zeros frame without holding the plaintext
frame = b"".join(c.process(b"\0" * (1 > 10, "MiB")
try:
 pmce.decompress_message_data(frame, max_output_len=1 > 10, "MiB")

Output:

421 bytes on the wire; peak RSS before: 36 MiB
PayloadExceededError | peak RSS after: 545 MiB

Impact

An unauthenticated WebSocket client can make a server that enabled permessage-brotli allocate hundreds of MB or more per tiny frame, regardless of the configured maxMessagePayloadSize. That is the memory-exhaustion DoS GHSA-hxp9-w8x3-p566 was meant to prevent.

Credit goes to Adrian Korwel.

Affected package ranges reported by GitHub: autobahn: >= 26.7.1, < 26.10.1

Scores and how they were measured

CVSS base severity

5.9

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.4%

Scored Percentile 36.7%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-pjh6-c8mg-3x7m (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-pjh6-c8mg-3x7m, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-116579, scored 2026-10-28. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 28). Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of GHSA-hxp9-w8x3-p566) (CVE-2026-116579). https://kevwire.com/record/CVE-2026-116579

MLA

"Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of GHSA-hxp9-w8x3-p566) (CVE-2026-116579)." KEVWire, 28 Oct. 2026, kevwire.com/record/CVE-2026-116579.

Chicago

KEVWire. "Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of GHSA-hxp9-w8x3-p566) (CVE-2026-116579)." Last modified October 28, 2026. https://kevwire.com/record/CVE-2026-116579.

Evidence summary (plain text)

Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of GHSA-hxp9-w8x3-p566) (Oct 28, 2026).
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.

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.
CVSS 5.9 (version 3.1; evidence as of Oct 28, 2026; https://github.com/advisories/GHSA-pjh6-c8mg-3x7m).
EPSS 0.4% for the following 30 days, scored Oct 28, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-116579&date=2026-10-28).
Sources:
GitHub-reviewed advisory GHSA-pjh6-c8mg-3x7m: https://github.com/advisories/GHSA-pjh6-c8mg-3x7m
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-116579&date=2026-10-28
Record ID: CVE-2026-116579.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 29, 2026, 02:16 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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