CVE-2026-116579VulnerabilityPublished 28 Oct 2026

Autobahn Python: permessage-brotli inflates a whole frame before maxMessagePayloadSize is checked (incomplete fix of

Affects autobahnSource GCVE Database

Authoritative record:CVE-2026-116579· Medium 5.9

Not known to be exploited · CVSS 5.9 Medium : upgrade to 26.10.1 when convenient.

Data as of 28 Oct 2026, 22:12 UTC

CVSS 3.1 base score

5.9Medium

How severe the flaw is on the 0–10 CVSS scale.

Patch: Available – fixed in 26.10.1

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

Attack Vector: Network
Attack Complexity: High
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: High: A successful attack depends on conditions beyond the attacker's control. That is, a successful attack cannot be accomplished at will, but requires the attacker to invest in some measurable amount of effort in preparation or execution against the vulnerable component before a successful attack can be expected. For example, a successful attack may require an attacker to: gather knowledge about the environment in which the vulnerable target/component exists; prepare the target environment to improve exploit reliability; or inject themselves into the logical network path between the target and the resource requested by the victim in order to read and/or modify network communications (e.g., a man in the middle attack).
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 26 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade autobahn to 26.10.1.

Affected software

Product	Affected	Fixed in
PyPIautobahn	Affected >=26.7.1 <26.10.1	Fixed in 26.10.1

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

Description

### 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 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 ``` ```python # 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 << 20)) for _ in range(256)) + c.finish() pmce = PerMessageBrotli(is_server=True, server_no_context_takeover=False, client_no_context_takeover=False) pmce.start_decompress_message() print(len(frame), "bytes on the wire; peak RSS before:", resource.getrusage(resource.RUSAGE_SELF).ru_maxrss >> 10, "MiB") try: pmce.decompress_message_data(frame, max_output_len=1 << 20) # remaining budget: 1 MiB except Exception as e: print(type(e).__name__, "| peak RSS after:", resource.getrusage(resource.RUSAGE_SELF).ru_maxrss >> 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.

CWE-409Improper Handling of Highly Compressed Data (Data Amplification) (opens in a new tab)
Base weakness
The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.2 (opens in a new tab)
V12.1.1 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-770Allocation of Resources Without Limits or Throttling (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #25
The product allocates a reusable resource or group of resources on behalf of an actor without imposing any intended restrictions on the size or number of resources that can be allocated.
Requirements
Clearly specify the minimum and maximum expectations for capabilities, and dictate which behaviors are acceptable when resource allocation reaches limits.
Architecture and Design
Limit the amount of resources that are accessible to unprivileged users. Set per-user limits for resources. Allow the system administrator to define these limits. Be careful to avoid CWE-410.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended.
OWASP Cheat Sheets
Abuse Case (opens in a new tab)
OWASP ASVS 4.0.3
V8.1.4 (opens in a new tab)
V13.4.1 (opens in a new tab)
V11.1.4 (opens in a new tab)
Attack patterns
XML Ping of the Death (opens in a new tab)
Exponential Data Expansion (opens in a new tab)
Serialized Data Parameter Blowup (opens in a new tab)
Serialized Data with Nested Payloads (opens in a new tab)
Oversized Serialized Data Payloads (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

WeaknessesCWE-409, CWE-770

Technical details

References

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