CVE
aiohttp denial of service in concatenated stream decompression
Affects aiohttp when it decompresses request bodies or WebSocket messages made of many concatenated gzip, deflate or zstd members. Fixed in 3.14.4.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- Exploited
- No known exploitation
- Fix
- Fixed in 3.14.4
- Who is exposed
- Not reported
Do first: Upgrade to a fixed version: 3.14.4.
- Last modified
- Whether it needs action now or is a reference record, by fixed rules, with why. Last checked is when cve.now last re-read it.What do these mean? →
- Reference
- Last checked
- FIRST's estimate of the chance attackers exploit it in the next 30 days, updated daily.What do these mean? →
- 0.52% · 44th percentile
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 26/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
AIOHTTP, a Python asynchronous HTTP client and server framework, can be made to use excessive CPU when it decompresses a payload made of many concatenated gzip, deflate, or zstd streams. A remote attacker can exploit this without logging in by sending a crafted compressed request body or WebSocket message, which blocks the server's event loop, and a malicious server can do the same to a client.
Original description
AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.4, aiohttp decompresses each member of a payload containing many concatenated gzip, deflate, or zstd streams by passing the entire remaining input to a new decompressor, causing quadratic CPU use. Empty members produce no output, so client_max_size and output backpressure do not limit the work. A remote attacker can block a server event loop with a crafted compressed request body or WebSocket message, and a malicious server can similarly block a client. This issue is fixed in version 3.14.4.
Who should check
Teams running aiohttp 3.14.3 or earlier as an HTTP or WebSocket server or client that decompresses compressed request bodies or messages.
Not fixed by: Client_max_size does not limit the work for empty members.
“When decompressing a payload made up of many concatenated compressed members (gzip, deflate or zstd)”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure17/25No login or user action needed, reachable over the network without a login · Unknown: Advisories don't say whether default setups are affected
- Reach?/20Unknown. No tracked technology or package matched
- Severity8.7/10CVSS 8.7
- No patch0/5No signal
What would change the score
- Would score about 71 if added to CISA KEV
- Would score about 66 if CISA's SSVC rates exploitation active
- Would score about 31 if advisories confirm default setups are exposed
What to do
Upgrade to a fixed version: 3.14.4.
Check your systems with an agent
Paste this into a coding or ops agent. It checks your systems read-only and reports a table per asset.
Check whether my systems are affected by CVE-2026-128131 (AIOHTTP: Concatenated compressed streams cause excessive CPU use), using the cve.now data below.
Rules: work read-only. Do not run exploit code, proof-of-concept scripts or scanners that send attack payloads, and do not change any configuration, package, image or deployment. Report findings only.
What cve.now has for CVE-2026-128131 (entry last updated 2026-11-13):
Affected:
- aio-libs aiohttp: fixed in 3.14.4
Fixed versions: 3.14.4.
Conditions (all must be true for the bug to be hit):
- Request body or WebSocket message contains many concatenated gzip, deflate or zstd compressed members (source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9)
Setup: unknown whether the bug needs a non-default feature or setting. Check the advisory.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-128131 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-128131 only. If you also check another CVE, look up its own KEV status; never assume it from this one.
How to check:
- Installed software: check what is actually running, not what a repo declares. Read the version from each host, container image and Kubernetes node (the binary's version output, the OS package database, image layers), and check whether the conditions above hold there. Distribution packages may carry a backported fix under an older version number; confirm against the distribution's own advisory.
Verify against the linked advisories before concluding, and cite the date you read each one:
- https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9
- https://github.com/aio-libs/aiohttp/pull/13362
- https://github.com/aio-libs/aiohttp/pull/13391
- https://github.com/aio-libs/aiohttp/commit/d041d4d0fd48c3f0832084d33be16cf1c4835f85
- https://github.com/aio-libs/aiohttp/commit/d925a80539ffd227da1d93ab4a5bff123cc777db
- https://github.com/aio-libs/aiohttp/releases/tag/v3.14.4
Do not conclude an asset is unaffected unless you checked it and the evidence covers both its version and the conditions above. If you could not check something, mark it unknown and say what is missing.
Report a table with one row per asset and these columns: Asset | Version or build | Status (affected, fixed, not applicable, unknown) | Compensating controls | Exposure (internet-facing, internal, none) | Evidence | Next action.
Entry: https://cve.now/CVE-2026-128131
For follow-up questions, use the cve.now MCP server at https://mcp.cve.now/mcp (no authentication), for example execute with name cve.get and arguments {"id": "CVE-2026-128131"}.Timeline
- CVE published
- First seen by cve.now
Affected versions
- aio-libs aiohttp
fixed in 3.14.4
Evidence 8
Vendor advisory · 1
Establishes: Affected and fixed versions.
- Advisory
CVE record · 2
Establishes: The identifier and description.
- Record
- Record
Other links · 5
- Repository
- Repository
- Repository
- Repository
- Repository
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-128131
Is CVE-2026-128131 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A specially crafted compressed body or WebSocket message makes aiohttp use CPU time that grows quadratically, which can block the event loop of a server, or of a client when a malicious server sends the data.Source: github.com
Which versions are affected by CVE-2026-128131?
aiohttp: before 3.14.4.Source: cve.now feed data
Am I affected, and how do I check?
The issue involves payloads made of many concatenated compressed members (gzip, deflate or zstd). Versions before 3.14.4 are the affected range, and the installed version can be checked with pip show aiohttp or python -c "import aiohttp; print(aiohttp.__version__)".Source: github.com
How do I fix CVE-2026-128131?
Upgrade aiohttp to 3.14.4 or later.Source: github.com