CVE
aiohttp denial of service in URL-encoded form parsing
Affects aiohttp servers whose handlers call Request.post() to read URL-encoded form bodies within the default client_max_size. 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
- Default setups
Do first: Check every install: default setups are affected.
- 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? →
- 31/100 · 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? →
- Default setups
- 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, an asynchronous HTTP client and server framework for Python, is affected. A remote attacker who can reach a handler that calls Request.post() can send one form POST with many URL-encoded fields, without logging in, and block the event loop, which causes a denial of service.
Original description
AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.4, Request.post() does not limit the number of fields in a URL-encoded form body, and inserting many repeated keys into MultiDict takes quadratic time on the event loop. A remote attacker can send a form POST within client_max_size to a handler that calls Request.post() and block the event loop. This issue is fixed in version 3.14.4.
Who should check
Teams running aiohttp 3.14.3 or earlier in web services whose handlers call Request.post() to read form bodies
“to any handler that calls Request.post()”
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- 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
What to do
Check every install: default setups are affected.
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-2026-128126 (AIOHTTP: Too many URL-encoded form fields block the event loop), 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-2026-128126 (entry last updated 2026-11-13):
Affected:
- aio-libs aiohttp: fixed in 3.14.4
Fixed versions: 3.14.4.
Vendor patches: 615fec3 (source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh). Check each asset for these patch ids, not only its version.
Conditions (all must be true for the bug to be hit):
- A handler calls Request.post() to read a URL-encoded form body (source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh)
- The form POST is within the default client_max_size (source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-xgw8-8ccx-rmqh)
Setup: the bug is reachable in a default setup; no special feature or setting is needed.
Attacker needs: network access, no login, no user action.
CISA KEV: CVE-2026-128126 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-128126 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-xgw8-8ccx-rmqh
- https://github.com/aio-libs/aiohttp/pull/13738
- https://github.com/aio-libs/aiohttp/pull/13741
- https://github.com/aio-libs/aiohttp/commit/615fec3d4f088f30d2ee72cad760152e92aa1c53
- https://github.com/aio-libs/aiohttp/commit/8b0100572936df28977b9de8a6cc7e464c0b521e
- 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-2026-128126
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-2026-128126"}.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-2026-128126
Is CVE-2026-128126 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 form POST with a very large number of repeated keys can block the asyncio event loop, which stalls the server. The advisory describes the impact as an attacker blocking the event loop this way.Source: github.com
Which versions are affected by CVE-2026-128126?
aiohttp: before 3.14.4.Source: cve.now feed data
Am I affected, and how do I check?
The advisory says the issue affects handlers that call Request.post() to read a URL-encoded form body, when the POST is within the default client_max_size. To check a setup, look at the installed aiohttp version (for example, with pip show aiohttp) and search handler code for Request.post() calls.Source: github.com
How do I fix CVE-2026-128126?
Upgrade aiohttp to 3.14.4 or later. The vendor's patch is commit 615fec3.Source: github.com