CVE
aiohttp denial of service in base64 multipart parsing
Affects aiohttp handlers that read multipart data with Content-Transfer-Encoding: base64 parts. 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? →
- 0.55% · 46th 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, an asynchronous HTTP client and server framework for Python, can be made to read a crafted base64-encoded multipart request past the configured client_max_size limit, which can use up memory or stall the event loop. A remote attacker can trigger this without logging in, against any handler that reads multipart data.
Original description
AIOHTTP is an asynchronous HTTP client/server framework for asyncio and Python. Prior to 3.14.4, BodyPartReader.read_chunk() processing a multipart part with Content-Transfer-Encoding set to base64 keeps reading until the chunk length is a multiple of four without enforcing a byte limit. A short Content-Length or a long run of whitespace can cause one call to read the remaining body into memory before the size check or spin on the event loop. A remote attacker can send such a multipart request to a handler that reads multipart data and bypass client_max_size. This issue is fixed in version 3.14.4.
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-2026-128127 (AIOHTTP: Base64 multipart parts bypass client_max_size), 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-2026-128127 (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):
- Multipart parts with Content-Transfer-Encoding set to base64 (source: https://github.com/aio-libs/aiohttp/security/advisories/GHSA-24c4-3p7v-29cm)
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-2026-128127 is not listed as of cve.now's last read.
This KEV status is for CVE-2026-128127 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-24c4-3p7v-29cm
- https://github.com/aio-libs/aiohttp/pull/13509
- https://github.com/aio-libs/aiohttp/pull/13513
- https://github.com/aio-libs/aiohttp/commit/2d9c5c1062eff0034fb3ab120897a6c1b3ff7041
- https://github.com/aio-libs/aiohttp/commit/a555ab698e08e9baea7386d5b4c0761cb8956fdd
- 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-2026-128127
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-2026-128127"}.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-2026-128127
Is CVE-2026-128127 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 crafted multipart request with base64-encoded parts can bypass the client_max_size limit. This can cause excessive memory use or block the event loop.Source: github.com
Which versions are affected by CVE-2026-128127?
aiohttp: before 3.14.4.Source: cve.now feed data
Am I affected, and how do I check?
aiohttp versions before 3.14.4 are affected, but only in handlers that read multipart data with Content-Transfer-Encoding: base64 parts. To check a version, run python -c "import aiohttp; print(aiohttp.__version__)" or pip show aiohttp, then search the code for handlers that read multipart requests.Source: github.com
How do I fix CVE-2026-128127?
Upgrade aiohttp to 3.14.4 or later.Source: github.com