CVE-2026-53539 — python-multipart
HIGHCVE-2026-53539 is a high-severity (CVSS 7.5) Uncontrolled Resource Consumption vulnerability in python-multipart. A fix is available for python-multipart — see the affected versions and patch details below.
Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-53539.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-53539 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 379,145 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
python-multipartReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
When parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk.
An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes.
Details
In python_multipart/multipart.py, both the FIELD_NAME and FIELD_DATA states located the next separator like this:
sep_pos = data.find(b"&", i)
if sep_pos == -1:
sep_pos = data.find(b";", i)
data.find(b"&", i) scans from i to the end of the buffer and returns -1 only when there is no & anywhere in the remainder. For a ; separated body with no &, this failed full buffer scan repeats once per field, making parsing quadratic in the body length.
For example, a 1 MiB url encoded body consisting of a; repeated ~500,000 times, submitted with Content-Type: application/x-www-form-urlencoded, causes the parser to perform on the order of 10^11 byte comparisons, consuming several seconds of CPU for a single request. Cost scales quadratically with chunk size.
The parser is reachable through the public QuerystringParser class and through the high level FormParser, create_form_parser, and parse_form APIs for url encoded bodies. It is also the parser Starlette and FastAPI use for application/x-www-form-urlencoded request bodies via request.form().
Impact
Uncontrolled CPU consumption (denial of service). Parsing is synchronous, so a single small crafted form body occupies the handling worker for seconds, blocking any other work on that worker until parsing finishes. Sustained concurrent requests keep workers continuously busy, degrading or denying service.
Mitigation
Upgrade to python-multipart 0.0.30 or later, which treats only & as a field separator (per the WHATWG URL standard) using a single bounded scan, making parsing linear in the body length.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | python-multipart | all versions | 0.0.30pip install --upgrade 'python-multipart==0.0.30' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for python-multipart, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update python-multipart to 0.0.30 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-53539 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-53539 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-53539. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
A flaw was found in python-multipart. When parsing `application/x-www-form-urlencoded` bodies, `QuerystringParser` used a two-step separator lookup that scanned the entire remaining buffer for `&` before falling back to `;`. For bodies using `;` as the separator with no `&`, this yields O(B²) byte comparisons per…
Upgrade to python-multipart 0.0.30 or later, which treats only & as a field separator per the WHATWG URL standard. ; is parsed as ordinary field data, matching urllib.parse, browsers, and other compliant parsers. Deployments that enforce request body size limits, rate limiting, and load balancing make exploitation much harder.Source: Red Hat security advisory for CVE-2026-53539 (CC BY 4.0)
Frequently Asked Questions
Is CVE-2026-53539 in your dependencies?
O3 Security finds CVE-2026-53539 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.