CVE-2026-53537 Python-Multipart: Content-Disposition parameter smuggling via RFC 2231/5987 extended parameters
Attackers can smuggle malicious filenames or field names past security filters by exploiting RFC 2231/5987 parameter decoding discrepancies in the Python-Multipart library.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-53537?
CVE-2026-53537 is a low-severity vulnerability in Kludex python-multipart, published on 22 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, parse_options_header parsed Content-Disposition (and Content-Type) headers with email.message.Message, which transparently applies RFC 2231/5987 decoding. The extended parameter syntax (filename*=charset'lang'value, name*=..., and the filename*0/filename*1 continuation form) is decoded and surfaced under the bare filename/name key, and overrides the plain parameter when both are present. RFC 7578 §4.2 explicitly forbids the filename* form in multipart/form-data. Components that follow RFC 7578, or that do not implement RFC 2231/5987 decoding for multipart/form-data (WAFs, proxies, gateways), may interpret such a header differently. An attacker can exploit that difference to smuggle a different field name or filename past an upstream inspector to the backend. This vulnerability is fixed in 0.0.30.
Which versions are affected by CVE-2026-53537?
3 open source packages ship an affected version, including python-multipart. It affects 1 product.
| Package | Affected | Fixed in |
|---|---|---|
python-multipart PyPI | All versions | 0.0.30 |
python-multipart PyPI | All versions | 0.0.30 |
python-multipart PyPI | All versions | 0.0.30 |
Products include Fastapiexpert Python-Multipart.
How do I fix CVE-2026-53537?
Upgrade python-multipart to 0.0.30 or later.
Is CVE-2026-53537 being exploited?
There is no known exploitation of CVE-2026-53537 in the wild and no public exploit code.
Other vulnerabilities in Kludex python-multipart
- CVE-2026-24486 Python-Multipart has Arbitrary File Write via Non-Default Configuration
- CVE-2026-53539 Python-Multipart: Quadratic-time querystring parsing with semicolon separators causes CPU denial of service
- CVE-2024-24762 python-multipart vulnerable to content-type header Regular expression Denial of Service
- CVE-2026-40347 Python-Multipart affected by Denial of Service via large multipart preamble or epilogue data
- CVE-2026-53538 Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling
Sources
Also known as CLEANSTART-2026-HR94860, CLEANSTART-2026-JY83445, DEBIAN-CVE-2026-53537, ECHO-7b4c-b0bf-6c71, GHSA-vffw-93wf-4j4q, MINI-29w8-g8f3-gx3f and 18 more.