Vulnerability · Published Jun 22, 2026 · Assigned by GitHub_M
CVE-2026-53538Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling
Low 3.7
In plain English
A low-severity security flaw in python-multipart from Kludex. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 17% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official description
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. This vulnerability is fixed in 0.0.30.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | python-multipart | no fix listed | DEBIAN-CVE-2026-53538 |
| Debian:13 | python-multipart | no fix listed | DEBIAN-CVE-2026-53538 |
| Debian:14 | python-multipart | 0.0.32-1 | DEBIAN-CVE-2026-53538 |
| PyPI | python-multipart | 0.0.30 | PYSEC-2026-3037 |
| PyPI | python-multipart | 0.0.30 | GHSA-6jv3-5f52-599m |
| Ubuntu:25.10 | python-multipart | no fix listed | UBUNTU-CVE-2026-53538 |
| Ubuntu:26.04:LTS | python-multipart | no fix listed | UBUNTU-CVE-2026-53538 |
| Ubuntu:26.10 | python-multipart | no fix listed | UBUNTU-CVE-2026-53538 |
Affected products (as reported)
| Vendor | Product | Versions |
|---|---|---|
| Kludex | python-multipart | < 0.0.30 |
Weakness: CWE-436, CWE-444 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N