Vulnerability in Fastapiexpert Python-Multipart
CVE-2026-53538
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.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 3.7 (Low). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N.
EPSS exploit prediction
EPSS: 0.003 (15.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 15th percentile — 15.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.003 · newest: 0.003 · change: 0.000
Affected products
- Fastapiexpert Python-Multipart
- Kludex Python-Multipart — versions < 0.0.30
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-53538?
- CVE-2026-53538 is a low-severity vulnerability in Fastapiexpert Python-Multipart, classified under CWE-436. CVSS score: 3.7/10. Published 2026-06-22.
- How severe is CVE-2026-53538?
- Low severity. CVSS v3 base score is 3.7 out of 10.