Resource exhaustion in Fastapiexpert Python-Multipart
CVE-2026-53539
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, 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. This vulnerability is fixed in 0.0.30.
Vulnerability class: DoS (Denial of Service)
Published · last modified .
CVSS v3 metric
CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
EPSS exploit prediction
EPSS: 0.005 (37.4th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 37th percentile — 37.4% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.005 · newest: 0.005 · 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-2026-53539?
- CVE-2026-53539 is a high-severity vulnerability in Fastapiexpert Python-Multipart, classified under Uncontrolled Resource Consumption. CVSS score: 7.5/10. Published 2026-06-22.
- How severe is CVE-2026-53539?
- High severity. CVSS v3 base score is 7.5 out of 10.