Vulnerability in Defnull Multipart
CVE-2026-28356
multipart is a fast multipart/form-data parser for python. Prior to 1.2.2, 1.3.1 and 1.4.0-dev, the parse_options_header() function in multipart.py uses a regular expression with an ambiguous alternation, which can cause exponential backtracking (ReDoS) when parsing maliciously crafted HTTP or multipart segment headers. This can be abused for denial of service (DoS) attacks against web applications using this library to parse request headers or multipart/form-data streams. The issue is fixed in 1.2.2, 1.3.1 and 1.4.0-dev.
Vulnerability class: ReDoS (Regular Expression 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: 0.010 (62.2th percentile), scored .
Low probability of exploitation in the next 30 days, but still tracked. 62nd percentile — 62.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.007 · newest: 0.010 · change: +0.003
Affected products
- Defnull Multipart — versions >= 1.3.0, < 1.3.1, < 1.2.2
Weakness classification (CWE)
References
- github.com/defnull/multipart/security/advisories/GHSA-p2m9-wcp5-6qw3
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
- 0b0ca135-0b70-47e7-9f44-1890c2a1c46c
Frequently asked questions
- What is CVE-2026-28356?
- CVE-2026-28356 is a high-severity vulnerability in Defnull Multipart, classified under Inefficient Regular Expression Complexity. CVSS score: 7.5/10. Published 2026-03-12.
- How severe is CVE-2026-28356?
- High severity. CVSS v3 base score is 7.5 out of 10.