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Vulnerability · Published Mar 12, 2026 · Assigned by GitHub_M

CVE-2026-28356ReDoS in multipart 1.3.0 - `parse_options_header()`

High 7.5

In plain English

A high-severity regular-expression denial of service in multipart from defnull. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.8% (higher than 56% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)7.5High
Exploit odds (30 days)0.8%top 44.5% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:13multipart1.2.1-2+deb13u1DEBIAN-CVE-2026-28356
Debian:13multipart1.2.1-2+deb13u1DSA-6161-1
Debian:14multipart1.3.1-1DEBIAN-CVE-2026-28356
PyPImultipart1.3.1GHSA-p2m9-wcp5-6qw3
PyPImultipart1.2.2GHSA-p2m9-wcp5-6qw3
PyPImultipart1.2.2, 1.3.1PYSEC-2026-2670
Ubuntu:25.10multipart1.2.1-2+deb13u1build0.25.10.1UBUNTU-CVE-2026-28356
Ubuntu:26.04:LTSmultipart1.3.0-3ubuntu0.1UBUNTU-CVE-2026-28356

Affected products (as reported)

VendorProductVersions
defnull multipart >= 1.3.0, < 1.3.1< 1.2.2

Weakness: CWE-1333 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

References