defnull multipart up to 1.2.1/1.3.0 multipart.py parse_options_header redos

Summaryinfo

A vulnerability was found in defnull multipart up to 1.2.1/1.3.0. It has been declared as problematic. The impacted element is the function parse_options_header of the file multipart.py. Such manipulation leads to redos. This vulnerability is referenced as CVE-2026-28356. It is possible to launch the attack remotely. No exploit is available. It is recommended to upgrade the affected component.

Detailsinfo

A vulnerability was found in defnull multipart up to 1.2.1/1.3.0. It has been rated as problematic. Affected by this issue is the function parse_options_header of the file multipart.py. The manipulation with an unknown input leads to a redos vulnerability. Using CWE to declare the problem leads to CWE-1333. The product uses a regular expression with an inefficient, possibly exponential worst-case computational complexity that consumes excessive CPU cycles. Impacted is availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability is handled as CVE-2026-28356 since 02/26/2026. The exploitation is known to be easy. The attack may be launched remotely. No form of authentication is required for exploitation. There are known technical details, but no exploit is available. The MITRE ATT&CK project declares the attack technique as T1449.003.

Upgrading to version 1.2.2 or 1.3.1 eliminates this vulnerability.

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Productinfo

Vendor

• defnull

Name

• multipart

Version

• 1.0
• 1.1
• 1.2
• 1.2.0
• 1.2.1
• 1.3.0

Website

• Product: https://github.com/defnull/multipart/

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