CVE-2026-28356

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.

Affected versions

All versions up to and including 1.3.0 are affected. The issue is fixed in 1.2.2, 1.3.1 and 1.4.0-dev.

Any WSGI or ASGI application using multipart.parse_form_data() directly or indirectly (e.g. while parsing multipart/form-data streams) is vulnerable. The slow-down is significant enough to block request handling threads for multiple seconds per request.

A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use. Typical impact: denial of service when input is crafted to trigger backtracking.

CVE-2026-28356 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.3.1, 1.2.2); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-28356? CVE-2026-28356 is a high-severity inefficient regular expression (ReDoS) vulnerability in multipart (pip), affecting versions >= 1.3.0, < 1.3.1. It is fixed in 1.3.1, 1.2.2. A regular expression with worst-case exponential or polynomial matching time is applied to untrusted input, causing excessive CPU use.
2. How severe is CVE-2026-28356? CVE-2026-28356 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of multipart are affected by CVE-2026-28356? multipart (pip) versions >= 1.3.0, < 1.3.1 is affected.
4. Is there a fix for CVE-2026-28356? Yes. CVE-2026-28356 is fixed in 1.3.1, 1.2.2. Upgrade to this version or later.
5. Is CVE-2026-28356 exploitable, and should I be worried? Whether CVE-2026-28356 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-28356 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-28356?
   • Upgrade multipart to 1.3.1 or later
   • Upgrade multipart to 1.2.2 or later