CVE-2026-40347

A denial of service vulnerability exists when parsing crafted multipart/form-data requests with large preamble or epilogue sections.

Details

Two inefficient multipart parsing paths could be abused with attacker-controlled input.

Before the first multipart boundary, the parser handled leading CR and LF bytes inefficiently while searching for the start of the first part. After the closing boundary, the parser continued processing trailing epilogue data instead of discarding it immediately. As a result, parsing time could grow with the size of crafted data placed before the first boundary or after the closing boundary.

Mitigation

Upgrade to version 0.0.26 or later, which skips ahead to the next boundary candidate when processing leading CR/LF data and immediately discards epilogue data after the closing boundary.

An attacker can send oversized malformed multipart bodies that consume excessive CPU time during request parsing, reducing request-handling capacity and delaying legitimate requests. This issue degrades availability but does not typically result in a complete denial of service for the entire application.

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2026-40347 has a CVSS score of 5.3 (Medium). 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 (0.0.26); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-40347? CVE-2026-40347 is a medium-severity uncontrolled resource consumption vulnerability in python-multipart (pip), affecting versions < 0.0.26. It is fixed in 0.0.26. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2026-40347? CVE-2026-40347 has a CVSS score of 5.3 (Medium). 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 python-multipart are affected by CVE-2026-40347? python-multipart (pip) versions < 0.0.26 is affected.
4. Is there a fix for CVE-2026-40347? Yes. CVE-2026-40347 is fixed in 0.0.26. Upgrade to this version or later.
5. Is CVE-2026-40347 exploitable, and should I be worried? Whether CVE-2026-40347 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-2026-40347 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-2026-40347? Upgrade python-multipart to 0.0.26 or later.