CVE-2026-42561

python-multipart has a denial of service vulnerability in multipart part header parsing. When parsing multipart/form-data, MultipartParser previously had no limit on the number of part headers or the size of an individual part header. An attacker could send a request with either many repeated headers without terminating the header block or a single very large header value, causing excessive CPU work before request rejection or completion.

Details

The affected parser states are HEADER_FIELD_START, HEADER_FIELD, HEADER_VALUE_START, HEADER_VALUE, and HEADER_VALUE_ALMOST_DONE. The issue can be triggered by:

• A multipart part with an oversized individual header value.
• A multipart part with many repeated header lines or an unterminated header block.

Both variants are addressed by enforcing default parser limits for maximum header count and maximum header size.

Mitigation

Upgrade to python-multipart 0.0.27 or later.

If upgrading is not immediately possible, reduce exposure by enforcing request body size limits at the server, proxy, or framework layer. This is only a mitigation; affected versions of python-multipart still parse multipart part headers without the default header count and header size limits.

Applications that parse attacker-controlled multipart/form-data with affected versions of python-multipart can experience CPU exhaustion. ASGI applications using Starlette, FastAPI, or other frameworks that invoke python-multipart may have worker or event-loop delays while processing malicious upload requests.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-42561 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 (0.0.27); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42561? CVE-2026-42561 is a high-severity allocation of resources without limits or throttling vulnerability in python-multipart (pip), affecting versions < 0.0.27. It is fixed in 0.0.27. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-42561? CVE-2026-42561 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 python-multipart are affected by CVE-2026-42561? python-multipart (pip) versions < 0.0.27 is affected.
4. Is there a fix for CVE-2026-42561? Yes. CVE-2026-42561 is fixed in 0.0.27. Upgrade to this version or later.
5. Is CVE-2026-42561 exploitable, and should I be worried? Whether CVE-2026-42561 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-2026-42561 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-2026-42561? Upgrade python-multipart to 0.0.27 or later.