CVE-2026-40347
Received Received - Intake
Denial of Service in Python-Multipart via Malformed Multipart Requests

Publication date: 2026-04-18

Last updated on: 2026-04-24

Assigner: GitHub, Inc.

Description
Python-Multipart is a streaming multipart parser for Python. Versions prior to 0.0.26 have a denial of service vulnerability when parsing crafted `multipart/form-data` requests with large preamble or epilogue sections. 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.
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Meta Information
Published
2026-04-18
Last Modified
2026-04-24
Generated
2026-06-16
AI Q&A
2026-04-18
EPSS Evaluated
2026-06-15
NVD
CVE-2026-40347
Affected Vendors & Products
Showing 1 associated CPE
Vendor Product Version / Range
fastapiexpert python-multipart to 0.0.26 (exc)
Helpful Resources
Exploitability
CWE
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KEV
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CWE ID Description
CWE-834 The product performs an iteration or loop without sufficiently limiting the number of times that the loop is executed.
CWE-400 The product does not properly control the allocation and maintenance of a limited resource.
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Executive Summary

This vulnerability exists in Python-Multipart, a streaming multipart parser for Python, in versions prior to 0.0.26. It is a denial of service (DoS) vulnerability that occurs when the parser processes specially crafted multipart/form-data requests containing large preamble or epilogue sections.

The issue arises because the parser does not efficiently handle leading CR/LF data or epilogue data, which can cause it to consume excessive resources or hang. The fix in version 0.0.26 and later involves skipping ahead to the next boundary candidate when processing leading CR/LF data and immediately discarding epilogue data after the closing boundary.

Impact Analysis

This vulnerability can lead to a denial of service condition in applications using vulnerable versions of Python-Multipart. An attacker can send crafted multipart/form-data requests with large preamble or epilogue sections to cause the parser to consume excessive resources or become unresponsive.

As a result, the affected application or service may become slow, unresponsive, or crash, potentially disrupting normal operations.

Mitigation Strategies

To mitigate this vulnerability, upgrade Python-Multipart to version 0.0.26 or later.

The fixed versions skip ahead to the next boundary candidate when processing leading CR/LF data and immediately discard epilogue data after the closing boundary, preventing the denial of service.

Compliance Impact

The vulnerability in Python-Multipart allows for a denial of service (DoS) attack when parsing crafted multipart/form-data requests with large preamble or epilogue sections.

Since the vulnerability impacts availability but does not affect confidentiality or integrity, it may have limited direct impact on compliance with standards like GDPR or HIPAA, which primarily focus on protecting personal data confidentiality and integrity.

However, denial of service incidents can indirectly affect compliance by disrupting service availability, which may be relevant under certain regulatory requirements that mandate system availability and resilience.

Upgrading to version 0.0.26 or later mitigates this issue by properly handling multipart data boundaries.

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