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Python MultipartApplication · Fastapiexpert

CVE-2026-53537

MEDIUM · 5.3 CVSS v3.1 Published 2026-06-22
Fix available
A fix is available. Upgrade to 0.0.30 or later.
See remediation →
62/100
Remediation priority · Elevated
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, parse_options_header parsed Content-Disposition (and Content-Type) headers with email.message.Message, which transparently applies RFC 2231/5987 decoding. The extended parameter syntax (filename*=charset'lang'value, name*=..., and the filename*0/filename*1 continuation form) is decoded and surfaced under the bare filename/name key, and overrides the plain parameter when both are present. RFC 7578 §4.2 explicitly forbids the filename* form in multipart/form-data. Components that follow RFC 7578, or that do not implement RFC 2231/5987 decoding for multipart/form-data (WAFs, proxies, gateways), may interpret such a header differently. An attacker can exploit that difference to smuggle a different field name or filename past an upstream inspector to the backend. This vulnerability is fixed in 0.0.30.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Python-Multipart versions before 0.0.30 use email.message.Message to parse Content-Disposition headers, which applies RFC 2231/5987 decoding and surfaces decoded filename*/name* parameters under the bare filename/name keys, overriding plain parameters. Since RFC 7578 §4.2 forbids filename* in multipart/form-data, WAFs and proxies that don't implement RFC 2231 decoding interpret headers differently than Python-Multipart, allowing attackers to smuggle malicious field names or filenames past upstream inspectors.

MitigationUpgrade Python-Multipart to version 0.0.30 or later to fix the RFC 2231 decoding behavior.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
Python MultipartApplication
Affected:< 0.0.30

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
Low
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Identify installed python-multipart version
    Run `pip show python-multipart` or check your requirements file for the installed version
    Affected if The version listed is before 0.0.30 (e.g., 0.0.29, 0.0.20, etc.)
  2. Confirm the library is actively imported
    Search your codebase for `import multipart` or `from multipart` statements, or check runtime dependencies
    Affected if The application imports and uses python-multipart for handling file uploads or form data
  3. Verify multipart form handling is in use
    Review your application code for FastAPI/Starlette endpoints accepting `UploadFile` or `File()` parameters, or multipart form submissions
    Affected if The application processes multipart/form-data requests using python-multipart

You are affected if python-multipart version 0.0.30 or higher is not installed AND your application uses the library to parse Content-Disposition headers from multipart form uploads.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 0.0.30 or later
Fixed in 0.0.30
Interim mitigation

Upgrade Python-Multipart to version 0.0.30 or later to fix the RFC 2231 decoding behavior.

Recommended fix High confidence

0.0.30

  1. Upgrade Python Multipart to version 0.0.30 or later by running: pip install python-multipart>=0.0.30

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Python Multipart Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-53537 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
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