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Python MultipartApplication · Fastapiexpert

CVE-2026-53539

HIGH · 7.5 CVSS v3.1 Published 2026-06-22
Fix available
A fix is available. Upgrade to 0.0.30 or later.
See remediation →
84/100
Remediation priority · High
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, when parsing application/x-www-form-urlencoded bodies, QuerystringParser located the field separator with a two step lookup: it first scanned the entire remaining buffer for &, and only when no & existed anywhere ahead did it fall back to scanning for ;. For a body that uses ; as the separator and contains no &, every field iteration performed a full failed & scan over the entire remaining buffer before locating the nearby ;. With N semicolon separated fields in a chunk of size B, this yields O(B^2) byte comparisons per chunk. An attacker can submit a small crafted body of the form a;a;a;... and cause the parser to spend seconds of CPU per request. A handful of concurrent requests can exhaust worker processes. 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's QuerystringParser has a quadratic-time complexity vulnerability when parsing application/x-www-form-urlencoded bodies using semicolon (;) as the field separator. The parser first scans the entire remaining buffer for ampersands (&) before falling back to semicolons, causing O(B²) byte comparisons when no ampersands exist. Attackers can submit small crafted bodies like 'a;a;a;...' to cause excessive CPU consumption and worker process exhaustion.

MitigationUpgrade python-multipart to version 0.0.30 or later to obtain the patched library, then verify form handling functionality remains intact.

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
None
Availability
High

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

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.txt/poetry.lock file for the installed version
    Affected if The version is below 0.0.30
  2. Confirm the application uses form parsing
    Search your codebase for RequestForm usage, Form(...) dependencies, or multipart/form-data handling in your FastAPI/Starlette routes
    Affected if The application processes form submissions via python-multipart's QuerystringParser
  3. Check for application/x-www-form-urlencoded usage
    Inspect your HTTP requests and form handling code to see if any endpoints accept urlencoded form data (content-type: application/x-www-form-urlencoded)
    Affected if Your application accepts urlencoded form bodies that get parsed by python-multipart
  4. Determine if semicolon separator is in use
    Review request payloads or test by submitting form data with semicolons as field separators (e.g., 'a;a;a;...')
    Affected if The application processes form bodies containing semicolons as field separators rather than ampersands

You are affected if python-multipart version is below 0.0.30 AND your application uses it to parse application/x-www-form-urlencoded bodies that may contain semicolons as separators.

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 obtain the patched library, then verify form handling functionality remains intact.

Recommended fix High confidence

0.0.30

  1. Check current installed version of python-multipart: pip show multipart or pip list | grep multipart
  2. Upgrade python-multipart to version 0.0.30 or later: pip install --upgrade 'multipart>=0.0.30' or pip install multipart==0.0.30
  3. Verify the upgrade was successful: pip show multipart to confirm version 0.0.30 or higher is installed
  4. If using a framework that depends on python-multipart (e.g., FastAPI, Starlette), restart the application to load the new version

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

Fix this in Python Multipart Scoped from the published advisory
  • Consultation2.0 h
  • Implementation1.0 h
  • Testing4.0 h
  • Review / QA1.0 h
8.0 hours of engineering $1,360
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,176.

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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-2026-53539 in production — separate from our analysis above.

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What this is

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