python-multipart — security advisory
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.
- Products
- python-multipart
- Affected versions
- 0.0.20-1, 0.0.20-1.1, 0.0.26-1, 0.0.32-1, 0.0.5-2, 0.0.5-2ubuntu0.1~esm1, 0.0.5-3, 0.0.6-1, 0.0.9-1, 0.0.9-1ubuntu0.1~esm1
- Fixed versions
- Not specified
- CVEs
- CVE-2026-53539
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 03:08 UTC · Revision 1
Source evidence
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.
osv:UBUNTU-CVE-2026-53539
python-multipart
Fixed versions: Not specified
python-multipart
Fixed versions: Not specified
python-multipart
Fixed versions: Not specified
python-multipart
Fixed versions: Not specified
python-multipart
Fixed versions: Not specified
Complete source record
{
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"summary": "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.",
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"modified": "2026-10-17T13:00:25.312916418Z",
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