Bug 2491441 (CVE-2026-53539)

Summary: CVE-2026-53539 python-multipart: Python-Multipart: Denial of Service via crafted form-urlencoded bodies
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security <prodsec-ir-bot>
Status: NEW --- QA Contact:
Severity: medium Docs Contact:
Priority: medium    
Version: unspecifiedCC: alinfoot, anpicker, anthomas, aprice, bbrownin, dfreiber, dkeler, drow, dschmidt, dtrifiro, ebourniv, ehelms, ggainey, hasun, jburrell, jfula, jlanda, jowilson, jpasqual, jsamir, juwatts, jwong, kaycoth, kshier, lgallett, mbarnett, mhulan, nmoumoul, nyancey, oezr, omaciel, ometelka, osousa, pcreech, prwatson, ptisnovs, rbryant, rchan, rjohnson, sbunciak, simaishi, smallamp, stcannon, suppawar, syedriko, teagle, thason, tmalecek, ttakamiy, vkumar, weaton, xdharmai, yguenane
Target Milestone: ---Keywords: Security
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Hardware: All   
OS: Linux   
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A flaw was found in Python-Multipart, a streaming multipart parser. A remote attacker can exploit this vulnerability by submitting a specially crafted `application/x-www-form-urlencoded` body. This can cause the `QuerystringParser` to perform inefficient processing, leading to excessive CPU consumption. Consequently, a small number of concurrent requests can exhaust worker processes, resulting in a Denial of Service (DoS) for the affected system.
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Description OSIDB Bzimport 2026-06-22 18:01:33 UTC 
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.