CVE-2026-53538
LowCVSS 3.7Exploitation Probability (EPSS)
Low risk7th percentile - higher than 7% of all known CVEs
Summary
Python-Multipart before version 0.0.30 incorrectly treated the semicolon (;) as a field separator in application/x-www-form-urlencoded bodies, while the WHATWG standard and modern browsers only recognize the & character. This parsing differential allows an attacker to smuggle extra form fields past an upstream body inspecting component.
Risk Assessment
The organization may be exposed to attacks involving smuggling of form fields, which could lead to bypassing security mechanisms such as input validation or access control.
Recommendation
Immediately update the Python-Multipart library to version 0.0.30 or later, which contains the fix for this vulnerability.
Other vulnerabilities in Python-Multipart przed wersją 0.0.30
See all- CVE-2026-53540Low
In the Python-Multipart library before version 0.0.31, the parse_form() function did not validate the Content-Length header before using it to bound its chunked read of the request body. A negative Content-Length turned the bounded read into a read-until-EOF, so the entire body was loaded into memory in a single read instead of in fixed-size chunks.
- CVE-2026-53539High
The vulnerability in Python-Multipart before version 0.0.30 causes quadratic computational complexity when parsing application/x-www-form-urlencoded bodies using semicolons as separators. An attacker can send a small crafted request with many semicolon-separated fields, leading to high CPU usage and potential resource exhaustion.
- CVE-2026-53537Low
The vulnerability in Python-Multipart before version 0.0.30 is that the parse_options_header function decodes Content-Disposition and Content-Type headers according to RFC 2231/5987, allowing the use of extended parameter syntax (e.g., filename*=charset'lang'value). An attacker can exploit this difference in header interpretation between components (e.g., WAF, proxy) and the backend to smuggle a different field name or filename, bypassing security inspection.
Original NVD description (English source)
Python-Multipart is a streaming multipart parser for Python. Prior to 0.0.30, QuerystringParser treated ; as a field separator in application/x-www-form-urlencoded bodies, in addition to &. The WHATWG URL standard, modern browsers, and Python's urllib.parse (since the CVE-2021-23336 fix) treat only & as a separator. This creates a parser differential: the same bytes are tokenized into different fields than a WHATWG compliant intermediary would produce, allowing an attacker to smuggle extra form fields past an upstream body inspecting component. This vulnerability is fixed in 0.0.30.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS