low3.7
CVE-2026-53538
python-multipart: Semicolon treated as querystring field separator enables parameter smuggling
Impact
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.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| pip | python-multipart | < 0.0.30 | 0.0.30 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Fastapiexpert | Python-Multipart | before 0.0.30 |