Werkzeug is a Web Server Gateway Interface web application library. Applications using werkzeug.formparser.MultiPartParser corresponding to a version of Werkzeug prior to 3.0.6 to parse multipart/form-data requests (e.g. all flask applications) are vulnerable to a relatively simple but effective resource exhaustion (denial of service) attack. A specifically crafted form submission request can cause the parser to allocate and block 3 to 8 times the upload size in main memory. There is no upper limit.

a single upload at 1 Gbit/s can exhaust 32 GB of RAM in less than 60 seconds. Werkzeug version 3.0.6 fixes this issue.