CVE-2026-26961
LowCVSS 3.7Exploitation Probability (EPSS)
Low risk17th percentile - higher than 17% of all known CVEs
Summary
A multipart content smuggling vulnerability was found in Rack (Ruby) before versions 2.2.23, 3.1.21, and 3.2.6. Rack selects the last boundary parameter instead of the first, which can bypass WAF filters.
Risk Assessment
An attacker can send malicious multipart data that is interpreted differently than by the WAF, potentially leading to code injection attacks.
Recommendation
Update Rack to version 2.2.23, 3.1.21, or 3.2.6.
Original NVD description (English source)
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first. In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.