What’s Vulnerable › Vulnerabilities
CVE-2026-26961: Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass
- Severity: medium (CVSS 5.3 v3.1)
- Exploited in the wild: not listed in CISA KEV
- EPSS: 0.3% probability of exploitation in the next 30 days
- Public exploit or detection template: none known
- Published: 2026-04-02 · updated 2026-07-24
- Weakness: CWE-436
Description
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.
Affected products
- rack: before 2.2.23; from 3.0.0 before 3.1.21; from 3.2.0 before 3.2.6 — fixed in 2.2.23, 3.1.21, 3.2.6
- Rubygems Rack: before 2.2.23; from 3.0.0.beta1 before 3.1.21; from 3.2.0 before 3.2.6; 0.1.0, 0.2.0, 0.3.0, 0.4.0, 0.9.0, 0.9.1 (+169) — fixed in 2.2.23, 3.1.21, 3.2.6
How to fix
Upgrade to a fixed version: rack 2.2.23, 3.1.21, 3.2.6; Rubygems Rack 2.2.23, 3.1.21, 3.2.6. Check the vendor advisory for mitigations.
References
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