{"cve":"CVE-2026-26961","nvd":{"published":"2026-04-02T17:16:21.973","modified":"2026-07-24T21:10:00.143","status":"Analyzed","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.","cvss_v4":null,"cvss_v3":{"version":"3.1","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N","score":5.3},"cwes":["CWE-436"],"references":["https://github.com/rack/rack/security/advisories/GHSA-vgpv-f759-9wx3"],"products":["rack:rack"],"source":"nvd","synced_at":"2026-10-19T22:28:18.033120106+00:00"},"epss":{"cve":"CVE-2026-26961","score":0.00262,"percentile":0.18097,"date":"2026-11-01"},"kev":null,"osv":[{"id":"GHSA-vgpv-f759-9wx3","url":"https://osv.dev/vulnerability/GHSA-vgpv-f759-9wx3","summary":"Rack's greedy multipart boundary parsing can cause parser differentials and WAF bypass.","modified":"2026-09-10T03:51:03.337006080Z","packages":[{"ecosystem":"RubyGems","name":"rack","purl":"pkg:gem/rack","ranges":["<2.2.23"],"fixed":["2.2.23"]},{"ecosystem":"RubyGems","name":"rack","purl":"pkg:gem/rack","ranges":[">=3.0.0.beta1 <3.1.21"],"fixed":["3.1.21"]},{"ecosystem":"RubyGems","name":"rack","purl":"pkg:gem/rack","ranges":[">=3.2.0 <3.2.6"],"fixed":["3.2.6"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}