Rack's greedy multipart boundary parsing can cause parser differentials and WAF bypass. — CVE-2026-26961

Summary

## Summary `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…

Product

rubygems: rack

What to do

General, cautious steps (verify details in the official source):

• Review exposure and plan remediation based on risk and environment.
• Identify affected product versions in your inventory and verify whether you are impacted.
• Apply vendor patches/updates or recommended mitigations as soon as available.
• Read the official advisory for exact affected versions and remediation steps.

Official advisory

• Read the official source
• CVE Record
• NVD

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