Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters — CVE-2026-34827

Summary

## Summary `Rack::Multipart::Parser#handle_mime_head` parses quoted multipart parameters such as `Content-Disposition: form-data; name="..."` using repeated `String#index` searches combined with `String#slice!` prefix deletion. For escape-heavy quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted `multipart/form-data` request containing many parts with long…

Product

rubygems: rack

What to do

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

• Prioritize patching or mitigation immediately (treat as actively risky).
• 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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