Unauthenticated CPU-exhaustion DoS via multipart parameter parsing
Published Apr 2, 2026 · Updated Apr 2, 2026
Algorithmic-complexity DoS in Rack 3.0.0.beta1 through 3.1.20 and 3.2.0 through 3.2.5 allows remote attackers to exhaust CPU via multipart forms. Rack::Multipart::Parser#handle_mime_head repeatedly searches quoted values with String#index and deletes prefixes with String#slice!, creating super-linear work for escape-heavy input. Any unauthenticated endpoint that accepts multipart/form-data can tie up application workers and reduce or deny service availability.
Summary
What happened
Algorithmic-complexity DoS in Rack 3.0.0.beta1 through 3.1.20 and 3.2.0 through 3.2.5 allows remote attackers to exhaust CPU via multipart forms. Rack::Multipart::Parser#handle_mime_head repeatedly searches quoted values with String#index and deletes prefixes with String#slice!, creating super-linear work for escape-heavy input. Any unauthenticated endpoint that accepts multipart/form-data can tie up application workers and reduce or deny service availability.
The record
- CVE
- CVE-2026-34827
- Published
- Apr 2, 2026
- Updated
- Apr 2, 2026
- Vendor
- Rack
- Product
- Rack
- Classifications
- CWE-770, CWE-400, CWE-407, T1499
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Apr 2, 2026CVE publishedPublication date reported by the CVE source.
- Apr 2, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=>= 3.0.0.beta1, < 3.1.21
- Affected versionversion=>= 3.2.0, < 3.2.6
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
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Technologies
Your stack
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Your stack
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