CVE-2026-34827
Published: 02 April 2026
Summary
CVE-2026-34827 is a high-severity Uncontrolled Resource Consumption (CWE-400) vulnerability in Rack Rack. Its CVSS base score is 7.5 (High).
Operationally, exploitation aligns with the MITRE ATT&CK technique Application or System Exploitation (T1499.004); ranked at the 6.3th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog.
The strongest mitigations our analysis identified are NIST 800-53 SC-5 (Denial-of-service Protection) and SI-2 (Flaw Remediation).
Threat & Defense at a Glance
Threat & Defense Details
Mitigating Controls (NIST 800-53 r5)AI
Flaw remediation directly mitigates this CVE by requiring timely patching of the vulnerable Rack versions (3.1.21 or 3.2.6) to fix the super-linear multipart parsing.
Denial-of-service protection implements mechanisms like rate limiting and resource quotas to block crafted multipart requests causing excessive CPU usage.
Information input validation detects and rejects oversized or escape-heavy multipart payloads, reducing the risk of CPU exhaustion during parsing.
MITRE ATT&CK Enterprise TechniquesAI
Why these techniques?
Vulnerability enables crafted multipart requests causing CPU exhaustion DoS in the web application parser, mapping directly to application/system exploitation for endpoint denial of service.
NVD Description
Rack is a modular Ruby web server interface. From versions 3.0.0.beta1 to before 3.1.21, and 3.2.0 to before 3.2.6, 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…
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quoted values, this causes super-linear processing. An unauthenticated attacker can send a crafted multipart/form-data request containing many parts with long backslash-escaped parameter values to trigger excessive CPU usage during multipart parsing. This results in a denial of service condition in Rack applications that accept multipart form data. This issue has been patched in versions 3.1.21 and 3.2.6.
Deeper analysisAI
Rack is a modular Ruby web server interface affected by CVE-2026-34827, a vulnerability in the Rack::Multipart::Parser#handle_mime_head method. Versions from 3.0.0.beta1 up to but not including 3.1.21, and 3.2.0 up to but not including 3.2.6, parse quoted multipart parameters—such as those in Content-Disposition: form-data; name="..."—using repeated String#index searches combined with String#slice! prefix deletions. This approach leads to super-linear processing time for escape-heavy quoted values, enabling resource exhaustion.
An unauthenticated attacker can exploit this by sending a crafted multipart/form-data request with numerous parts containing long backslash-escaped parameter values. The parsing process consumes excessive CPU resources, resulting in a denial-of-service condition for Rack applications that process multipart form data.
The Rack security advisory at https://github.com/rack/rack/security/advisories/GHSA-v6x5-cg8r-vv6x confirms the issue has been patched in Rack versions 3.1.21 and 3.2.6, recommending immediate upgrades for affected applications.
Details
- CWE(s)