Rack: Algorithmic-Complexity DoS in Rack::Multipart::Parser
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- Denial of service
- Confidentiality · integrity · availability
- None · None · High
- Fix status
- Partial Red Hat: 2 without a fix
Attack profile from CVSS 3.1 · CNA. Fix status from CNA, GitHub and Red Hat. Impact type from CWE-400.
Summary
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 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.
Why it matters now
SSVC decision points
CISA ADP · SSVC 2.0.3
- Exploitation
- None
- Automatable
- Yes
- Technical impact
- Partial
The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.
Affected products
| Vendor | Product | Affected versions | Fixed in | Status | Source |
|---|---|---|---|---|---|
| rack | rack | 3.0.0.beta1 to before 3.1.21; 3.2.0 to before 3.2.6 | 3.1.21, 3.2.6 | Patch available | CNA |
| — | rack (RubyGems) | 3.0.0.beta1 to before 3.1.21 | 3.1.21 | Patch available | GHSA-v6x5-cg8r-vv6x |
| — | rack (RubyGems) | 3.2.0 to before 3.2.6 | 3.2.6 | Patch available | GHSA-v6x5-cg8r-vv6x |
- rack Vendorrack Affected 3.0.0.beta1 to before 3.1.21; 3.2.0 to before 3.2.6 Fixed in 3.1.21, 3.2.6 Patch available CNA
- rack (RubyGems) Vendor— Affected 3.0.0.beta1 to before 3.1.21 Fixed in 3.1.21 Patch available GHSA-v6x5-cg8r-vv6x
- rack (RubyGems) Vendor— Affected 3.2.0 to before 3.2.6 Fixed in 3.2.6 Patch available GHSA-v6x5-cg8r-vv6x
Remediation
- Fix · CNA rack (rack) Fixed in 3.1.21, 3.2.6
- Fix · GHSA-v6x5-cg8r-vv6x rack (RubyGems) Fixed in 3.1.21 Advisory →
- Fix · GHSA-v6x5-cg8r-vv6x rack (RubyGems) Fixed in 3.2.6 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Important. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Logging Subsystem for Red Hat OpenShift | Affected | No fix yet |
| Red Hat 3scale API Management Platform 2 | Affected | No fix planned: Will not fix |
Not affected: 4 products
- Red Hat Enterprise Linux 10
- Red Hat Enterprise Linux 7
- Red Hat Enterprise Linux 8
- Red Hat Enterprise Linux 9
Timeline
- 2026-10-02 Threatist record created from CVE-2026-34827
- 2026-10-03 Alias added: GHSA-v6x5-cg8r-vv6x
- 2026-10-11 EPSS moved to the 50th–90th percentile
- 2026-10-25 EPSS moved to the below the 50th percentile
- 2026-10-25 Priority changed: MEDIUM → LOW
References
More Rack vulnerabilities
- TI-2024-16580 Rack ReDos in content type parsing (2nd degree polynomial) CVSS 3.1HIGH 7.5
- TI-2024-17236 Possible Denial of Service Vulnerability in Rack Header Parsing CVSS 3.1HIGH 7.5
- TI-2024-17231 Possible DoS Vulnerability with Range Header in Rack CVSS 3.1HIGH 7.5
- TI-2023-12254 Uncontrolled resource consumption in https://github.com/rack/rack CVSS 3.1HIGH 7.5
- TI-2025-38165 Rack's multipart parser buffers unbounded per-part headers, enabling DoS (memory exhaustion) CVSS 3.1HIGH 7.5
- TI-2025-38163 Rack's unbounded multipart preamble buffering enables DoS (memory exhaustion) CVSS 3.1HIGH 7.5
Sources
- CVE-2026-34827 · CVE Program
- CVE-2026-34827 · NVD
- FIRST EPSS
- GHSA-v6x5-cg8r-vv6x · GitHub Advisory Database · CC BY 4.0
- CVE-2026-34827 · Red Hat security data · CC BY 4.0
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