CVE-2026-34827
Rack's multipart header parsing allows Denial of Service via escape-heavy quoted parameters
Impact
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.
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| rubygems | rack | >= 3.0.0.beta1, < 3.1.21 | 3.1.21 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Rack | Rack | from 3.0.0 before 3.1.21 from 3.2.0 before 3.2.6 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/advisories/GHSA-v6x5-cg8r-vv6x
- https://github.com/rack/rack/security/advisories/GHSA-v6x5-cg8r-vv6x
- https://nvd.nist.gov/vuln/detail/CVE-2026-34827
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-34827.yml
- https://access.redhat.com/security/cve/CVE-2026-34827
- https://bugzilla.redhat.com/show_bug.cgi?id=2454501
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-34827.json