Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.
Key facts
- Attack vector
- Network
- Privileges required
- None
- User interaction
- None
- Attack complexity
- Low
- Impact type
- —
- Confidentiality · integrity · availability
- Low · Low · None
- Fix status
- Partial Red Hat: 6 without a fix
Attack profile from CVSS 3.1 · NVD. Fix status from CNA, GitHub and Red Hat.
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::Request parses the Host header using an AUTHORITY regular expression that accepts characters not permitted in RFC-compliant hostnames, including /, ?, #, and @. Because req.host returns the full parsed value, applications that validate hosts using naive prefix or suffix checks can be bypassed. This can lead to host header poisoning in applications that use req.host, req.url, or req.base_url for link generation, redirects, or origin validation. 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
- No
- 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-g2pf-xv49-m2h5 |
| — | rack (RubyGems) | 3.2.0 to before 3.2.6 | 3.2.6 | Patch available | GHSA-g2pf-xv49-m2h5 |
- 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-g2pf-xv49-m2h5
- rack (RubyGems) Vendor— Affected 3.2.0 to before 3.2.6 Fixed in 3.2.6 Patch available GHSA-g2pf-xv49-m2h5
Remediation
- Fix · CNA rack (rack) Fixed in 3.1.21, 3.2.6
- Fix · GHSA-g2pf-xv49-m2h5 rack (RubyGems) Fixed in 3.1.21 Advisory →
- Fix · GHSA-g2pf-xv49-m2h5 rack (RubyGems) Fixed in 3.2.6 Advisory →
Vendor assessments
Red Hat
Red Hat rating: Moderate. Red Hat's page for this CVE →
| Product | Status | Plan or fix |
|---|---|---|
| Logging Subsystem for Red Hat OpenShift | Affected | No fix yet: Fix deferred |
| Red Hat 3scale API Management Platform 2 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 10 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 7 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 8 | Affected | No fix yet: Fix deferred |
| Red Hat Enterprise Linux 9 | Affected | No fix yet: Fix deferred |
Timeline
- 2026-10-02 Threatist record created from CVE-2026-34835
- 2026-10-03 Alias added: GHSA-g2pf-xv49-m2h5
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-2026-34835 · CVE Program
- CVE-2026-34835 · NVD
- FIRST EPSS
- GHSA-g2pf-xv49-m2h5 · GitHub Advisory Database · CC BY 4.0
- CVE-2026-34835 · Red Hat security data · CC BY 4.0
This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences