Vulnerability · Published Apr 2, 2026 · Assigned by GitHub_M
CVE-2026-34835Rack: `Rack::Request` accepts invalid Host characters, enabling host allowlist bypass.
Medium 4.8
In plain English
A medium-severity security flaw in rack from rack. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.2% (higher than 10% of all known vulnerabilities). Ready-made attack tooling exists: public proof-of-concept code. Fixed versions are available - upgrade the affected package.
Official 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::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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:13 | ruby-rack | 3.1.20-0+deb13u2 | DEBIAN-CVE-2026-34835 |
| Debian:14 | ruby-rack | 3.2.6-2 | DEBIAN-CVE-2026-34835 |
| RubyGems | rack | 3.1.21 | GHSA-g2pf-xv49-m2h5 |
| RubyGems | rack | 3.2.6 | GHSA-g2pf-xv49-m2h5 |
| Ubuntu:25.10 | ruby-rack | 3.1.16-0.1ubuntu0.3 | UBUNTU-CVE-2026-34835 |
| Ubuntu:26.04:LTS | ruby-rack | 3.2.4-1ubuntu1.1 | USN-8989-1 |
| Ubuntu:26.04:LTS | ruby-rack | 3.2.4-1ubuntu1.1 | UBUNTU-CVE-2026-34835 |
Affected products (as reported)
Weakness: CWE-1286 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
References
- https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5 x_refsource_CONFIRM