Cyber Resilience

CVE-2026-34835

Rack 3.0.0 – 3.1.21

Public PoC
Published
02 April 2026
Modified
24 July 2026
Patch / advisory
View advisory
CVSS Score v3.1 4.8
Click a component to see what it means
Raw vectorCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
EPSS Score 0.0019 9th percentile
Risk Priority 37 floored blend · peak EPSS

Summary

CVE-2026-34835 is a medium-severity Improper Validation of Syntactic Correctness of Input (CWE-1286) vulnerability in Rack Rack. Its CVSS base score is 4.8 (Medium).

Operationally, exploitation aligns with the MITRE ATT&CK technique Exploit Public-Facing Application (T1190); ranked at the 9th percentile by exploit likelihood (below the median); it is not currently listed in the CISA KEV catalog; a public proof-of-concept is referenced.

The strongest mitigations our analysis identified map to SI-10 (Information Input Validation) — see the control section below for these in your framework.

EU & UK References

Vulnerability Data

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 /, ?,…

more

#, 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.

CWE(s)
CWE-1286

Related Threats

MITRE ATT&CK Enterprise Techniques

T1190 Exploit Public-Facing Application Initial Access
Adversaries may attempt to exploit a weakness in an Internet-facing host or system to initially access a network.
attack.mitre.org →
Derived from this CVE’s CWE(s) via the direct CWE→ATT&CK cross-walk.

CVEs Like This One

CVE-2026-34763Same product: Rack Rack
CVE-2026-34830Same product: Rack Rack
CVE-2026-32762Same product: Rack Rack
CVE-2026-34786Same product: Rack Rack
CVE-2026-26961Same product: Rack Rack
CVE-2026-22860Same product: Rack Rack
CVE-2025-27610Same product: Rack Rack
CVE-2026-26962Same product: Rack Rack
CVE-2025-27111Same product: Rack Rack
CVE-2025-25184Same product: Rack Rack

Affected Assets

rack
rack
3.0.0 — 3.1.21 · 3.2.0 — 3.2.6

Mitigating Controls

Control response

Prevent
Stop it (NIST 800-53)

—

Detect
Catch it (NIST detect / respond)

—

Harden
Shrink the surface (DISA STIG)

—

Validate
Prove the fix (OWASP ASVS)
  • V1.1.1
  • V1.4.2
  • V2.1.1
  • V2.2.2

Mitigating Controls (NIST 800-53 r5) AI

SI-10 Information Input Validation good match

SI-10 requires validation of information inputs for correctness, directly stopping syntactic validation failures from being introduced or exploitable.

Mitigating Controls (NIST CSF 2.0) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→CSF cross-walk (authority under review) — links open the control.

PR.PS-06 mostly match
prevents

Secure SDLC practices directly require syntactic input validation to prevent malformed data handling.

Mitigating Controls (ISO/IEC 27001:2022 Annex A) AI

Derived directly from the weakness types (CWEs) cited in the NVD entry via our AI-authored CWE→ISO cross-walk (authority under review) — links open the control.

A.8.29 Security testing in development and acceptance mostly match
finds

Security testing in development catches missing or incorrect syntax validation.

A.8.25 Secure development life cycle partial match
prevents

Secure development life cycle mandates input validation to ensure syntactic correctness.

A.8.26 Application security requirements partial match
prevents

Application security requirements explicitly call for validation of input syntax.

A.8.27 Secure system architecture and engineering principles partial match
prevents

Secure architecture principles reduce the likelihood of missing syntactic checks.

A.8.28 Secure coding partial match
prevents

Secure coding standards directly require proper syntactic validation of all inputs.

References