Content-Length Mismatch in Rack Web Server Interface Affecting Multiple Versions
CVE-2026-34831

4.8MEDIUM

Key Information:

Vendor

Rack

Status
Rack
Vendor
CVE Published:
2 April 2026

What is CVE-2026-34831?

The Rack web server interface, utilized extensively within Ruby applications, has encountered a flaw in its response handling. Prior to the patched versions (2.2.23, 3.1.21, and 3.2.6), the function Rack::Files#fail incorrectly set the Content-Length response header using String#size, leading to a discrepancy when multibyte UTF-8 characters were included in the response body. This flaw permits attackers to exploit 404 responses by leveraging percent-encoded UTF-8 strings, which can cause response desynchronization in environments dependent on the inaccurate Content-Length value. This issue has been resolved in the latest versions.

Affected Version(s)

rack < 2.2.23 < 2.2.23

rack >= 3.0.0.beta1, < 3.1.21 < 3.0.0.beta1, 3.1.21

rack >= 3.2.0, < 3.2.6 < 3.2.0, 3.2.6

References

https://github.com/rack/rack/security/advisories/GHSA-q2w...
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CVSS V3.1

Score:
4.8
Severity:
MEDIUM
Confidentiality:
Low
Integrity:
Low
Availability:
Low
Attack Vector:
Network
Attack Complexity:
High
Privileges Required:
None
User Interaction:
None
Scope:
Unchanged

Timeline

  • Vulnerability published

  • Vulnerability Reserved

.