Rack Static Header Bypass via URL-Encoded PATH before 2.2.23/3.1.21/3.2.6
CVE-2026-34786 Published on April 2, 2026
Rack: Rack::Static header_rules bypass via URL-encoded paths
Rack is a modular Ruby web server interface. Prior to versions 2.2.23, 3.1.21, and 3.2.6, Rack::Static#applicable_rules evaluates several header_rules types against the raw URL-encoded PATH_INFO, while the underlying file-serving path is decoded before the file is served. As a result, a request for a URL-encoded variant of a static path can serve the same file without the headers that header_rules were intended to apply. In deployments that rely on Rack::Static to attach security-relevant response headers to static content, this can allow an attacker to bypass those headers by requesting an encoded form of the path. This issue has been patched in versions 2.2.23, 3.1.21, and 3.2.6.
Vulnerability Analysis
CVE-2026-34786 is exploitable with network access, and does not require authorization privileges or user interaction. This vulnerability is considered to have a low attack complexity. The potential impact of an exploit of this vulnerability is considered to have no impact on confidentiality, with no impact on integrity, and no impact on availability.
Weakness Type
Incorrect Behavior Order: Validate Before Canonicalize
The software validates input before it is canonicalized, which prevents the software from detecting data that becomes invalid after the canonicalization step. This can be used by an attacker to bypass the validation and launch attacks that expose weaknesses that would otherwise be prevented, such as injection.
Products Associated with CVE-2026-34786
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Affected Versions
rack:- Version < 2.2.23 is affected.
- Version >= 3.0.0.beta1, < 3.1.21 is affected.
- Version >= 3.2.0, < 3.2.6 is affected.
Exploit Probability
EPSS (Exploit Prediction Scoring System) scores estimate the probability that a vulnerability will be exploited in the wild within the next 30 days. The percentile shows you how this score compares to all other vulnerabilities.