Bug 2529161 (CVE-2026-86251)

Summary: CVE-2026-86251 h3: h3: Information disclosure via path traversal due to double-decoding flaw
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Priority: medium    
Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in h3. A remote attacker could exploit a path traversal vulnerability in the serveStatic utility. This vulnerability arises from a double-decoding flaw, where specially crafted, double-encoded dot sequences in a request path are incorrectly processed. This allows an attacker to bypass security checks and read arbitrary files from backend storage systems, leading to information disclosure.
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Description OSIDB Bzimport 2026-09-06 12:11:48 UTC 
h3 versions before 1.15.9 contain a path traversal vulnerability in the serveStatic utility. A double-decoding flaw allows a request path containing double-encoded dot sequences (e.g. %252e%252e) to be decoded to %2e%2e, which survives resolveDotSegments() because that function only checks for literal '.' characters. When the resulting asset ID is resolved by URL-based backends (CDN, S3, object storage), %2e%2e is interpreted as '..' per RFC 3986, enabling path traversal to read arbitrary files from the backend.