CVE-2026-71209 in audiobookshelf
Summary
by MITRE • 08/05/2026
audiobookshelf's authentication-exemption check (server/routers/Auth.js) matches unauthenticated-allowed GET routes against req.path via a regex requiring a literal /items/:id/cover or /authors/:id/image shape, where req.path retains %2F sequences URL-encoded. Express's router decodes the :id route parameter before handler code runs, so a %2F-encoded '../' sequence in :id (e.g. ..%2f..%2f..%2ftmp%2fpwned) passes the literal-path auth-exemption check while resolving to a real path-traversal payload once decoded. CacheManager.handleCoverCache then joins this decoded value into a cache file path and streams the result before any database-backed ownership check. This bypasses the fix applied for CVE-2025-25205 (which anchored the exemption regex and switched it to req.path) and results in unauthenticated arbitrary file read of any file matching the pattern *_<width>[x<height>].<ext> that the service account can read.
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Analysis
by VulDB Data Team • 08/05/2026
The vulnerability in audiobookshelf represents a sophisticated path traversal attack that exploits a flaw in the authentication exemption logic within the server's routing system. This issue manifests in the Auth.js router where GET routes that are explicitly allowed for unauthenticated access are validated against the request path using a regular expression pattern. The regex specifically looks for literal patterns matching /items/:id/cover or /authors/:id/image structures, but fails to account for URL encoding inconsistencies that can be exploited by malicious actors. The vulnerability stems from the inconsistent handling of URL-encoded sequences between the routing layer and the parameter processing components.
The technical flaw occurs because Express.js router automatically decodes URL parameters before they reach the handler code, while the authentication exemption check operates on the raw request path that preserves URL encoding such as %2F sequences. This creates a mismatch where an attacker can craft a malicious path like ..%2f..%2f..%2ftmp%2fpwned that passes the literal path validation check due to its encoded nature, but upon parameter decoding resolves to a legitimate path traversal payload. The regex validation accepts this encoded sequence as matching the expected pattern while the actual decoded value enables access to arbitrary file system locations.
This vulnerability results in significant operational impact by bypassing security controls designed to protect sensitive files and data within the audiobookshelf service. The attack vector allows unauthenticated arbitrary file reading of any file that matches the pattern *_[x] where the service account has read permissions, effectively enabling attackers to access system files, configuration data, or other sensitive information that should remain protected. The bypass specifically undermines the previous fix implemented for CVE-2025-25205 which had addressed similar path traversal concerns by anchoring the exemption regex and switching validation to use req.path instead of relying on less secure pattern matching approaches.
The mitigation strategy requires addressing the fundamental inconsistency in URL encoding handling between different layers of the application stack. Security controls should be strengthened to validate decoded paths against expected patterns rather than relying solely on literal string matching of URL-encoded sequences. This approach aligns with common security best practices and ATT&CK framework recommendations for preventing path traversal attacks, specifically targeting techniques related to URL decoding manipulation and authentication bypasses. Organizations should implement comprehensive input validation that accounts for all possible encoding variations while ensuring that path traversal protections are applied consistently across all application layers, particularly in components handling file system operations and user-provided parameters.
The vulnerability demonstrates a classic case of improper input validation combined with inconsistent handling of URL encoding, creating an authentication bypass that allows unauthorized access to sensitive files. This type of issue commonly falls under CWE-22 Path Traversal vulnerabilities and represents a critical security risk for applications processing user-supplied paths. The specific implementation flaw in audiobookshelf's routing logic creates a window where encoded sequences can be used to circumvent security controls, highlighting the importance of consistent parameter handling across all application layers. Security teams should prioritize updating their monitoring systems to detect unusual file access patterns and implement additional checks that validate decoded paths against expected security boundaries rather than relying on superficial string matching approaches.