CVE-2026-16615 in librest
Summary
by MITRE • 07/22/2026
A flaw was found in librest. The PKCE implementation for OAuth authorization uses the GRand function from the GLib API, a cryptographically insecure pseudo-random number generator. Because the generated "code verifier" lacks sufficient cryptographic entropy, a malicious actor can reverse-engineer the pseudo-random number generator (PRNG) seed to predict or reconstruct the code verifier string, allowing an attacker to bypass PKCE protections and successfully impersonate the client during the OAuth 2.0 authorization flow.
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Analysis
by VulDB Data Team • 07/29/2026
The vulnerability exists within the librest library's implementation of Proof Key for Code Exchange (PKCE) for OAuth authorization protocols. This flaw represents a critical weakness in the cryptographic foundation of the authentication mechanism, specifically targeting the generation of code verifiers that are essential for protecting against authorization code interception attacks. The issue stems from the use of the GRand function from the GLib API, which is classified as a cryptographically insecure pseudo-random number generator according to industry standards and best practices.
The technical implementation flaw involves the direct utilization of GRand instead of a cryptographically secure random number generator for creating the code verifier component within the PKCE framework. This design choice fundamentally undermines the security guarantees that PKCE is intended to provide, as the generated values lack sufficient entropy to resist cryptographic attacks. According to CWE-330, this represents an insufficient entropy vulnerability where the randomness source is predictable and vulnerable to reverse engineering, making it susceptible to mathematical analysis and pattern recognition.
The operational impact of this vulnerability extends beyond simple authentication bypass scenarios and creates a significant risk for any system relying on librest for OAuth 2.0 implementations. Attackers who can predict or reconstruct the code verifier through PRNG seed reconstruction can successfully impersonate legitimate clients during the authorization flow, potentially gaining unauthorized access to protected resources, user data, or service endpoints. This vulnerability directly maps to ATT&CK technique T1566 where adversaries exploit weak authentication mechanisms to gain initial access.
The security implications are particularly severe given that PKCE is designed specifically to prevent authorization code interception attacks by ensuring that the code verifier cannot be guessed or predicted by an attacker who intercepts the authorization code. When the code verifier generation becomes predictable, the entire purpose of PKCE is defeated, leaving systems vulnerable to man-in-the-middle attacks and session hijacking scenarios. Organizations using affected versions of librest should immediately evaluate their OAuth 2.0 implementations for potential compromise and implement emergency mitigations such as upgrading to patched versions or implementing additional authentication layers.
The vulnerability demonstrates a fundamental misunderstanding of cryptographic requirements in security-sensitive contexts, where the choice between cryptographically secure and insecure random number generators can determine the entire security posture of an authentication system. This flaw represents a classic example of how seemingly minor implementation decisions can have catastrophic security consequences, particularly when dealing with protocols that require strong randomness properties for their integrity. The use of insecure PRNG functions in cryptographic contexts violates established security frameworks and best practices that mandate the use of cryptographically secure random number generators for all security-sensitive operations.
Mitigation efforts should focus on immediate replacement of the GRand function with a proper cryptographically secure random number generator, such as those provided by the OpenSSL library or other established cryptographic libraries. Organizations should also conduct comprehensive audits of their OAuth implementations to identify any other potential uses of insecure randomness sources and ensure that all cryptographic material generation meets current security standards and recommendations from NIST SP 800-90A for random number generation requirements.