CVE-2025-40918 in Authen::SASL::Perl::DIGEST_MD5
Summary
by MITRE • 07/16/2025
Authen::SASL::Perl::DIGEST_MD5 versions 2.04 through 2.1800 for Perl generates the cnonce insecurely.
The cnonce (client nonce) is generated from an MD5 hash of the PID, the epoch time and the built-in rand function. The PID will come from a small set of numbers, and the epoch time may be guessed, if it is not leaked from the HTTP Date header. The built-in rand function is unsuitable for cryptographic usage.
According to RFC 2831, "The cnonce-value is an opaque quoted string value provided by the client and used by both client and server to avoid chosen plaintext attacks, and to provide mutual authentication. The security of the implementation depends on a good choice. It is RECOMMENDED that it contain at least 64 bits of entropy."
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Analysis
by VulDB Data Team • 04/09/2026
The vulnerability identified as CVE-2025-40918 affects the Authen::SASL::Perl::DIGEST_MD5 Perl module version 2.04 through 2.1800, specifically targeting the insecure generation of client nonces within the DIGEST-MD5 authentication mechanism. This flaw resides in the cryptographic implementation of the SASL (Simple Authentication and Security Layer) protocol, which is widely used for securing network communications. The module's failure to generate cryptographically secure nonces creates a significant security weakness that directly impacts the integrity and confidentiality of authentication processes relying on this implementation. The vulnerability stems from the use of predictable and insufficiently random sources for nonce generation, fundamentally undermining the security assurances that DIGEST-MD5 is designed to provide.
The technical flaw manifests in how the cnonce value is constructed using a combination of predictable elements that severely limit its entropy. Specifically, the implementation utilizes an MD5 hash incorporating the process identifier (PID), epoch timestamp, and Perl's built-in rand() function, which is inherently unsuitable for cryptographic purposes. The PID typically originates from a limited range of values, making it easily guessable, while the epoch time can often be inferred from network timing information or HTTP Date headers, particularly when these are not properly obfuscated or randomized. The inclusion of Perl's rand() function, which is designed for general programming tasks rather than cryptographic security, introduces additional weaknesses that make the generated nonces vulnerable to brute force attacks and pattern recognition. This approach directly violates the cryptographic requirements established in RFC 2831, which explicitly recommends that cnonce values contain at least 64 bits of entropy to ensure adequate security against various attack vectors.
The operational impact of this vulnerability extends beyond simple authentication failures, creating opportunities for sophisticated attacks that can compromise the entire authentication infrastructure. Attackers capable of predicting or generating valid cnonce values can potentially perform credential stuffing attacks, session hijacking, or even bypass authentication entirely by exploiting the predictable nature of the nonce generation. The weakness becomes particularly dangerous in environments where attackers can observe or infer timing information, as they can then reconstruct the nonce values used in authentication exchanges. This vulnerability affects systems that rely on DIGEST-MD5 for authentication, which includes various network services, web applications, and enterprise systems that have not properly migrated away from this older authentication mechanism. The attack surface is further expanded when considering that many legacy systems continue to support DIGEST-MD5 due to compatibility requirements, making these environments prime targets for exploitation.
Mitigation strategies for CVE-2025-40918 must prioritize immediate module updates to versions that properly implement cryptographically secure nonce generation. Organizations should implement comprehensive vulnerability scanning to identify all systems running affected versions of the Authen::SASL::Perl::DIGEST_MD5 module, particularly in environments where DIGEST-MD5 authentication is actively used. The recommended approach includes upgrading to patched versions that utilize proper cryptographic random number generators such as /dev/urandom or equivalent secure sources for nonce creation, ensuring that all generated values meet the minimum entropy requirements specified in RFC 2831. Additionally, system administrators should consider disabling DIGEST-MD5 authentication where possible and migrating to more secure authentication mechanisms such as OAuth, JWT tokens, or modern SASL mechanisms that provide better cryptographic guarantees. Security monitoring should be enhanced to detect anomalous authentication patterns that might indicate nonce prediction attacks, and access controls should be reviewed to minimize the impact of potential compromise. This vulnerability aligns with CWE-330 (Use of Insufficiently Random Values) and represents a clear violation of the principle of least privilege in authentication security, as described in ATT&CK technique T1566 for credential access and T1550 for valid accounts.