CVE-2025-40918: Perl SASL Auth Bypass Vulnerability

CVE-2025-40918 Overview

CVE-2025-40918 affects Authen::SASL::Perl::DIGEST_MD5 versions 2.04 through 2.1800, a Perl module implementing DIGEST-MD5 Simple Authentication and Security Layer (SASL) authentication. The module generates the client nonce (cnonce) using an MD5 hash of the process identifier (PID), the epoch time, and Perl's built-in rand function. Each input source provides limited entropy. The PID falls within a small numeric range, epoch time can be guessed or leaked via the HTTP Date header, and rand is not cryptographically secure. RFC 2831 recommends at least 64 bits of entropy in the cnonce to resist chosen plaintext attacks and support mutual authentication [CWE-338].

Critical Impact
An attacker who predicts the cnonce can undermine mutual authentication guarantees and mount chosen plaintext attacks against DIGEST-MD5 SASL exchanges.

Affected Products

Authen::SASL Perl distribution, module Authen::SASL::Perl::DIGEST_MD5
Versions 2.04 through 2.1800
Fixed in Authen-SASL release 2.1900

Discovery Timeline

2025-07-16 - CVE-2025-40918 published to the National Vulnerability Database
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-40918

Vulnerability Analysis

DIGEST-MD5 SASL authentication relies on the client-supplied cnonce to prevent replay and chosen plaintext attacks. RFC 2831 specifies that this value must be opaque and unpredictable, with at least 64 bits of entropy. The affected Perl module constructs the cnonce from three predictable or weak inputs before hashing them with MD5. Hashing predictable inputs does not add entropy — the output remains searchable across a small keyspace.

Because the underlying inputs are guessable, an attacker who observes a DIGEST-MD5 exchange can brute force the cnonce offline. Once the cnonce is known, mutual authentication assurances break down and the server response can be validated or forged in scenarios that depend on cnonce unpredictability.

Root Cause

The root cause is insecure random number generation [CWE-338]. The implementation in lib/Authen/SASL/Perl/DIGEST_MD5.pm combines the process PID, the current epoch time, and Perl's rand() output. Perl's rand is a non-cryptographic pseudo-random function whose internal state can be recovered from a small number of outputs. Combining low-entropy inputs through MD5 does not create cryptographic strength.

Attack Vector

The attack requires network access to observe or participate in a DIGEST-MD5 SASL authentication exchange. An attacker collects the challenge, response, and any timing information such as an HTTP Date header. The attacker then enumerates plausible PID values, likely epoch timestamps, and rand outputs, computing MD5 candidates until the observed cnonce matches. See the MetaCPAN Authen-SASL Source and the GitHub Pull Request Discussion for the vulnerable code path and remediation.

Detection Methods for CVE-2025-40918

Indicators of Compromise

Presence of Authen-SASL versions 2.04 through 2.1800 in Perl @INC paths or CPAN inventories
DIGEST-MD5 SASL clients emitting cnonce values that decode to predictable PID and epoch inputs
Unexpected DIGEST-MD5 authentication successes preceded by repeated failed attempts targeting the same account

Detection Strategies

Inventory installed Perl modules with cpan -l or perl -MAuthen::SASL -e 'print $Authen::SASL::VERSION' and flag versions at or below 2.1800
Search source repositories and container images for pinned dependencies on vulnerable Authen-SASL releases
Review application logs for DIGEST-MD5 authentication over cleartext transports where cnonce values could be captured

Monitoring Recommendations

Monitor package management events that install or update the Authen-SASL CPAN distribution
Alert on Perl-based services performing SASL authentication without Transport Layer Security (TLS) protection
Track outbound authentication traffic from mail, LDAP, and XMPP clients that historically used DIGEST-MD5

How to Mitigate CVE-2025-40918

Immediate Actions Required

Upgrade Authen-SASL to version 2.1900 or later on all systems using Perl SASL clients
Apply the vendor patch CVE-2025-40918-r1.patch from MetaCPAN Authen-SASL CVE-2025 Patch where an immediate upgrade is not feasible
Prefer stronger SASL mechanisms such as SCRAM-SHA-256 over DIGEST-MD5 for new deployments

Patch Information

The fix replaces the weak cnonce construction with a cryptographically secure random source. Details are documented in the MetaCPAN Authen-SASL Changes and the corresponding GitHub Pull Request Discussion. Distribution maintainers should track the vendor advisory posted on the OpenWall OSS-Security List Post.

Workarounds

Wrap DIGEST-MD5 SASL exchanges inside TLS to prevent an attacker from observing the cnonce and challenge on the wire
Disable DIGEST-MD5 on server components that support stronger mechanisms and require SCRAM or GSSAPI instead
Rotate credentials used with vulnerable clients after upgrading, in case earlier sessions were observed

bash

# Configuration example
cpan install Authen::SASL@2.1900
perl -MAuthen::SASL -e 'print "Authen::SASL $Authen::SASL::VERSION\n"'