CVE-2025-5372: Libssh Information Disclosure Vulnerability

CVE-2025-5372 Overview

A critical cryptographic vulnerability has been identified in libssh versions built with OpenSSL versions older than 3.0. The flaw exists in the ssh_kdf() function responsible for key derivation, where inconsistent interpretation of return values between OpenSSL and libssh leads to the use of uninitialized cryptographic key buffers. OpenSSL uses 0 to indicate failure while libssh interprets 0 as success, causing the function to mistakenly return a success status even when key derivation fails. This results in SSH sessions potentially using uninitialized memory for cryptographic operations, compromising session confidentiality, integrity, and availability.

Critical Impact

SSH sessions may use uninitialized cryptographic keys, allowing attackers to potentially decrypt communications, modify data in transit, or cause denial of service through corrupted sessions.

Affected Products

• libssh (all versions built with OpenSSL < 3.0)
• Red Hat OpenShift Container Platform 4.0
• Red Hat Enterprise Linux 6.0, 7.0, 8.0, 9.0, 10.0

Discovery Timeline

• 2025-07-04 - CVE-2025-5372 published to NVD
• 2025-12-10 - Last updated in NVD database

Technical Details for CVE-2025-5372

Vulnerability Analysis

This vulnerability stems from a fundamental API contract mismatch between libssh and OpenSSL regarding function return value semantics. When libssh is compiled against OpenSSL versions prior to 3.0, the ssh_kdf() function incorrectly interprets the return value from OpenSSL's key derivation routines. In the C programming convention used by OpenSSL, a return value of 0 typically indicates an error condition, while libssh's internal convention treats 0 as a success indicator.

This semantic mismatch means that when OpenSSL's key derivation function fails and returns 0 to signal the error, libssh's ssh_kdf() function interprets this as a successful operation and proceeds with subsequent cryptographic operations. The consequence is that the key buffers, which were never properly initialized with derived key material, contain arbitrary memory contents that are then used for encryption, decryption, and authentication operations in the SSH session.

The vulnerability is classified under CWE-682 (Incorrect Calculation), as the core issue involves incorrect processing of computational results that leads to security-relevant errors in cryptographic operations.

Root Cause

The root cause is an incorrect interpretation of return values in the ssh_kdf() function when interfacing with OpenSSL's key derivation API. OpenSSL versions prior to 3.0 return 0 on failure, but libssh's code incorrectly treats this value as success, leading to the use of uninitialized key material. This represents a classic API contract violation where assumptions about return value semantics do not match the actual behavior of the underlying library.

Attack Vector

The vulnerability is network-exploitable with low attack complexity. An attacker positioned to intercept or participate in SSH session establishment could potentially exploit this flaw to:

1. Intercept Communications: If key derivation fails silently, the resulting session keys may be predictable or weak, allowing passive eavesdropping on SSH traffic
2. Session Manipulation: Uninitialized key buffers may contain predictable patterns, potentially allowing attackers to forge or modify SSH packets
3. Denial of Service: Corrupted cryptographic state can cause session failures, authentication errors, or application crashes

The vulnerability requires low privileges to exploit, as the attacker needs only the ability to establish SSH connections to affected systems. The attack does not require user interaction.

Detection Methods for CVE-2025-5372

Indicators of Compromise

• Unusual SSH session failures or authentication errors that cannot be attributed to credential issues
• SSH connections that establish successfully but exhibit corrupted data transmission
• Log entries indicating cryptographic operation failures or checksum mismatches in SSH sessions
• Increased frequency of SSH session renegotiations or unexpected disconnections

Detection Strategies

• Monitor for libssh library versions compiled against OpenSSL versions < 3.0 in your environment
• Implement SSH session anomaly detection to identify sessions with unusual behavior patterns
• Review system logs for cryptographic errors originating from libssh-based applications
• Deploy network monitoring to detect SSH traffic anomalies that may indicate exploitation attempts

Monitoring Recommendations

• Enable verbose logging for SSH services using libssh to capture key derivation events
• Implement file integrity monitoring on libssh library files to track version changes
• Configure alerting for repeated SSH authentication failures that may indicate exploitation attempts
• Monitor for unusual patterns in SSH session establishment latency that may indicate key derivation issues

How to Mitigate CVE-2025-5372

Immediate Actions Required

• Identify all systems running libssh compiled against OpenSSL versions older than 3.0
• Prioritize patching for internet-facing SSH services and critical infrastructure
• Consider temporarily disabling affected SSH services in high-security environments until patches are applied
• Apply vendor-provided patches from Red Hat advisories RHSA-2025:21977 and RHSA-2025:23024

Patch Information

Red Hat has released security advisories addressing this vulnerability:

• Red Hat Security Advisory RHSA-2025:21977 - Contains patches for affected Red Hat products
• Red Hat Security Advisory RHSA-2025:23024 - Additional security updates for enterprise deployments
• Red Hat CVE Review CVE-2025-5372 - Official vulnerability assessment and patch information
• Red Hat Bug Report #2369388 - Technical details and patch tracking

Workarounds

• Upgrade OpenSSL to version 3.0 or later, which uses a different return value convention that is correctly handled by libssh
• Recompile libssh against a newer OpenSSL version if source-based deployment is used
• Implement network segmentation to limit exposure of vulnerable SSH services
• Deploy additional authentication layers (such as certificate-based authentication) to reduce attack surface

# Check OpenSSL version on affected systems
openssl version

# Verify libssh linking against OpenSSL
ldd /usr/lib*/libssh.so* | grep -i ssl

# Update libssh and dependencies on RHEL-based systems
sudo yum update libssh openssl

# Restart SSH services after patching
sudo systemctl restart sshd