CVE-2023-2283: Libssh Auth Bypass Vulnerability

CVE-2023-2283 Overview

A vulnerability was found in libssh where the authentication check of the connecting client can be bypassed in the pki_verify_data_signature function due to memory allocation problems. This issue may occur when there is insufficient memory or memory usage is limited. The flaw allows unauthenticated attackers to potentially bypass client authentication mechanisms under specific memory pressure conditions.

Critical Impact

Authentication bypass in libssh allows attackers to circumvent cryptographic verification under memory-constrained conditions, potentially granting unauthorized access to SSH-protected systems.

Affected Products

• libssh libssh (versions prior to patch)
• Fedora Project Fedora 37
• Red Hat Enterprise Linux 8.0 and 9.0

Discovery Timeline

• 2023-05-26 - CVE CVE-2023-2283 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2023-2283

Vulnerability Analysis

The vulnerability exists in the pki_verify_data_signature function within libssh, a widely-used library implementing the SSH protocol. The flaw is a classic case of improper error handling combined with variable initialization issues that can lead to authentication bypass.

The core issue stems from the return value variable rc, which is initialized to SSH_ERROR at the beginning of the function. This variable is subsequently overwritten to store the return value of the pki_key_check_hash_compatible function call. However, the value of rc is not properly reset or checked between this function call and the subsequent cryptographic signature verification step.

When memory allocation fails or memory usage is constrained during execution, any error occurring between the pki_key_check_hash_compatible call and the cryptographic verification causes the code to jump to an error handler via goto error. Due to the flawed logic, this error path incorrectly returns SSH_OK instead of the expected SSH_ERROR, effectively signaling successful authentication when verification was never completed.

Root Cause

The root cause is improper error handling in the authentication flow. The variable rc is reused without proper reinitialization before the cryptographic verification step. This creates a code path where memory allocation failures or resource exhaustion conditions can cause the function to return success (SSH_OK) without actually completing the signature verification, constituting CWE-287 (Improper Authentication).

Attack Vector

The attack vector is network-based and requires no privileges or user interaction. An attacker could potentially exploit this vulnerability by:

1. Connecting to an SSH server that uses a vulnerable version of libssh
2. Triggering memory pressure conditions during the authentication handshake
3. Exploiting the flawed error handling to bypass signature verification
4. Gaining unauthorized access without valid credentials

The vulnerability is exploitable when the target system experiences memory constraints, either naturally or through deliberate resource exhaustion by the attacker.

The vulnerable code path in the pki_verify_data_signature function improperly handles the return value flow, allowing authentication to succeed when cryptographic verification fails due to memory issues. For detailed technical analysis, refer to the libssh CVE Advisory.

Detection Methods for CVE-2023-2283

Indicators of Compromise

• Unexpected successful SSH authentication events from unknown or suspicious source IPs
• Anomalous memory allocation patterns or out-of-memory events on SSH servers running libssh
• Authentication logs showing successful connections without corresponding valid credential usage
• Increased failed connection attempts followed by successful authentication under memory pressure

Detection Strategies

• Monitor SSH authentication logs for anomalous patterns, particularly successful authentications during periods of high memory usage
• Implement network-level detection for SSH connection attempts that exhibit unusual handshake patterns
• Deploy application-level monitoring on libssh-based services to detect memory allocation failures during authentication
• Use SentinelOne Singularity Platform to detect exploitation attempts through behavioral analysis of SSH daemon processes

Monitoring Recommendations

• Enable verbose logging on all SSH services using libssh to capture detailed authentication events
• Configure memory usage alerts on systems running vulnerable libssh versions to identify potential exploitation conditions
• Implement network segmentation and monitoring for SSH traffic to critical infrastructure
• Deploy endpoint detection and response (EDR) solutions to identify post-exploitation activities

How to Mitigate CVE-2023-2283

Immediate Actions Required

• Update libssh to the latest patched version immediately on all affected systems
• Review SSH authentication logs for any signs of unauthorized access or anomalous authentication patterns
• Implement network-level access controls to restrict SSH access to trusted IP ranges
• Consider temporary service restrictions for critical systems until patches are applied

Patch Information

The libssh project has released security updates addressing this vulnerability. Administrators should apply the latest patches from the libssh CVE Advisory. Additional vendor-specific patches are available from Red Hat, Fedora, Gentoo, and NetApp.

Workarounds

• Implement additional authentication mechanisms such as multi-factor authentication (MFA) for SSH access
• Configure system resource limits to ensure adequate memory is available for SSH authentication processes
• Deploy intrusion detection systems (IDS) to monitor for SSH authentication bypass attempts
• Use network firewalls to limit SSH access to trusted networks and IP addresses only

# Example: Configure memory limits for SSH processes on Linux
# Add to /etc/security/limits.conf to ensure SSH has adequate memory
sshd            hard    memlock         unlimited
sshd            soft    memlock         unlimited

# Verify libssh version on the system
rpm -qa | grep libssh   # For RHEL/Fedora systems
dpkg -l | grep libssh   # For Debian/Ubuntu systems