CVE-2023-1667: Libssh Denial of Service Vulnerability

CVE-2023-1667 Overview

A NULL pointer dereference vulnerability was discovered in libssh during re-keying operations with algorithm guessing. This security flaw allows an authenticated client to trigger a denial of service condition by exploiting improper handling of pointer operations during the SSH key exchange process. The vulnerability affects the core SSH protocol implementation within libssh, making it a significant concern for systems and applications relying on this library for secure communications.

Critical Impact

Authenticated attackers can crash SSH services and connections, causing denial of service to legitimate users and disrupting secure communications infrastructure.

Affected Products

• libssh (versions prior to the security patch)
• Fedora 37
• Debian Linux 10.0
• Red Hat Enterprise Linux 8.0 and 9.0

Discovery Timeline

• May 26, 2023 - CVE-2023-1667 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-1667

Vulnerability Analysis

This vulnerability is classified under CWE-476 (NULL Pointer Dereference), representing a fundamental memory safety issue in the libssh codebase. The flaw occurs during the SSH re-keying process when algorithm guessing is employed. During this operation, the library fails to properly validate pointer states before dereferencing, leading to a crash when encountering unexpected conditions.

The vulnerability is network-accessible, requiring only low privileges (an authenticated session) to exploit. No user interaction is needed, making automated exploitation feasible once an attacker has established an authenticated connection. While the confidentiality and integrity of data remain unaffected, the availability impact is significant as it can crash SSH services entirely.

Root Cause

The root cause lies in improper pointer validation within the re-keying mechanism of libssh. When the library performs algorithm negotiation during key exchange renewal, it fails to check whether certain data structures are properly initialized before accessing them. Specifically, during algorithm guessing—a process where the client and server negotiate supported cryptographic algorithms—the code path can encounter a situation where a pointer expected to reference a valid algorithm structure is NULL.

This occurs because the re-keying process assumes that prior initialization steps have successfully populated all necessary structures. However, under certain timing conditions or specific algorithm negotiation sequences, these assumptions do not hold, resulting in a NULL pointer dereference when the code attempts to access algorithm-specific data.

Attack Vector

The attack vector for CVE-2023-1667 is network-based, requiring an authenticated SSH session. An attacker with valid credentials can exploit this vulnerability by:

1. Establishing a legitimate SSH connection to a vulnerable libssh-based server
2. Initiating a re-key request during the session
3. Manipulating the algorithm negotiation process to trigger the vulnerable code path
4. Causing the NULL pointer dereference, which crashes the SSH service

Since the vulnerability requires authentication, it is most likely to be exploited by insider threats, compromised accounts, or as part of a multi-stage attack where credentials have been previously obtained.

The vulnerability manifests during the SSH re-keying algorithm negotiation phase. When the re-key process is initiated and algorithm guessing is performed, improper validation of internal data structures can lead to a NULL pointer being dereferenced. This causes the SSH service or client to crash immediately. For detailed technical information, see the LibSSH Security Advisory.

Detection Methods for CVE-2023-1667

Indicators of Compromise

• Unexpected SSH service crashes or restarts, particularly during established sessions
• Core dumps or crash logs indicating NULL pointer dereference in libssh components
• Multiple SSH connection terminations coinciding with re-key timing windows
• Log entries showing abnormal re-keying patterns or frequencies from specific authenticated users

Detection Strategies

• Monitor SSH daemon logs for crash events and correlate with connection metadata
• Implement application crash monitoring to detect repeated libssh-related failures
• Deploy network traffic analysis to identify unusual SSH re-key request patterns
• Use endpoint detection to flag libssh process crashes with NULL dereference signatures

Monitoring Recommendations

• Enable detailed logging for SSH services to capture re-keying events and connection states
• Configure crash dump collection for forensic analysis of potential exploitation attempts
• Set up alerting thresholds for SSH service availability and restart frequency
• Monitor authenticated user behavior for anomalous session patterns that may indicate exploitation attempts

How to Mitigate CVE-2023-1667

Immediate Actions Required

• Update libssh to the latest patched version available from your distribution or upstream
• Review and limit SSH access to only trusted, necessary users to reduce attack surface
• Implement network segmentation to restrict SSH access from untrusted network segments
• Enable enhanced monitoring and alerting for SSH service health and availability

Patch Information

Security patches addressing CVE-2023-1667 are available from multiple sources. Organizations should apply updates according to their deployment:

• libssh upstream: Refer to the LibSSH Security Advisory for official patch information
• Red Hat Enterprise Linux: Updates available via Red Hat Security CVE page
• Debian Linux: Security updates announced via Debian LTS Announcement
• Fedora: Patches available through Fedora Package Announcement
• Gentoo: Updates detailed in Gentoo GLSA Report

Workarounds

• Restrict SSH access to only authenticated users with verified need for access
• Implement connection rate limiting to reduce the potential impact of repeated exploitation attempts
• Consider deploying SSH connection proxies or bastion hosts with additional monitoring capabilities
• If possible, disable re-keying or extend re-key intervals as a temporary measure (note: this may have security implications for long-running sessions)

# Example: Restrict SSH access by user group in sshd_config
# Add to /etc/ssh/sshd_config to limit SSH access
AllowGroups ssh-users admin
MaxStartups 10:30:60
LoginGraceTime 30