CVE-2025-5987 Overview
A critical flaw was discovered in libssh when using the ChaCha20 cipher with the OpenSSL library. If an attacker manages to exhaust the heap space, this error is not detected and may lead to libssh using a partially initialized cipher context. This occurs because the OpenSSL error code returned aliases with the SSH_OK code, resulting in libssh not properly detecting the error returned by the OpenSSL library. This issue can lead to undefined behavior, including compromised data confidentiality and integrity or crashes.
Critical Impact
This vulnerability can compromise the confidentiality and integrity of SSH communications by causing libssh to operate with a partially initialized cipher context, potentially exposing encrypted data or leading to application crashes.
Affected Products
- libssh (all versions using ChaCha20 cipher with OpenSSL)
- Applications and services built on libssh
- Systems running Red Hat Enterprise Linux with affected libssh packages
Discovery Timeline
- July 7, 2025 - CVE-2025-5987 published to NVD
- January 13, 2026 - Last updated in NVD database
Technical Details for CVE-2025-5987
Vulnerability Analysis
This vulnerability stems from an improper error handling mechanism within libssh's cryptographic initialization routines when using the ChaCha20 cipher in conjunction with OpenSSL. The core issue is a return value aliasing problem (CWE-393: Return of Wrong Status Code) where OpenSSL's error codes are incorrectly interpreted by libssh.
When heap memory allocation fails during cipher context initialization, OpenSSL returns an error code that inadvertently matches libssh's SSH_OK success indicator. This aliasing causes libssh to proceed with cryptographic operations using a cipher context that was not properly initialized, creating a dangerous condition where the encryption state is undefined.
The network-accessible nature of this vulnerability means remote attackers could potentially trigger it by causing heap exhaustion conditions on target systems. While exploitation requires creating specific memory pressure conditions (indicated by high attack complexity), successful exploitation could allow attackers to compromise the confidentiality and integrity of SSH-secured communications without requiring any authentication or user interaction.
Root Cause
The root cause is a return value aliasing issue (CWE-393) where OpenSSL error codes collide with libssh's internal status codes. Specifically, when the EVP_CIPHER_CTX_new() or related OpenSSL functions fail due to heap exhaustion, the returned error value is misinterpreted as SSH_OK by libssh's error checking logic. This allows the library to continue execution with an uninitialized or partially initialized EVP_CIPHER_CTX structure, leading to undefined cryptographic behavior.
Attack Vector
The attack vector is network-based, requiring an attacker to first create heap exhaustion conditions on the target system. This could be achieved through:
- Memory pressure attacks: Sending specially crafted requests or payloads that cause excessive memory allocation on the target
- Resource exhaustion: Combining this vulnerability with other flaws to gradually deplete available heap memory
- Timing attacks: Exploiting race conditions during high-load scenarios when memory is already constrained
Once heap exhaustion is achieved during an SSH session establishment using ChaCha20 cipher, the uninitialized cipher context could result in:
- Predictable or weak encryption output
- Disclosure of sensitive data in transit
- Application crashes causing denial of service
- Potential memory corruption leading to further exploitation
The vulnerability affects the ChaCha20 cipher specifically when used with OpenSSL as the underlying cryptographic provider, meaning systems configured to use this cipher combination are at risk.
Detection Methods for CVE-2025-5987
Indicators of Compromise
- Unexpected SSH connection failures or crashes during session establishment
- Memory exhaustion events coinciding with SSH connection attempts
- Abnormal cipher negotiation patterns in SSH handshake logs
- Application crashes in libssh-dependent services with memory-related error messages
Detection Strategies
- Monitor system memory utilization and correlate spikes with SSH connection activity
- Implement logging for OpenSSL initialization failures in applications using libssh
- Deploy intrusion detection rules to identify potential heap spray or memory exhaustion attacks targeting SSH services
- Audit SSH cipher suite configurations to identify systems using ChaCha20 with OpenSSL-backed libssh
Monitoring Recommendations
- Enable verbose logging for libssh-based applications to capture initialization errors
- Configure SIEM alerts for memory exhaustion events on systems running SSH services
- Monitor for unusual patterns in SSH connection rates that could indicate exploitation attempts
- Track system stability metrics for services dependent on libssh
How to Mitigate CVE-2025-5987
Immediate Actions Required
- Update libssh to the latest patched version available from your distribution
- Apply Red Hat security updates if running affected RHEL systems (see RHSA advisories below)
- Consider temporarily disabling ChaCha20 cipher in SSH configurations as a short-term mitigation
- Monitor affected systems for signs of memory exhaustion attacks
Patch Information
Multiple security advisories have been released to address this vulnerability:
- Red Hat Security Advisory RHSA-2025:23483
- Red Hat Security Advisory RHSA-2025:23484
- Red Hat Security Advisory RHSA-2026:0427
- Red Hat Security Advisory RHSA-2026:0428
- Red Hat Security Advisory RHSA-2026:0430
- Red Hat Security Advisory RHSA-2026:0431
Additional details are available in the Red Hat CVE Analysis for CVE-2025-5987 and Red Hat Bug Report #2376219.
Workarounds
- Configure SSH to use alternative ciphers (e.g., AES-GCM) instead of ChaCha20 until patches are applied
- Implement memory limits and resource controls for SSH-related processes to reduce heap exhaustion risk
- Deploy network-level protections to rate-limit SSH connection attempts from untrusted sources
# Configuration example - Disable ChaCha20 cipher in SSH config
# Edit /etc/ssh/sshd_config or libssh configuration
# Remove chacha20-poly1305@openssh.com from cipher list
# For OpenSSH sshd_config:
Ciphers aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes256-ctr,aes192-ctr,aes128-ctr
# Restart SSH service after configuration change
sudo systemctl restart sshd
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
