CVE-2019-16905 in OpenSSH
Summary
by MITRE
OpenSSH 7.7 through 7.9 and 8.x before 8.1, when compiled with an experimental key type, has a pre-authentication integer overflow if a client or server is configured to use a crafted XMSS key. This leads to memory corruption and remote code execution because of an error in the XMSS key parsing algorithm. NOTE: the XMSS implementation is considered experimental in all released OpenSSH versions, and there is no supported way to enable it when building portable OpenSSH.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 04/17/2025
The vulnerability identified as CVE-2019-16905 represents a critical pre-authentication integer overflow flaw within OpenSSH versions ranging from 7.7 through 7.9 and 8.x prior to 8.1. This vulnerability specifically manifests when the software is compiled with experimental key type support and encounters a crafted XMSS (eXtended Merkle Signature Scheme) key during the authentication process. The integer overflow occurs within the XMSS key parsing algorithm, creating a condition where memory corruption can occur, potentially enabling remote code execution. The flaw exists at the protocol level before authentication is completed, making it particularly dangerous as it can be exploited without requiring valid credentials or prior access to the system.
The technical implementation of this vulnerability stems from improper handling of integer arithmetic within the XMSS key parsing routine. When processing a maliciously constructed XMSS key, the algorithm fails to properly validate input parameters, leading to an integer overflow condition that subsequently corrupts adjacent memory locations. This memory corruption can be leveraged by remote attackers to execute arbitrary code on the target system. The vulnerability is categorized under CWE-190, which specifically addresses integer overflow conditions, and aligns with ATT&CK technique T1190, which covers exploitation of remote services through the manipulation of input data. The XMSS implementation in OpenSSH has been marked as experimental since its introduction, indicating that it was not intended for production use and lacked the comprehensive security testing that production-ready cryptographic implementations require.
The operational impact of this vulnerability extends beyond simple remote code execution, as it represents a complete compromise of the target system's security posture. Attackers can exploit this vulnerability to gain unauthorized access to systems running vulnerable OpenSSH versions, potentially leading to data theft, system takeover, or further lateral movement within a network. The pre-authentication nature of the exploit means that the vulnerability can be triggered simply by connecting to the affected service, without requiring any prior authentication attempts. This characteristic makes it particularly attractive to attackers who seek to exploit systems without raising immediate detection flags. Organizations running vulnerable OpenSSH versions are at risk of complete system compromise, especially in environments where SSH access is exposed to untrusted networks or where the software is configured to accept experimental key types.
Mitigation strategies for CVE-2019-16905 primarily focus on immediate version upgrades to OpenSSH 8.1 or later, where the vulnerability has been addressed through proper integer overflow protections in the XMSS key parsing algorithm. System administrators should also consider disabling experimental key types in their OpenSSH configurations, as these features were never intended for production use according to the OpenSSH development team. The implementation of network segmentation and access controls can provide additional defense-in-depth measures, limiting the potential attack surface for systems running vulnerable versions. Security monitoring should be enhanced to detect unusual SSH connection patterns or malformed key exchanges that might indicate exploitation attempts. Organizations should also conduct comprehensive inventory assessments to identify all systems running affected OpenSSH versions and prioritize remediation efforts based on risk assessment and system criticality. The vulnerability serves as a reminder of the importance of avoiding experimental cryptographic features in production environments and the necessity of thorough security validation for all cryptographic implementations.