CVE-2019-3863 is a vulnerability identified in the libssh2 library, specifically in versions prior to 1.8.1. libssh2 is a widely used client-side C library implementing the SSH2 protocol, enabling secure communication and file transfers over encrypted channels. The vulnerability arises from improper handling of multiple keyboard-interactive response messages sent by a server. In this scenario, a malicious or compromised SSH server can send multiple keyboard-interactive response messages whose combined length exceeds the maximum value representable by an unsigned char (typically 255). This length value is then used as an index for copying memory without proper bounds checking, leading to an out-of-bounds memory write condition. This type of vulnerability is classified under CWE-190 (Integer Overflow or Wraparound) and CWE-787 (Out-of-bounds Write). The out-of-bounds write can corrupt memory, potentially causing application crashes, data corruption, or enabling arbitrary code execution if exploited effectively. Since libssh2 is a client library, the attack vector requires the client to connect to a malicious or compromised SSH server that sends the crafted messages. No known exploits have been reported in the wild, and the vulnerability was publicly disclosed in March 2019. The flaw is mitigated in libssh2 version 1.8.1 and later, where proper bounds checking and validation have been implemented to prevent this memory corruption issue.

For European organizations, the impact of this vulnerability depends largely on the extent to which libssh2 is integrated into their infrastructure and software products. libssh2 is commonly embedded in various client applications, automation tools, and network management systems that rely on SSH for secure communication. Exploitation could allow a malicious SSH server to cause client application crashes or potentially execute arbitrary code on client machines, leading to confidentiality breaches, integrity violations, or availability disruptions. This is particularly critical for organizations that use automated SSH connections to manage critical infrastructure, cloud environments, or sensitive data transfers. The vulnerability could be leveraged in targeted attacks where adversaries control or impersonate SSH servers, such as in supply chain attacks or compromised bastion hosts. Although no public exploits exist, the risk remains for organizations with high exposure to untrusted SSH servers or those using custom or legacy software embedding vulnerable libssh2 versions. The medium severity rating reflects the requirement for a malicious server and the client-side nature of the vulnerability, limiting widespread exploitation but still posing a significant risk in targeted scenarios.

1. Upgrade all libssh2 instances to version 1.8.1 or later to ensure the vulnerability is patched. 2. Conduct an inventory of software and systems using libssh2, including embedded devices, automation scripts, and third-party applications, to identify and remediate vulnerable versions. 3. Implement strict SSH server validation policies, including the use of known host keys and certificate-based authentication, to reduce the risk of connecting to malicious or compromised SSH servers. 4. Employ network segmentation and firewall rules to limit SSH client connections to trusted servers only, minimizing exposure to potentially hostile servers. 5. Monitor SSH client logs and network traffic for unusual or unexpected keyboard-interactive authentication attempts that could indicate exploitation attempts. 6. For critical environments, consider deploying application-level sandboxing or memory protection mechanisms to mitigate potential impacts of memory corruption vulnerabilities. 7. Engage with software vendors and suppliers to confirm that their products have addressed this vulnerability if they incorporate libssh2.