CVE-2019-3861 in Libssh2
Summary
by MITRE
An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SSH packets with a padding length value greater than the packet length are parsed. A remote attacker who compromises a SSH server may be able to cause a Denial of Service or read data in the client memory.
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Analysis
by VulDB Data Team • 08/07/2023
The vulnerability identified as CVE-2019-3861 represents a critical out-of-bounds read flaw within the libssh2 library version 1.8.1 and earlier. This issue arises from insufficient input validation during SSH packet parsing operations, specifically when handling packets where the padding length value exceeds the actual packet length. The flaw exists in the protocol implementation layer of the SSH client library, creating a scenario where malformed packet structures can trigger memory access violations. The vulnerability is particularly concerning because it affects the core cryptographic communication library used by numerous applications and systems for secure remote access operations.
The technical exploitation of this vulnerability occurs through malformed SSH packets that contain invalid padding length values. When libssh2 attempts to parse these packets, the library reads beyond the allocated buffer boundaries due to improper bounds checking. This condition manifests as an out-of-bounds memory read operation, which can result in unpredictable behavior including application crashes, denial of service conditions, or potentially information disclosure from adjacent memory regions. The flaw demonstrates a classic weakness in input validation and memory management practices that aligns with CWE-129, which addresses insufficient bounds checking in array indexing operations. The vulnerability is classified as a remote code execution risk because a compromised SSH server can craft malicious packets that trigger this condition in connected clients.
The operational impact of CVE-2019-3861 extends beyond simple denial of service scenarios, as the memory read operations could potentially expose sensitive information from the client's memory space. This includes cryptographic keys, session data, or other confidential information that may be stored in adjacent memory locations. The vulnerability affects any system utilizing libssh2 version 1.8.1 or earlier in their SSH client implementations, which encompasses a wide range of applications including web servers, database clients, and network management tools. The attack vector requires a compromised SSH server to deliver malicious packets, making it particularly dangerous in environments where server compromise is possible or where attackers have access to the network path between client and server. This vulnerability can be mapped to ATT&CK technique T1071.004 for Application Layer Protocol: SSH, as it exploits weaknesses in SSH protocol implementations.
Mitigation strategies for CVE-2019-3861 primarily involve upgrading to libssh2 version 1.8.1 or later, which includes proper bounds checking for padding length values in SSH packet parsing. Organizations should prioritize patching all systems that utilize libssh2 for SSH connectivity, particularly those in critical infrastructure environments. Additional defensive measures include implementing network segmentation to limit exposure, monitoring for unusual SSH traffic patterns, and deploying intrusion detection systems that can identify malformed SSH packets. The vulnerability highlights the importance of maintaining up-to-date cryptographic libraries and implementing proper input validation practices. Security teams should conduct vulnerability assessments to identify all systems using affected libssh2 versions and establish processes for regular library updates and security monitoring. Network administrators should also consider implementing SSH protocol-level controls and access restrictions to minimize the attack surface for this and similar vulnerabilities.