CVE-2019-3857 in Libssh2
Summary
by MITRE
An integer overflow flaw which could lead to an out of bounds write was discovered in libssh2 before 1.8.1 in the way SSH_MSG_CHANNEL_REQUEST packets with an exit signal are parsed. A remote attacker who compromises a SSH server may be able to execute code on the client system when a user connects to the server.
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Analysis
by VulDB Data Team • 08/07/2023
The vulnerability identified as CVE-2019-3857 represents a critical integer overflow flaw within the libssh2 library version 1.8.0 and earlier, specifically affecting the parsing of SSH_MSG_CHANNEL_REQUEST packets containing exit signal information. This flaw exists in the client-side implementation of the SSH protocol library that is widely used across numerous applications and systems for establishing secure remote connections. The vulnerability stems from inadequate input validation and arithmetic handling when processing certain SSH protocol messages, creating a scenario where maliciously crafted packets can trigger unexpected behavior in the affected software components.
The technical exploitation of this vulnerability occurs through a precise manipulation of integer arithmetic during packet parsing operations. When a client processes an SSH_MSG_CHANNEL_REQUEST packet that contains an exit signal, the libssh2 library performs calculations to determine buffer sizes and memory allocation requirements. An integer overflow occurs when the calculation exceeds the maximum value that can be represented by the integer type, causing the result to wrap around to a much smaller value. This overflowed value then gets used to determine memory allocation or buffer boundaries, leading to a situation where subsequent memory writes occur beyond the intended buffer limits, resulting in an out-of-bounds write condition.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates a remote code execution vector that allows attackers to compromise client systems. When a victim user connects to a malicious SSH server, the attacker can craft a specially formatted exit signal packet that triggers the integer overflow condition. This condition enables the attacker to overwrite adjacent memory locations with controlled data, potentially allowing arbitrary code execution on the victim's system with the privileges of the user running the affected SSH client application. The vulnerability affects any software that relies on libssh2 for SSH functionality, including but not limited to SSH clients, VPN applications, and various network management tools.
The vulnerability aligns with CWE-190, Integer Overflow or Wraparound, which describes situations where integer arithmetic operations produce results that exceed the maximum value that can be represented by the data type. From an adversarial perspective, this flaw maps to several ATT&CK techniques including T1059 Command and Scripting Interpreter and T1078 Valid Accounts, as successful exploitation would allow attackers to execute arbitrary commands on compromised systems and potentially maintain persistent access. The vulnerability also relates to T1566 Phishing, as attackers might use social engineering to trick users into connecting to malicious SSH servers. Organizations should prioritize patching affected systems and implementing network monitoring to detect suspicious SSH traffic patterns that might indicate exploitation attempts.
Mitigation strategies should focus on immediate patch application to libssh2 version 1.8.1 or later, which includes proper integer overflow checks and bounds validation in the packet parsing routines. Additionally, network administrators should implement SSH protocol monitoring and anomaly detection to identify potentially malicious exit signal packets. System hardening measures including restricted user privileges for SSH client applications and network segmentation can help limit the potential impact of successful exploitation. Regular security assessments of systems using libssh2 should include vulnerability scanning and penetration testing to identify any remaining exposure risks. Organizations should also consider implementing SSH key management best practices and alternative authentication methods to reduce the attack surface associated with SSH connections.