CVE-2019-3860 in Libssh2
Summary
by MITRE
An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SFTP packets with empty payloads 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 • 01/15/2025
The vulnerability identified as CVE-2019-3860 represents a critical out-of-bounds read flaw within the libssh2 library version 1.8.0 and earlier, which is widely utilized for implementing secure shell communications in numerous applications and systems. This issue specifically manifests during the parsing of SFTP (SSH File Transfer Protocol) packets that contain empty payloads, creating a scenario where memory access violations can occur. The flaw stems from inadequate input validation and boundary checking within the SFTP packet processing mechanism, which is a fundamental component of the SSH protocol implementation. The vulnerability is particularly concerning because it can be exploited by remote attackers who have compromised a SSH server, effectively allowing them to manipulate the memory access patterns of client applications that utilize libssh2 for secure communications. This type of vulnerability falls under the CWE-125 Out-of-Bounds Read classification, which is categorized as a common weakness in software development practices related to memory safety and input validation. The attack vector leverages the SFTP protocol's handling of malformed packets, where the absence of proper payload validation creates an opportunity for attackers to cause unpredictable behavior in the target system.
The technical exploitation of CVE-2019-3860 occurs when a malicious SSH server sends specially crafted SFTP packets with empty payloads to a vulnerable client application using libssh2. During the parsing process, the library fails to properly validate the length of the payload before attempting to read from memory locations that correspond to the expected packet contents. This results in the client application attempting to access memory beyond the allocated bounds, which can lead to either a denial of service condition where the application crashes or more insidiously, allows for potential information disclosure through memory read operations. The vulnerability is particularly dangerous because it operates at the protocol parsing layer, meaning that any application relying on libssh2 for SFTP functionality becomes susceptible to this attack. The flaw demonstrates a classic memory safety issue where buffer boundaries are not properly enforced during packet processing, which aligns with the ATT&CK technique T1059.007 for Command and Scripting Interpreter and T1499.004 for Endpoint Denial of Service. The impact extends beyond simple service disruption to potentially exposing sensitive information stored in memory, as attackers can manipulate the out-of-bounds read to retrieve data from adjacent memory locations.
The operational impact of CVE-2019-3860 is significant across various network infrastructure components and enterprise applications that depend on SSH and SFTP communications. Systems utilizing libssh2 for file transfer operations, remote administration, and secure data exchange become vulnerable to both availability and confidentiality attacks. When exploited, this vulnerability can cause widespread service disruption across organizations that rely on SSH-based file transfers, potentially affecting critical business operations and data processing workflows. The vulnerability affects not only direct SSH clients but also applications that incorporate libssh2 as a library component, including various network management tools, backup systems, and automated deployment frameworks. Organizations using older versions of libssh2 may experience unexpected application crashes, leading to service interruptions that can cascade through dependent systems. The attack can be executed remotely without requiring authentication to the target system, making it particularly dangerous in environments where SSH servers might be compromised or where attackers can manipulate network traffic. From a threat modeling perspective, this vulnerability represents a medium to high severity risk that aligns with the ATT&CK tactic TA0040 for Defense Evasion and TA0006 for Credential Access, as it can potentially be used as a stepping stone for further exploitation.
Mitigation strategies for CVE-2019-3860 focus primarily on upgrading the libssh2 library to version 1.8.1 or later, which contains the necessary patches to address the out-of-bounds read vulnerability. Organizations should conduct comprehensive inventory assessments to identify all systems and applications utilizing vulnerable versions of libssh2, including both direct usage and indirect dependencies through other software packages. The patching process should be prioritized based on the criticality of affected systems and the potential impact of exploitation. Additional defensive measures include implementing network segmentation to limit exposure of SSH services, configuring firewalls to restrict SSH access to trusted networks, and monitoring for suspicious SFTP packet patterns that may indicate exploitation attempts. Security teams should also consider implementing intrusion detection systems capable of identifying malformed SFTP packets and establishing incident response procedures for handling potential exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date cryptographic libraries and implementing robust software supply chain security practices. Organizations should also consider implementing application whitelisting and mandatory code review processes for third-party libraries to prevent similar vulnerabilities from being introduced into production environments. Regular vulnerability assessments and penetration testing should be conducted to identify potential attack vectors and ensure that security controls remain effective against evolving threats.