CVE-2015-3146 in libssh
Summary
by MITRE
The (1) SSH_MSG_NEWKEYS and (2) SSH_MSG_KEXDH_REPLY packet handlers in package_cb.c in libssh before 0.6.5 do not properly validate state, which allows remote attackers to cause a denial of service (NULL pointer dereference and crash) via a crafted SSH packet.
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Analysis
by VulDB Data Team • 07/24/2022
The vulnerability identified as CVE-2015-3146 resides within the libssh library version 0.6.4 and earlier, specifically affecting the SSH_MSG_NEWKEYS and SSH_MSG_KEXDH_REPLY packet handlers located in the package_cb.c file. This flaw represents a critical state validation failure that exposes the library to remote exploitation scenarios. The vulnerability stems from insufficient input validation mechanisms within the cryptographic key exchange process, which is fundamental to secure SSH communications. When processing specially crafted SSH packets, the library fails to properly verify the operational state of the SSH connection, creating a pathway for malicious actors to manipulate the protocol flow.
The technical implementation of this vulnerability involves a NULL pointer dereference condition that occurs during the handling of key exchange messages. During the SSH key exchange process, the library expects specific state transitions and packet sequences to maintain connection integrity. However, the absence of proper state validation in the packet handlers allows attackers to send malformed packets that trigger unexpected behavior in the connection state machine. The SSH_MSG_NEWKEYS packet is typically used to signal that both parties have completed the key exchange and are ready to switch to the new encryption keys, while SSH_MSG_KEXDH_REPLY contains the Diffie-Hellman key exchange response. When these packets are crafted with malicious payloads, they can cause the library to attempt to access null pointers, leading to immediate process termination and system crash.
From an operational impact perspective, this vulnerability enables remote attackers to execute denial of service attacks against systems running vulnerable versions of libssh. The crash occurs during the critical key exchange phase of SSH connections, effectively preventing legitimate users from establishing secure connections to affected systems. This vulnerability particularly impacts SSH servers and clients that rely on libssh for their cryptographic operations, potentially affecting a wide range of network infrastructure components including routers, switches, firewalls, and server applications. The vulnerability can be exploited without authentication requirements, making it particularly dangerous as attackers can trigger the denial of service condition simply by connecting to the target system and sending crafted packets. The impact extends beyond individual system compromise to potentially affecting entire network services that depend on SSH connectivity.
The vulnerability aligns with CWE-476 which identifies NULL pointer dereference as a common software weakness, and it maps to ATT&CK technique T1499.004 which covers network denial of service attacks. Organizations using vulnerable libssh implementations should prioritize immediate patching to address this issue. The recommended mitigation strategy involves upgrading to libssh version 0.6.5 or later, where proper state validation has been implemented in the packet handling routines. System administrators should also consider implementing network-level protections such as rate limiting and connection monitoring to detect and prevent exploitation attempts. Additionally, organizations should conduct comprehensive vulnerability assessments to identify all systems using vulnerable libssh versions and ensure proper patch management procedures are in place to prevent similar issues in the future. The vulnerability demonstrates the critical importance of proper state management in cryptographic libraries and highlights the potential for seemingly minor implementation flaws to result in significant service disruption.