CVE-2023-48795 is a vulnerability in the SSH transport protocol as implemented in OpenSSH versions prior to 9.6 and a wide array of other SSH clients, servers, and libraries. The root cause lies in the SSH Binary Packet Protocol (BPP) extensions mishandling the handshake phase and sequence numbers. During the SSH handshake, extension negotiation messages are exchanged to establish security parameters. Due to improper handling, some packets can be omitted without detection, effectively bypassing integrity checks. This flaw enables an attacker to downgrade or disable certain security features on the connection, such as the use of ChaCha20-Poly1305 and CBC with Encrypt-then-MAC (EtM) algorithms. The attack, known as the Terrapin attack, exploits the fact that the protocol does not properly verify the completeness and order of extension negotiation packets, allowing selective omission. Affected products include OpenSSH before 9.6, PuTTY before 0.80, Dropbear through 2022.83, libssh before 0.10.6, libssh2 through 1.11.0, Paramiko before 3.4.0, WinSCP before 6.2.2, and many others across different programming languages and platforms. The vulnerability does not require authentication or user interaction, making it remotely exploitable over the network. Although no public exploits are known yet, the broad impact surface and the critical role of SSH in secure communications make this a significant threat. The CVSS v3.1 base score is 5.9 (medium), reflecting network attack vector, high attack complexity, no privileges required, no user interaction, and impact limited to integrity compromise without confidentiality or availability loss. The CWE classification is CWE-354 (Improper Integrity Check).

For European organizations, this vulnerability poses a risk to the integrity of SSH connections used for remote administration, secure file transfers, and automated processes. Attackers exploiting this flaw could cause silent downgrades or disable security features, potentially allowing interception or modification of data in transit without detection. This undermines trust in SSH tunnels, which are foundational for securing critical infrastructure, cloud environments, and enterprise networks. Sectors such as finance, energy, telecommunications, and government agencies in Europe that rely heavily on SSH for secure remote access and management are particularly vulnerable. The integrity bypass could facilitate further attacks, including lateral movement or data manipulation, increasing the risk of espionage or sabotage. Although confidentiality and availability are not directly impacted, the integrity compromise can lead to significant operational and reputational damage. The medium severity score suggests a moderate but non-trivial risk, especially given the widespread use of affected SSH implementations across European IT environments.

1. Immediately inventory all SSH clients, servers, and libraries in use across the organization to identify affected versions. 2. Prioritize patching to the latest versions of OpenSSH (9.6 or later) and other affected products as soon as vendor updates become available. 3. For environments where immediate patching is not feasible, consider restricting SSH access to trusted networks and hosts using network segmentation and firewall rules to reduce exposure. 4. Monitor SSH traffic for anomalies that may indicate tampering or downgrade attempts, leveraging advanced network detection tools capable of inspecting SSH handshake behavior. 5. Enforce strict cryptographic policies that disable weaker or deprecated algorithms and prefer robust, well-vetted cipher suites. 6. Employ multi-factor authentication and robust logging to detect suspicious SSH session activities. 7. Engage with vendors and open-source communities to track patch releases and vulnerability disclosures. 8. Conduct penetration testing and red team exercises simulating this attack vector to validate defenses. 9. Educate system administrators and security teams about the specific nature of this vulnerability and the importance of timely updates. 10. Consider deploying SSH proxies or gateways that can enforce protocol compliance and integrity checks as an additional layer of defense.