CVE-2024-31497 is a cryptographic vulnerability found in PuTTY versions 0.68 through 0.80 prior to 0.81, as well as in related software such as FileZilla, WinSCP, TortoiseGit, and TortoiseSVN. The core issue lies in biased nonce generation during ECDSA signing using the NIST P-521 curve. ECDSA requires a fresh, uniformly random nonce for each signature; bias or reuse of this nonce can leak the private key. In this case, the nonce bias allows an attacker to recover the victim's private key after collecting approximately 60 signatures. The attack scenarios include: (1) an adversary accessing signed messages publicly available in Git repositories where commit signing is used, especially when signatures are generated via Pageant with agent forwarding; (2) a malicious SSH server operator to which the victim connects using the same private key, enabling the attacker to derive the private key from signatures generated during authentication. Once the private key is compromised, the attacker can impersonate the victim, gain unauthorized access to other services using the same key, and potentially conduct supply-chain attacks by injecting malicious code into software repositories. The vulnerability is classified under CWE-338 (Use of Cryptographically Weak Pseudo-Random Number Generator) and has a CVSS v3.1 score of 5.9, indicating medium severity. The attack vector is network-based with high attack complexity, no privileges or user interaction required, and impacts confidentiality but not integrity or availability. The vulnerability affects a broad range of widely used SSH and file transfer clients, making it a significant risk for environments relying on these tools for secure communications and code signing.

For European organizations, the impact of CVE-2024-31497 is considerable due to the widespread use of PuTTY and related clients in enterprise environments for SSH access, file transfers, and Git commit signing. The ability to recover private keys compromises the confidentiality of authentication credentials and code signing keys, enabling attackers to impersonate users and inject malicious code into software supply chains. This can lead to unauthorized access to critical infrastructure, intellectual property theft, and disruption of software development pipelines. Organizations with public or semi-public Git repositories are particularly vulnerable, as attackers may harvest signatures without direct network access. The risk is amplified in environments where private keys are reused across multiple services, increasing the attack surface. Supply-chain attacks facilitated by this vulnerability could have cascading effects on software integrity and trustworthiness across European industries, including finance, manufacturing, and government sectors. Additionally, the involvement of SSH agent forwarding in the attack vector highlights risks in remote work and cloud-based development workflows common in Europe. Although the CVSS score is medium, the potential for key compromise and subsequent unauthorized access elevates the threat's seriousness for organizations relying heavily on these tools.

To mitigate CVE-2024-31497, European organizations should immediately update all affected software to versions that have patched the biased nonce generation issue: PuTTY 0.81 or later, FileZilla 3.67.0 or later, WinSCP 6.3.3 or later, TortoiseGit 2.15.0.1 or later, and TortoiseSVN versions beyond 1.14.6. Organizations should audit their use of NIST P-521 ECDSA keys, especially those used for SSH authentication and Git commit signing, and consider generating new keys with secure nonce generation. Avoid reusing the same private key across multiple services to limit the impact of a potential compromise. Disable SSH agent forwarding where not strictly necessary, or restrict it to trusted servers only, to reduce exposure to rogue SSH servers. Review and restrict access to signed messages stored in public or semi-public repositories to minimize signature leakage. Implement monitoring for unusual SSH authentication patterns and unauthorized Git commits that could indicate key compromise. Educate developers and system administrators about the risks of key reuse and the importance of timely patching. Finally, consider adopting alternative cryptographic algorithms or key types less susceptible to nonce-related vulnerabilities, such as Ed25519, where feasible.