Pierluigi Paganini April 16, 2024

The PuTTY Secure Shell (SSH) and Telnet client are impacted by a critical vulnerability that could be exploited to recover private keys.

PuTTY tools from 0.68 to 0.80 inclusive are affected by a critical vulnerability, tracked as CVE-2024-31497, that resides in the code that generates signatures from ECDSA private keys which use the NIST P521 curve.

An attacker can exploit the vulnerability to recover NIST P-521 private keys.

“The effect of the vulnerability is to compromise the private key. An attacker in possession of a few dozen signed messages and the public key has enough information to recover the private key, and then forge signatures as if they were from you, allowing them to (for instance) log in to any servers you use that key for.” reads the advisory. “To obtain these signatures, an attacker need only briefly compromise any server you use the key to authenticate to, or momentarily gain access to a copy of Pageant holding the key. (However, these signatures are not exposed to passive eavesdroppers of SSH connections.)”

The vulnerability was discovered by researchers Fabian Bäumer and Marcus Brinkmann from the Ruhr University Bochum. Bäumer explained that the vulnerability stems from the generation of biased ECDSA cryptographic nonces, which could allow full secret key recovery.

“The PuTTY client and all related components generate heavily biased ECDSA nonces in the case of NIST P-521. To be more precise, the first 9 bits of each ECDSA nonce are zero. This allows for full secret key recovery in roughly 60 signatures by using state-of-the-art techniques. These signatures can either be harvested by a malicious server (man-in-the-middle attacks are not possible given that clients do not transmit their signature in the clear) or from any other source, e.g. signed git commits through forwarded agents.” Baumer explained. “The nonce generation for other curves is slightly biased as well. However, the bias is negligible and far from enough to perform lattice-based key recovery attacks (not considering cryptanalytical advancements).”

The following products include an affected PuTTY version and are therefore are also impacted by the flaw:

• FileZilla (3.24.1 – 3.66.5)
• WinSCP (5.9.5 – 6.3.2)
• TortoiseGit (2.4.0.2 – 2.15.0)
• TortoiseSVN (1.10.0 – 1.14.6)

The flaw has been fixed in PuTTY 0.81, FileZilla 3.67.0, WinSCP 6.3.3, and TortoiseGit 2.15.0.1. TortoiseSVN users are recommended to configure TortoiseSVN to use Plink from the latest PuTTY 0.81 release when accessing a SVN repository via SSH until a patch becomes available.

Any product or component using ECDSA NIST-P521 keys impacted by the flaw CVE-2024-31497 should be deemed compromised. These keys should be revoked by removing them from authorized_keys, GitHub repositories, and any other relevant platforms.

Pierluigi Paganini

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(SecurityAffairs – hacking, PuTTY Secure Shell (SSH))