Widely-Used PuTTY SSH Client Found Vulnerable to Key Recovery Attack
Vulnerabilities mentionedAll →
| CVE | Vulnerability | CVSS | EPSS | Flags | Affected | Exposure | Published |
|---|---|---|---|---|---|---|---|
| CVE-2024-31497 | In PuTTY 0.68 through 0.80 before 0.81, biased ECDSA nonce generation allows an attacker to recover a user's NIST P-521 secret key via a quick attack in approxi In PuTTY 0.68 through 0.80 before 0.81, biased ECDSA nonce generation allows an attacker to recover a user's NIST P-521 secret key via a quick attack in approximately 60 signatures. This is especially important in a scenario where an adversary is able to read messages signed by PuTTY or Pageant. The required set of signed messages may be publicly readable because they are stored in a public Git service that supports use of SSH for commit signing, and the signatures were made by Pageant through an agent-forwarding mechanism. In other words, an adversary may already have enough signature information to compromise a victim's private key, even if there is no further use of vulnerable PuTTY versions. After a key compromise, an adversary may be able to conduct supply-chain attacks on software maintained in Git. A second, independent scenario is that the adversary is an operator of an SSH server to which the victim authenticates (for remote login or file copy), even though this server is not fully trusted by the victim, and the victim uses the same private key for SSH connections to other services operated by other entities. Here, the rogue server operator (who would otherwise have no way to determine the victim's private key) can derive the victim's private key, and then use it for unauthorized access to those other services. If the other services include Git services, then again it may be possible to conduct supply-chain attacks on software maintained in Git. This also affects, for example, FileZilla before 3.67.0, WinSCP before 6.3.3, TortoiseGit before 2.15.0.1, and TortoiseSVN through 1.14.6. NVD description · AI analysis pending | 5.9 | 6% |
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Full article467 words · extracted from thehackernews.com · click to collapse
Ravie LakshmananApr 16, 2024Encryption / Network Security
The maintainers of the PuTTY Secure Shell (SSH) and Telnet client are alerting users of a critical vulnerability impacting versions from 0.68 through 0.80 that could be exploited to achieve full recovery of NIST P-521 (ecdsa-sha2-nistp521) private keys.
The flaw has been assigned the CVE identifier CVE-2024-31497, with the discovery credited to researchers Fabian Bäumer and Marcus Brinkmann from the Ruhr University Bochum.
"The effect of the vulnerability is to compromise the private key," the PuTTY project said in an advisory.
"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."
However, in order to obtain the signatures, an attacker will have to compromise the server for which the key is used to authenticate to.
In a message posted on the Open Source Software Security (oss-sec) mailing list, Bäumer described the flaw as stemming from the generation of biased ECDSA cryptographic nonces, which could enable the recovery of the private key.
"The first 9 bits of each ECDSA nonce are zero," Bäumer explained. "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."
Besides impacting PuTTY, it also affects other products that incorporate a vulnerable version of the software -
- 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)
Following responsible disclosure, the issue has been addressed in PuTTY 0.81, FileZilla 3.67.0, WinSCP 6.3.3, and TortoiseGit 2.15.0.1. Users of TortoiseSVN are recommended to use Plink from the latest PuTTY 0.81 release when accessing an SVN repository via SSH until a patch becomes available.
Specifically, it has been resolved by switching to the RFC 6979 technique for all DSA and ECDSA key types, abandoning its earlier method of deriving the nonce using a deterministic approach that, while avoiding the need for a source of high-quality randomness, was susceptible to biased nonces when using P-521.
This older approach, PuTTY developers said, was devised at a time when Microsoft Windows lacked native support for a cryptographic random number generator.
ECDSA NIST-P521 keys used with any of the vulnerable components should be considered compromised and consequently revoked by removing them from ~/.ssh/authorized_keys files and their equivalents in other SSH servers.
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Text extracted automatically; images, tables and formatting may be missing. Original: https://thehackernews.com/2024/04/widely-used-putty-ssh-client-found.html