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Date: Mon, 15 Apr 2024 21:15:22 +0200
From: Fabian Bäumer <fabian.baeumer@....de>
To: oss-security@...ts.openwall.com
Cc: Marcus Brinkmann <marcus.brinkmann@...r-uni-bochum.de>
Subject: CVE-2024-31497: Secret Key Recovery of NIST P-521 Private Keys
Through Biased ECDSA Nonces in PuTTY Client
### Summary
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. 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).
### Affected Products
- PuTTY 0.68 - 0.80
The following (not necessarily complete) list of products bundle an
affected PuTTY version and are therefore vulnerable as well:
- 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
### Impact
The nonce bias allows for full secret key recovery of NIST P-521 keys
after a malicious actor has seen roughly 60 valid ECDSA signatures
generated by any PuTTY component under the same key. Luckily, client
signatures are transmitted within the secure channel of SSH, requiring a
malicious server to acquire such signatures. If the key has been used to
sign arbitrary data (e.g., git commits by forwarding Pageant to a
development host), the publicly available signatures (e.g., on GitHub)
can be used as well.
All NIST P-521 client keys used with PuTTY must be considered
compromised, given that the attack can be carried out even after the
root cause has been fixed in the source code (assuming that ~60
pre-patch signatures are available to an adversary).
### Mitigations
This vulnerability has been fixed in PuTTY 0.81, FileZilla 3.67.0,
WinSCP 6.3.3, and TortoiseGit 2.15.0.1. Users of TortoiseSVN are advised
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.
ECDSA NIST-P521 keys used with any vulnerable product / component should
be considered compromised and consequently revoked by removing them from
authorized_keys, GitHub, ...
### CVE
This vulnerability has been assigned CVE-2024-31497.
--
M. Sc. Fabian Bäumer
Chair for Network and Data Security
Ruhr University Bochum
Universitätsstr. 150, Building MC 4/145
44780 Bochum
Germany
Download attachment "smime.p7s" of type "application/pkcs7-signature" (5977 bytes)
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