During an X25519 key exchange, the client’s private is generated with System.Random:
var rnd = new Random();
_privateKey = new byte[MontgomeryCurve25519.PrivateKeySizeInBytes];
rnd.NextBytes(_privateKey);
Source: KeyExchangeECCurve25519.cs
Source commit: sshnet/SSH.NET@b58a11c
System.Random is not a cryptographically secure random number generator, it must therefore not be used for cryptographic purposes.
Impact
When establishing an SSH connection to a remote host, during the X25519 key exchange, the private key is generated with
a weak random number generator whose seed can be bruteforced. This allows an attacker able to eavesdrop the
communications to decrypt them.
Workarounds
To ensure you're not affected by this vulnerability, you can disable support for curve25519-sha256 and curve25519-sha256@libssh.org key exchange algorithms by invoking the following method before a connection is established:
private static void RemoveUnsecureKEX(BaseClient client)
{
client.ConnectionInfo.KeyExchangeAlgorithms.Remove("curve25519-sha256");
client.ConnectionInfo.KeyExchangeAlgorithms.Remove("curve25519-sha256@libssh.org");
}Thanks
This issue was initially reported by Siemens AG, Digital Industries, shortly followed by @yaumn-synacktiv.
References
yaumn-synacktiv Analyst
During an X25519 key exchange, the client’s private is generated with System.Random:
Source: KeyExchangeECCurve25519.cs
Source commit: sshnet/SSH.NET@b58a11c
System.Random is not a cryptographically secure random number generator, it must therefore not be used for cryptographic purposes.
Impact
When establishing an SSH connection to a remote host, during the X25519 key exchange, the private key is generated with
a weak random number generator whose seed can be bruteforced. This allows an attacker able to eavesdrop the
communications to decrypt them.
Workarounds
To ensure you're not affected by this vulnerability, you can disable support for
curve25519-sha256andcurve25519-sha256@libssh.orgkey exchange algorithms by invoking the following method before a connection is established:Thanks
This issue was initially reported by Siemens AG, Digital Industries, shortly followed by @yaumn-synacktiv.
References