How to mitigate against SSH Vulnerability CVE-2023-48795 with Maverick Legacy Client

Posted on by Lee Painter

There are several options available to mitigate the risk of exposure to CVE-2023-48795 for Maverick Legacy Client-enabled applications

In code

Use the code below before making any call to SshConnector.connect.

SshConnector ssh = SshConnector.createInstance();
Ssh2Context ctx = ssh.getContext();
        
ctx.supportedCiphersCS().remove(Ssh2Context.CIPHER_CHACHA20_POLY1305);
ctx.supportedCiphersSC().remove(Ssh2Context.CIPHER_CHACHA20_POLY1305);
        
ctx.supportedMacsCS().remove(Ssh2Context.HMAC_SHA256_ETM);
ctx.supportedMacsCS().remove(Ssh2Context.HMAC_SHA512_ETM);
ctx.supportedMacsCS().remove(Ssh2Context.HMAC_SHA1_ETM);
ctx.supportedMacsCS().remove(Ssh2Context.HMAC_MD5_ETM);
        
ctx.supportedMacsSC().remove(Ssh2Context.HMAC_SHA256_ETM);
ctx.supportedMacsSC().remove(Ssh2Context.HMAC_SHA512_ETM);
ctx.supportedMacsSC().remove(Ssh2Context.HMAC_SHA1_ETM);
ctx.supportedMacsSC().remove(Ssh2Context.HMAC_MD5_ETM);

Using System Properties

You do not need to make any code changes if you have a sensitive deployment. By adding the JVM arguments below, you can disable the vulnerable algorithms using system properties.

-Ddisable.chacha20-poly1305@openssh.com -Dmaverick.enableETM=false

Using the Adaptive Configuration File

Another option to configure the client without making code changes is to use the Adaptive Configuration file.

With this file, like the OpenSSH ssh_config file, you can configure which algorithms will be used. Therefore, to mitigate against this vulnerability, you should remove the algorithms from the list below from the Cipher and Mac configuration.

Ciphers:
chacha20-poly1305@openssh.com

HMACs:
hmac-sha2-256-etm@openssh.com
hmac-sha2-512-etm@openssh.com
hmac-sha1-etm@openssh.com
hmac-md5-etm@openssh.com

To do this, create a file called maverick.cfg in the working directory of your application. Add the following lines, ensuring that if you change the algorithms in the example below, you DO NOT use any of the algorithms listed above. Using the file below will mitigate the risk if used unchanged.

Ciphers      aes256-ctr,aes256-gcm@openssh.com,aes128-gcm@openssh.com,aes192-ctr,aes128-ctr
Compressions none,zlib,zlib@openssh.com
Kex          diffie-hellman-group18-sha512,curve25519-sha256,curve25519-sha256@libssh.org,diffie-hellman-group17-sha512,diffie-hellman-group16-sha512,diffie-hellman-group15-sha512,diffie-hellman-group14-sha256,diffie-hellman-group-exchange-sha256,rsa2048-sha256,ecdh-sha2-nistp521,ecdh-sha2-nistp384,ecdh-sha2-nistp256
Macs         hmac-sha2-512,hmac-sha2-512-96,hmac-sha2-256,hmac-sha2-256-96
Publickeys   ssh-ed25519,rsa-sha2-512,rsa-sha2-256,x509v3-rsa2048-sha256,ecdsa-sha2-nistp521-cert-v01@openssh.com,ecdsa-sha2-nistp384-cert-v01@openssh.com,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp521,ecdsa-sha2-nistp384,ecdsa-sha2-nistp256