Disable default SSH algorithms

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Purpose

Network administrators may wish to disable certain algorithms (ciphers, macs, key exchanges) for their SSH traffic. 

Since the client selects the algorithms after a negotiation phase the only way to disable certain algorithms is to completely exclude them from the available algorithms list on the server side.

Solution

  • Find the available algorithm names by starting Stash with debug logging enabled and looking in the atlassian-stash.log file for messages of the form:
    Available SSH CIPHER: [aes128-ctr, arcfour128, aes128-cbc, 3des-cbc, blowfish-cbc]
    Available SSH KEY_EXCHANGE: [diffie-hellman-group-exchange-sha256, diffie-hellman-group-exchange-sha1, ecdh-sha2-nistp256, ecdh-sha2-nistp384, ecdh-sha2-nistp521, diffie-hellman-group14-sha1, diffie-hellman-group1-sha1]
    Available SSH MAC: [hmac-sha2-256, hmac-sha2-512, hmac-sha1, hmac-sha1-96, hmac-md5-96]
  • Add the algorithm names you wish to disable to the plugin.ssh.disabled.ciphers, plugin.ssh.disabled.key.exchanges, and plugin.ssh.disabled.macs properties (available in Stash 3.9+) as specified in Stash config properties, and restart Stash.
  • You can verify the algorithms used by executing a remote SSH command and reading the available and negotiated algorithms in the verbose output.
    • ssh -vvv -p <stash ssh port> git@<stash host> whoami 2>&1

    • Look for debug2: kex_parse_kexinit: for available algorithms and debug1: kex: server->client for negotiated result or execute:

    • ssh -vvv -p <stash ssh port> git@<stash host> whoami 2>&1 | grep kex_parse_kexinit

    • ssh -vvv -p <stash ssh port> git@<stash host> whoami 2>&1 | grep server-\>client

  • To enable additional stronger algorithms see List ciphers used by JVM to install Java Cryptography Extension (JCE) Unlimited Strength Jurisdiction Policy Files.

Last modified on Feb 26, 2016

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