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1.6.5 Ensure system wide crypto policy disables cbc for ssh

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Information

Cypher Block Chaining (CBC) is an algorithm that uses a block cipher.

A vulnerability exists in SSH messages that employ CBC mode that may allow an attacker to recover plaintext from a block of ciphertext. If exploited, this attack can potentially allow an attacker to recover up to 32 bits of plaintext from an arbitrary block of ciphertext from a connection secured using the SSH protocol.

Solution

Note:

 - The commands below are written for the included DEFAULT system-wide crypto policy. If another policy is in use and follows local site policy, replace DEFAULT with the name of your system-wide crypto policy.
 - CBC can be turned off globally by using the argument cipher opposed to cipher@SSH
 - Multiple subpolicies may be assigned to a policy as a colon separated list. e.g. DEFAULT:NO-SHA1:NO-SSHCBC
 - Subpolicies:

 - Not included in the update-crypto-policies --set command will not be applied to the system wide crypto policy.
 - must exist before they can be applied to the system wide crypto policy.
 - .pmod file filenames must be in all upper case, upper case, e.g. NO-SSHCBC.pmod, or they will not be read by the update-crypto-policies --set command.
Create or edit a file in /etc/crypto-policies/policies/modules/ ending in .pmod and add or modify one of the the following lines:

cipher@SSH = -*-CBC # Disables the CBC cipher for SSH

Example:

# printf '%s\n' "# This is a subpolicy to disable all CBC mode ciphers" "# for the SSH protocol (libssh and OpenSSH)" "cipher@SSH = -*-CBC" >> /etc/crypto-policies/policies/modules/NO-SSHCBC.pmod

Run the following command to update the system-wide cryptographic policy

# update-crypto-policies --set <CRYPTO_POLICY>:<CRYPTO_SUBPOLICY1>:<CRYPTO_SUBPOLICY2>:<CRYPTO_SUBPOLICY3>

Example:

update-crypto-policies --set DEFAULT:NO-SHA1:NO-WEAKMAC:NO-SSHCBC

Run the following command to reboot the system to make your cryptographic settings effective for already running services and applications:

# reboot

Impact:

CBC ciphers might be the only common cyphers when connecting to older SSH clients and servers

See Also

https://workbench.cisecurity.org/benchmarks/18211

Item Details

References: CSCv7|14.4

Plugin: Unix

Control ID: 85de8d9265a82b1e6bb22718e9a7ae9ed2d0d470360311ad2ab19ec4ceeda5ee