6.2.11 Use Only Approved Ciphers in Counter Mode - Ciphers aes128-ctr,aes192-ctr,aes256-ctr

Information

This variable limits the types of ciphers that SSH can use during communication.

Rationale:

Based on research conducted at various institutions, it was determined that the symmetric portion of the SSH Transport Protocol (as described in RFC 4253) has security weaknesses that allowed recovery of up to 32 bits of plaintext from a block of ciphertext that was encrypted with the Cipher Block Chaining (CBC) method. From that research, new Counter mode algorithms (as described in RFC4344) were designed that are not vulnerable to these types of attacks and these algorithms are now recommended for standard use.

Solution

Edit the /etc/ssh/sshd_config file to set the parameter as follows:

Ciphers aes128-ctr,aes192-ctr,aes256-ctr

Default Value:

OS Default: No

See Also

https://workbench.cisecurity.org/files/3096

Item Details

Category: CONFIGURATION MANAGEMENT

References: 800-53|CM-7b., CSCv7|9.2

Plugin: Unix

Control ID: 4c3622248999c61f8450a246c035b0af08e2ed5fc439dcba0ff7b083a75c04f4