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1.6.6 Ensure system wide crypto policy disables EtM for ssh

Information

Encrypt-then-MAC (EtM) - The ciphertext is generated by encrypting the plaintext and then appending a MAC of the encrypted plaintext

There is an effective attack against SSH's use of Cypher-Block-Chaining (CBC) with Encrypt-then-MAC as referenced in CVE-2023-48795

Solution

- IF - CVE-2023-48795 (the Terrapin attack) has been addressed - for example, the system runs OpenSSH 9.6 or later - and this meets local site policy, this recommendation may be skipped.

Create or edit a file in /etc/crypto-policies/policies/modules/ ending in .pmod and add or modify the following line:

etm@SSH = DISABLE_ETM # This disables EtM for openSSH and libssh

Example:

# printf '%s\n' "# This is a subpolicy to disable Encrypt then MAC" "# for the SSH protocol (libssh and OpenSSH)" "etm@SSH = DISABLE_ETM" >> /etc/crypto-policies/policies/modules/NO-SSHETM.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:NO-SSHCHACHA20:NO-SSHETM

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

# reboot

Impact:

Disabling Encrypt-then-MAC (EtM) for SSH may cause interoperability problems with clients or servers that only offer EtM MAC modes.

See Also

https://workbench.cisecurity.org/benchmarks/27890

Item Details

Category: ACCESS CONTROL, IDENTIFICATION AND AUTHENTICATION, SYSTEM AND COMMUNICATIONS PROTECTION

References: 800-53|AC-17(2), 800-53|IA-5, 800-53|IA-5(1), 800-53|SC-8, 800-53|SC-8(1), CSCv7|14.4

Plugin: Unix

Control ID: 565e5035abc553142419ff8524a63b767f230932334c8a2c896d17e31c307140