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1.6.3 Ensure system wide crypto policy disables sha1 hash and signature support

Information

SHA-1 (Secure Hash Algorithm) is a cryptographic hash function that produces a 160 bit hash value.

The SHA-1 hash function has an inherently weak design, and advancing cryptanalysis has made it vulnerable to attacks. The most significant danger for a hash algorithm is when a "collision" which happens when two different pieces of data produce the same hash value occurs. This hashing algorithm has been considered weak since 2005.

Note: The use of SHA-1 with hashbased message authentication codes (HMAC) do not rely on the collision resistance of the corresponding hash function, and therefore the recent attacks on SHA-1 have a significantly lower impact on the use of SHA-1 for HMAC. Because of this, the recommendation does not disable the hmac-sha1 MAC.

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.
 - 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-SHA1.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 inpmod and add or modify the following lines:

hash = -SHA1
sign = -*-SHA1
sha1_in_certs = 0

Example:

# printf '%s
' "# This is a subpolicy dropping the SHA1 hash and signature support" "hash = -SHA1" "sign = -*-SHA1" "sha1_in_certs = 0" >> /etc/crypto-policies/policies/modules/NO-SHA1.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

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

# reboot

See Also

https://workbench.cisecurity.org/benchmarks/18210

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: 5cabb677cfdb69d114558af09d60fa59429b6bad3c00d3d42536f3bd35c56f60