sha256crypt and sha512crypt through 0.6 allow attackers to cause a denial of service (CPU consumption) because the algorithm's runtime is proportional to the square of the length of the password.

References

• https://nvd.nist.gov/vuln/detail/CVE-2016-20013
• https://akkadia.org/drepper/SHA-crypt.txt
• https://pthree.org/2018/05/23/do-not-use-sha256crypt-sha512crypt-theyre-dangerous/
• https://twitter.com/solardiz/status/795601240151457793