SHA-1 is a hash function that is two generations old, no longer considered secure for all uses and should only be used for backward compatibility.
SHA-1 is a cryptographic hash function that is no longer considered collision-resistant and should only be used for backward compatibility.
In 2006, NIST issued a policy stating that:
"Federal agencies should stop using SHA-1 ... as soon as practical, and must use the SHA-2 family of hash functions for these applications after 2010."
Since 2012, NIST recommends that new applications should use either sha-2 or (since 2015) the new sha-3 hash function instead of SHA-1.
In 2015, the first freestart collisions in SHA-1 were found.
In early 2017, Google found a SHA-1 collision in practice with two different PDF documents that have the same hash. See also: https://shattered.io/
