Why Are MD5 and SHA-1 Considered Insecure for Most Modern Applications?

MD5 and SHA-1 are considered insecure primarily because practical collision attacks have been demonstrated against them. For MD5, collisions can be found in seconds.

For SHA-1, a chosen-prefix collision attack has been proven feasible with significant but manageable resources. This failure in collision resistance means they cannot be reliably used for digital signatures or integrity checks, as an attacker could forge documents with the same hash.