It's not secure for many applications. There are various practical collision attacks on it. However, to rewrite history you need a preimage attack. In other

It's not secure for many applications. There are various practical collision attacks on it. However, to rewrite history you need a preimage attack. In other words, you need to find a value X which hashes to the same hash value y which already exists h(X)=y These are not currently practical. So SHA1 is certainly not recommended, but isn't completely broken yet.