browserify-sign upper bound check issue in `dsaVerify` leads to a signature forgery attack

Summary

An upper bound check issue in dsaVerify function allows an attacker to construct signatures that can be successfully verified by any public key, thus leading to a signature forgery attack.

Details

In dsaVerify function, it checks whether the value of the signature is legal by calling function checkValue, namely, whether r and s are both in the interval [1, q - 1]. However, the second line of the checkValue function wrongly checks the upper bound of the passed parameters, since the value of b.cmp(q) can only be 0, 1 and -1, and it can never be greater than q.

In this way, although the values of s cannot be 0, an attacker can achieve the same effect as zero by setting its value to q, and then send (r, s) = (1, q) to pass the verification of any public key.

Impact

All places in this project that involve DSA verification of user-input signatures will be affected by this vulnerability.

Fix PR:

Since the temporary private fork was deleted, here's a webarchive of the PR discussion and diff pages: PR webarchive.zip