musl’s crypt() returns the string * as an error indicator when the key is over 256 bytes long. * is also exactly what the stored field contains for every account with no password, so the “hash” matches and the login succeeds.

Does anyone know why musl does this? Are they the only C library that behaves this way?

For a C library that famously tries to adhere to POSIX as much as possible, this seems like a crazy choice. From POSIX standard:

Upon successful completion, crypt() shall return a pointer to the encoded string. The first two bytes of the returned value shall be those of the salt argument. Otherwise, it shall return a null pointer and set errno to indicate the error.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/c...