> If you didn’t, here’s how the exploit works. The comparison operator, “!==”, is vulnerable to a timing attack. The string comparison compares the two keys one

> If you didn’t, here’s how the exploit works. The comparison operator, “!==”, is vulnerable to a timing attack. The string comparison compares the two keys one byte at a time, stopping as soon as it finds an offset where the two strings do not match. As a result, checkApiKey will take longer if the two keys start with the same bytes. It’s sort of like if the error message itself said “wrong key, but you got the first 2 bytes right!”. There was a similar vulnerability but using space instead of time on TOPS-10 on the DEC PDP-10. That was back in the day when passwords were stored in plain text. When the operating system had to check a password it just did a string compare of the string supplied by the caller and the plain text string. Crucially, it did not copy the caller's string into kernel memory before comparing. It compared the plain text in kernel memory with the caller string in user memory. The trick then to password cracking was to start by guessing first characters. To test a first character you place it at some memory address where the next address is invalid, and you install a handler for access faults. Then check the password, telling the kernel your guess is longer than 1 character. If you get an access fault you know that first character was right and the string comparison continued on into the invalid address. So try first characters until you get an access fault. Once you have the first character, repeat but now starting the guess 2 characters before the invalid memory, with the right first character and you guess for the second character, and tell the kernel the password is at least 3 characters. Same for the rest of the characters. (I don't remember if the kernel's string compare checked length first. If it did, add a first step above of determining the password length. Same idea...start near invalid memory, and guess lengths until you find the one that gives an access fault).