Microsoft said it has steadily worked over the past decade to deprecate RC4, but that the task wasn’t easy.

No salt, no iteration? Really?

“The problem though is that it’s hard to kill off a cryptographic algorithm that is present in every OS that’s shipped for the last 25 years and was the default algorithm for so long, Steve Syfuhs, who runs Microsoft’s Windows Authentication team, wrote on Bluesky. “See,” he continued, “the problem is not that the algorithm exists. The problem is how the algorithm is chosen, and the rules governing that spanned 20 years of code changes.”

Over those two decades, developers discovered a raft of critical RC4 vulnerabilities that required “surgical” fixes. Microsoft considered deprecating RC4 by this year, but ultimately “punted” after discovering vulnerabilities that required still more fixes. During that time Microsoft introduced some “minor improvements” that favored the use of AES, and as a result, usage dropped by “orders of magnitude.”

“Within a year we had observed RC4 usage drop to basically nil. This is not a bad thing and in fact gave us a lot more flexibility to kill it outright because we knew it genuinely wasn’t going to break folks, because folks weren’t using it.”

Syfuhs went on to document additional challenges Microsoft encountered and the approach it took to solving them.

While RC4 has known cipher weaknesses that make it insecure, Kerberoasting exploits a separate weakness. As implemented in Active Directory authentication, it uses no cryptographic salt and a single round of the MD4 hashing function. Salt is a technique that adds random input to each password before it is hashed. That requires hackers to invest considerable time and resources into cracking the hash. MD4, meanwhile, is a fast algorithm that requires modest resources. Microsoft’s implementation of AES-SHA1 is much slower and iterates the hash to further slow down cracking efforts. Taken together, AES-Sha1-hashed passwords require about 1,000 times the time and resources to be cracked.

Windows admins would do well to audit their networks for any usage of RC4. Given its wide adoption and continued use industry-wide, it may still be active, much to the surprise and chagrin of those charged with defending against hackers.