Password cracking experts have reversed a secret cryptographic formula recently added to Cisco devices. Ironically, the encryption type 4 algorithm leaves users considerably more susceptible to password cracking than an older alternative, even though the new routine was intended to enhance protections already in place.

It turns out that Cisco's new method for converting passwords into one-way hashes uses a single iteration of the SHA256 function with no cryptographic salt. The revelation came as a shock to many security experts because the technique requires little time and computing resources. As a result, relatively inexpensive computers used by crackers can try a dizzying number of guesses when attempting to guess the corresponding plain-text password. For instance, a system outfitted with two AMD Radeon 6990 graphics cards that run a soon-to-be-released version of the Hashcat password cracking program can cycle through more than 2.8 billion candidate passwords each second.

By contrast, the type 5 algorithm the new scheme was intended to replace used 1,000 iterations of the MD5 hash function. The large number of repetitions forces cracking programs to work more slowly and makes the process more costly to attackers. Even more important, the older function added randomly generated cryptographic "salt" to each password, preventing crackers from tackling large numbers of hashes at once.

"In my eyes, for such an important company, this is a big fail," Jens Steube, the creator of ocl-Hashcat-plus said of the discovery he and beta tester Philipp Schmidt made last week. "Nowadays everyone in the security/crypto/hash scene knows that password hashes should be salted, at least. By not salting the hashes we can crack all the hashes at once with full speed."

Cisco officials acknowledged the password weakness in an advisory published Monday. The bulletin didn't specify the specific Cisco products that use the new algorithm except to say that they ran "Cisco IOS and Cisco IOS XE releases based on the Cisco IOS 15 code base." It warned that devices that support Type 4 passwords lose the capacity to create more secure Type 5 passwords. It also said "backward compatibility problems may arise when downgrading from a device running" the latest version.

The advisory said that Type 4 protection was designed to use the Password-Based Key Derivation Function version 2 standard to SHA256 hash passwords 1,000 times. It was also designed to append a random 80-bit salt to each password.

"Due to an implementation issue, the Type 4 password algorithm does not use PBKDF2 and does not use a salt, but instead performs a single iteration of SHA256 over the user-provided plaintext password," the Cisco advisory stated. "This approach causes a Type 4 password to be less resilient to brute-force attacks than a Type 5 password of equivalent complexity."

The weakness threatens anyone whose router configuration data may be exposed in an online breach. Rather than store passwords in clear text, the algorithm is intended to store passwords as a one-way hash that can only be reversed by guessing the plaintext that generated it. The risk is exacerbated by the growing practice of including configuration data in online forums. Steube found the hash "luSeObEBqS7m7Ux97dU4qPfW4iArF8KZI2sQnuwGcoU" posted here and had little trouble cracking it. (Ars isn't publishing the password in case it's still being used to secure the Cisco gear.)

While Steube and Schmidt reversed the Type 4 scheme, word of the weakness they uncovered recently leaked into other password cracking forums. An e-mail posted on Saturday to a group dedicated to the John the Ripper password cracker, for instance, noted that the secret to the Type 4 password scheme "is it's base64 SHA256 with character set './0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz'." Armed with this knowledge, crackers have everything they need to crack hundreds of thousands, or even millions, of hashes in a matter of hours.

It's hard to fathom an implementation error of this magnitude being discovered only after the new hashing mechanism went live. The good news is that Cisco is openly disclosing the weakness early in its life cycle. Ars strongly recommends that users consider the pros and cons before upgrading their Cisco gear.