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Cisco has released a security bulletin warning that a move to strengthen passwords on some devices resulted in making passwords much easier to crack.

The vulnerability affects recently released versions of the IOS and IOS XE operating system that runs on Cisco routers and switches. The problem stems from Cisco's use of the Type 4 password algorithm in the operating system, and its failure to salt – add random bits to -- passwords or use PBKDF2, which randomizes passphrases and adds a salt, thus deriving a cryptographic key that's more difficult to crack.

"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 SHA-256 over the user-provided plaintext password," said the Cisco security warning. "This approach causes a Type 4 password to be less resilient to brute-force attacks than a Type 5 password of equivalent complexity."

The flaw affects only devices that have been upgraded to versions of IOS or IOS XE that support Type 4 passwords, and only affects those devices' "'enable secret ' and 'username secret ' commands," according to the security advisory. "No other Cisco IOS or IOS XE features use this algorithm to hash passwords or keys." Previous versions of IOS and IOS XE used the Type 5 algorithm.

Some caveats about the vulnerability: Cisco said that Type 4 passwords must have been purposefully generated, because no preexisting passwords on its devices would have been automatically updated. "There is no automatic conversion of existing Type 5 passwords to Type 4 passwords after upgrading a device to a Cisco IOS or Cisco IOS XE release that has support for Type 4 passwords," said Cisco.