crypt() function potential insecurities with invalid salts

So, yesterday I discovered quite a serious vulnerability in an application using the crypt() function (it was in perl, but perl just calls through to the system library so this application design flaw may be found in anything using the crypt() function for authentication).

Firstly why use crypt() at all? It’s well known that the DES-style crypt is very weak, for example the salt is only 2 characters and it only takes the first 8 characters of the password and ignores everything after that. However the modern glibc implementation of crypt() include a number of very secure hashing functions prefixed with $, particularly $6$ which is hashed SHA-512 and I’d advise everyone to use.

Anyway, back to the issue at hand. Your standard crypt() password check would look like this (taken from perl’s Catalyst::Authentication::Credential::Password module):

ie you use as the salt the encrypted version of the password to encrypt the user specified password, and then check that against the crypted password itself. If they match it means the password was the same.

However in this application for certain pre-created accounts or accounts that had only been logged in to using an oauth mechanism (eg facebook login) the user’s password field was an empty string. This seems reasonable enough – crypt() of a blank password should always return something non-blank (unless you’re using an older version of mysql that has a crypt() inconsistency that I reported 2 years ago). eg

Unless that is, that you specify an invalid (or blank) salt:

D’oh. This basically means that if you are using a blank password column to specify no password login allowed in reality someone can log in with ANY password! So in the case of the app in question if you knew the registered email address of someone who had a precreated (but locked out) account, or the address that someone who logged in using oauth you could do a login with any password. I’m guessing that because this is not particularly widely known amongst developers there are probably a number of apps where this is possible today but no-one has tested it.

Some workarounds/mitigations:

  • Set your database password field to default to something that is non-blank (eg ‘a’) – even if crypt() classes the salt as invalid it will return blank which will not match this field.
  • Use something that overrides crypt() to auto-generate a salt if none is specified (eg Crypt::Password::Util’s crypt function). This won’t cover you on the case where the specified salt is invalid and so crypt() just returns a blank.
  • Assert that the user’s password field is not blank before allowing login (but you need to make sure you do this everywhere in your application).
  • For language designers: Either make sure that your crypt() function doesn’t ever return blank (throws an error on invalid output for example), OR that it automatically generates a valid salt regardless of whether or not a salt is specified (including the case where a salt is specified but is invalid).

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