CWE-521: Weak Password Requirements

A product's design should require adherence to an appropriate password policy. Specific password requirements depend strongly on contextual factors, but it is recommended to contain the following attributes:

- Enforcement of a minimum and maximum length

- Restrictions against password reuse

- Restrictions against using common passwords

- Restrictions against using contextual string in the password (e.g., user id, app name)

Depending on the threat model, the password policy may include several additional attributes.

- Complex passwords requiring mixed character sets (alpha, numeric, special, mixed case)

- Increasing the range of characters makes the password harder to crack and may be appropriate for systems relying on single factor authentication.

- Unfortunately, a complex password may be difficult to memorize, encouraging a user to select a short password or to incorrectly manage the password (write it down).

- Another disadvantage of this approach is that it often does not result in a significant increase in overall password complexity due to people's predictable usage of various symbols.

- Large Minimum Length (encouraging passphrases instead of passwords)

- Increasing the number of characters makes the password harder to crack and may be appropriate for systems relying on single factor authentication.

- A disadvantage of this approach is that selecting a good passphrase is not easy and poor passwords can still be generated. Some prompting may be needed to encourage long un-predictable passwords.

- Randomly Chosen Secrets

- Generating a password for the user can help make sure that length and complexity requirements are met, and can result in secure passwords being used.

- A disadvantage of this approach is that the resulting password or passphrase may be too difficult to memorize, encouraging them to be written down. However, there is growing adoption of password managers that allow users to manage many passwords with varying complexity requirements.

See NIST 800-63B [REF-1053] for further information on password requirements.

Consider a second
authentication factor beyond the password, which prevents the
password from being a single point of failure. See CWE-308 for
further information.

Consider implementing a password complexity meter to inform users when a chosen password meets the required attributes.

Previously, "password expiration" was
widely advocated as a defense-in-depth approach to
minimize the risk of weak passwords, and it has become
a common practice. Password expiration requires a
password to be changed within a fixed time window (such
as every 90 days). However, this approach has
significant limitations in the current threat
landscape, and its utility has been reduced in light of
the adoption of related protection mechanisms (such as
password complexity and computational effort), along
with the recognition that regular password changes
often caused users to generate more predictable
passwords. As a result, this is now a Discouraged
Common Practice [REF-1488] [REF-1489], especially as
the sole factor in protecting passwords. It is still
strongly encouraged to force password changes in case
of evidence of compromise, but this is not the same as
a forced "expiration" on an arbitrary time
frame.