Overly Restrictive Account Lockout Mechanism

The software contains an account lockout protection mechanism, but the mechanism is too restrictive and can be triggered too easily, which allows attackers to deny service to legitimate users by causing their accounts to be locked out.


Account lockout is a security feature often present in applications as a countermeasure to the brute force attack on the password based authentication mechanism of the system. After a certain number of failed login attempts, the users' account may be disabled for a certain period of time or until it is unlocked by an administrator. Other security events may also possibly trigger account lockout. However, an attacker may use this very security feature to deny service to legitimate system users. It is therefore important to ensure that the account lockout security mechanism is not overly restrictive.


The following examples help to illustrate the nature of this weakness and describe methods or techniques which can be used to mitigate the risk.

Note that the examples here are by no means exhaustive and any given weakness may have many subtle varieties, each of which may require different detection methods or runtime controls.

Example One

A famous example of this type of weakness being exploited is the eBay attack. eBay always displays the user id of the highest bidder. In the final minutes of the auction, one of the bidders could try to log in as the highest bidder three times. After three incorrect log in attempts, eBay password throttling would kick in and lock out the highest bidder's account for some time. An attacker could then make their own bid and their victim would not have a chance to place the counter bid because they would be locked out. Thus an attacker could win the auction.

See Also

Lockout Mechanism Errors

Weaknesses in this category are related to a software system's lockout mechanism. Frequently these deal with scenarios that take effect in case of multiple failed atte...

Authentication Errors

Weaknesses in this category are related to authentication components of a system. Frequently these deal with the ability to verify that an entity is indeed who it clai...

Lock Computer

Weaknesses in this category are related to the design and architecture of a system's lockout mechanism. Frequently these deal with scenarios that take effect in case o...

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