Missing Check for Certificate Revocation after Initial Check

Description

If the revocation status of a certificate is not checked before each action that requires privileges, the system may be subject to a race condition. If a certificate is revoked after the initial check, all subsequent actions taken with the owner of the revoked certificate will lose all benefits guaranteed by the certificate. In fact, it is almost certain that the use of a revoked certificate indicates malicious activity.

Demonstrations

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

The following code checks a certificate before performing an action.

if (cert = SSL_get_peer_certificate(ssl)) {

  foo=SSL_get_verify_result(ssl);
  if (X509_V_OK==foo)


    //do stuff
    foo=SSL_get_verify_result(ssl);
    //do more stuff without the check.

While the code performs the certificate verification before each action, it does not check the result of the verification after the initial attempt. The certificate may have been revoked in the time between the privileged actions.

See Also