It was reported  that vncviewer could prompt for, and send, authentication credentials to a remote server without first properly validating the X.509 certificate. This could allow a malicious server to obtain a client's credentials because the client does not indicate to the user that a certificate is bad or missing.
A proposed patch  is being discussed.
This flaw does not affect older versions of tigervnc due to X.509 certificate handling being a new feature (new to the forthcoming 1.1.0):
Created tigervnc tracking bugs for this issue
Affects: fedora-15 [bug 702672]
This has been assigned the name CVE-2011-1775.
Confirmed Fedora 14 is not vulnerable, they ship 1.0.90 which is labeled "1.0.90 (1.1beta1)" on tigervnc download sit, but the 1.0.90 they ship is older (svn4123) and does not have the X509 support.
This issue has been addressed in following products:
Red Hat Enterprise Linux 6
Via RHSA-2011:0871 https://rhn.redhat.com/errata/RHSA-2011-0871.html