  Web password recovery, enabling a user who forgets their password to
re-establish a shared secret with a website, is very widely implemented.
However, use of such a fall-back system brings with it additional
vulnerabilities to user authentication. This paper provides a framework within
which such systems can be analysed systematically, and uses this to help gain a
better understanding of how such systems are best implemented. To this end, a
model for web password recovery is given, and existing techniques are
documented and analysed within the context of this model. This leads naturally
to a set of recommendations governing how such systems should be implemented to
maximise security. A range of issues for further research are also highlighted.
