Firefox before 1.0.1 and Mozilla before 1.7.6 allows remote malicious web sites to overwrite arbitrary files by tricking the user into downloading a .LNK (link) file twice, which overwrites the file that was referenced in the first .LNK file.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A100037
https://euvd.enisa.europa.eu/vulnerability/EUVD-2005-0588
http://www.securityfocus.com/bid/12659
http://www.novell.com/linux/security/advisories/2006_04_25.html