Interpretation conflict in phpBB 2.0.17, with remote avatars and avatar uploading enabled, allows remote authenticated users to inject arbitrary web script or HTML via an HTML file with a GIF or JPEG file extension, which causes the HTML to be executed by a victim who views the file in Internet Explorer, which renders malformed image types as HTML, enabling cross-site scripting (XSS) attacks.  NOTE: it could be argued that this vulnerability is due to a design flaw in Internet Explorer (CVE-2005-3312) and the proper fix should be in that browser; if so, then this should not be treated as a vulnerability in phpBB.