An anonymous reader writes: Flickr offers a fairly comprehensive web-service API that allows programmers to create applications that can perform almost any function a user on the Flickr site can do. Users should be authenticated using the Flickr Authentication API. Any applications wishing to use the Flickr Authentication API must have already obtained a Flickr's API Key. An 8-byte long 'shared secret' for the API Key is then issued by Flickr and cannot be changed by the users. This secret is used in the signing process, which is required for all API calls using an authentication token. This advisory describes a vulnerability in the signing process that allows an attacker to generate valid signatures without knowing the shared secret. By exploiting this vulnerability, an attacker can send valid arbitrary requests on behalf of any application using Flickr's API.