The official Twitter blog has published an article that describes the security threat, named Silhouette, and how they have addressed the issue on their service.
We have published a paper that describes the threat and possible countermeasures against it. We hope the blog article will let other social web developers be aware of the threat and take appropriate actions against it. The study was conducted by Mr. Takuya Watanabe who is now working at NTT while studying at Waseda University as a PhD student. The study is a joint work with the NTT Secure Platform Laboratories, NTT corporation.
The source of the image: Twitter official blog.