Use-After-FreeMail: Generalizing the Use-After-Free Problem and Applying it to Email Services

Use-after-free is a type of vulnerability commonly present in software written in memory-unsafe languages like C or C++, where a program frees a memory buffer too early. By placing counterfeit structures at the freed memory location, an attacker can leak information or gain execution control upon subsequent access. In this paper, we show that the concept of use-after-free can be generalized to any environment and situation where resources can be silently exchanged. As an instance of our generalization we demonstrate Use-After-FreeMail attacks. Use-After-FreeMail attacks gather email addresses from publicly available database leaks. The fully automated quantitative analysis brought to light that 33.5% of all free-mail addresses we tested are not valid anymore. In two user studies with 100 and 31 participants we found that 11-19% of users are affected by our attack. In qualitative case studies we investigated what information can be gained in Use-After-FreeMail attacks, e.g., payment information, and how far currently used accounts can be compromised (identity theft). Finally, drawing the connection between mitigations against traditional use-after-free scenarios and the Use-After-FreeMail scenario, we provide a concise list of recommendations to free-mail providers and users as a protection against use-after-free attacks.