OpenSSL 1.0.2 introduced the "multiblock" performance improvement. This feature only applies on 64-bit x86 architecture platforms that support AES NI instructions. A defect in the implementation of "multiblock" can cause OpenSSL's internal write buffer to become incorrectly set to NULL when using non-blocking I/O. Typically, when the user application is using a socket BIO for writing, this will only result in a failed connection. However if some other BIO is used, then it is likely that a segmentation fault will be triggered, thus enabling a potential denial of service attack.

This issue affects OpenSSL version 1.0.2, and is fixed in version 1.0.2a.

Acknowledgements:

Red Hat would like to thank the OpenSSL project for reporting this issue. Upstream acknowledges Daniel Danner and Rainer Mueller as the original reporters.