curl 7.61.0 through 7.76.1 suffers from exposure of data element to wrong session due to a mistake in the code for CURLOPTSSLCIPHERLIST when libcurl is built to use the Schannel TLS library. The selected cipher set was stored in a single "static" variable in the library, which has the surprising side-effect that if an application sets up multiple concurrent transfers, the last one that sets the ciphers will accidentally control the set used by all transfers. In a worst-case scenario, this weakens transport security significantly.

A use-after-free flaw was found in the way curl handled TLS session data. The curl versions using the OpenSSL library as their TLS backend could use freed memory after TLS session renegotiation was performed by the OpenSSL library. A malicious TLS server could use this flaw to crash or, possibly, execute arbitrary code with the privileges of a client application using the curl library.