CVE-2021-22901: Use After Free

A vulnerability was found in curl, where libcurl can be tricked into using already freed memory when a new TLS session is negotiated or a client certificate is requested on an existing connection. For example, this can happen when a TLS server requests a client certificate on a connection that was established without one. A malicious server can use this in rare unfortunate circumstances to potentially reach remote code execution in the client.

— Red Hat

curl 7.75.0 through 7.76.1 suffers from a use-after-free vulnerability resulting in already freed memory being used when a TLS 1.3 session ticket arrives over a connection. A malicious server can use this in rare unfortunate circumstances to potentially reach remote code execution in the client. When libcurl at run-time sets up support for TLS 1.3 session tickets on a connection using OpenSSL, it stores pointers to the transfer in-memory object for later retrieval when a session ticket arrives. If the connection is used by multiple transfers (like with a reused HTTP/1.1 connection or multiplexed HTTP/2 connection) that first transfer object might be freed before the new session is established on that connection and then the function will access a memory buffer that might be freed. When using that memory, libcurl might even call a function pointer in the object, making it possible for a remote code execution if the server could somehow manage to get crafted memory content into the correct place in memory.