This vulnerability arises when a server using CPython's SSL module assigns a new SSLContext in the sni_callback without maintaining a reference to the original SSLContext. If the original SSLContext is freed while still in use, a remote TLS client can trigger a crash or use-after-free condition. Servers that create or replace SSLContext objects per connection are vulnerable, whereas those wrapping their listening socket with a persistent SSLContext are not affected.

A remote, unauthenticated attacker can cause a denial of service by crashing the server or potentially execute code through use-after-free of SSLContext objects. The vulnerability does not affect TLS clients.

Keep a reference to every SSLContext that sets sni_callback for the lifetime of the server to prevent the original SSLContext from being freed prematurely. This mitigation is effective and no official patch is indicated in the provided data. TLS clients require no action.