int ssl_do_client_cert_cb(SSL *s, X509 **px509, EVP_PKEY **ppkey)
{
    int i = 0;
#ifndef OPENSSL_NO_ENGINE
    if (s->ctx->client_cert_engine) {
        i = ENGINE_load_ssl_client_cert(s->ctx->client_cert_engine, s,
                                        SSL_get_client_CA_list(s),
                                        px509, ppkey, NULL, NULL, NULL);
        if (i != 0)
            return i;
    }
#endif
    if (s->ctx->client_cert_cb)
        i = s->ctx->client_cert_cb(s, px509, ppkey);
    return i;
}

This weakness pertains to the use of cryptographic functions that are weak, misconfigured, or outdated, which undermines the intended protection of encrypted data and communications.