Code: int ssl3_get_new_session_ticket(SSL *s)
{
    int ok, al, ret = 0, ticklen;
    long n;
    const unsigned char *p;
    unsigned char *d;

    n = s->method->ssl_get_message(s,
                                   SSL3_ST_CR_SESSION_TICKET_A,
                                   SSL3_ST_CR_SESSION_TICKET_B,
                                   SSL3_MT_NEWSESSION_TICKET, 16384, &ok);

    if (!ok)
        return ((int)n);

    if (n < 6) {
        /* need at least ticket_lifetime_hint + ticket length */
        al = SSL_AD_DECODE_ERROR;
        SSLerr(SSL_F_SSL3_GET_NEW_SESSION_TICKET, SSL_R_LENGTH_MISMATCH);
        goto f_err;
     }
 
     p = d = (unsigned char *)s->init_msg;
     n2l(p, s->session->tlsext_tick_lifetime_hint);
     n2s(p, ticklen);
     /* ticket_lifetime_hint + ticket_length + ticket */
    if (ticklen + 6 != n) {
        al = SSL_AD_DECODE_ERROR;
        SSLerr(SSL_F_SSL3_GET_NEW_SESSION_TICKET, SSL_R_LENGTH_MISMATCH);
        goto f_err;
    }
    OPENSSL_free(s->session->tlsext_tick);
    s->session->tlsext_ticklen = 0;
    s->session->tlsext_tick = OPENSSL_malloc(ticklen);
    if (!s->session->tlsext_tick) {
        SSLerr(SSL_F_SSL3_GET_NEW_SESSION_TICKET, ERR_R_MALLOC_FAILURE);
        goto err;
    }
    memcpy(s->session->tlsext_tick, p, ticklen);
    s->session->tlsext_ticklen = ticklen;
    /*
     * There are two ways to detect a resumed ticket session. One is to set
     * an appropriate session ID and then the server must return a match in
     * ServerHello. This allows the normal client session ID matching to work
     * and we know much earlier that the ticket has been accepted. The
     * other way is to set zero length session ID when the ticket is
     * presented and rely on the handshake to determine session resumption.
     * We choose the former approach because this fits in with assumptions
     * elsewhere in OpenSSL. The session ID is set to the SHA256 (or SHA1 is
     * SHA256 is disabled) hash of the ticket.
     */
    EVP_Digest(p, ticklen,
               s->session->session_id, &s->session->session_id_length,
               EVP_sha256(), NULL);
    ret = 1;
    return (ret);
 f_err:
    ssl3_send_alert(s, SSL3_AL_FATAL, al);
 err:
    s->state = SSL_ST_ERR;
    return (-1);
}

Vulnerability Type: DoS 
CWE ID: CWE-362
Summary: Race condition in the ssl3_get_new_session_ticket function in ssl/s3_clnt.c in OpenSSL before 0.9.8zg, 1.0.0 before 1.0.0s, 1.0.1 before 1.0.1n, and 1.0.2 before 1.0.2b, when used for a multi-threaded client, allows remote attackers to cause a denial of service (double free and application crash) or possibly have unspecified other impact by providing a NewSessionTicket during an attempt to reuse a ticket that had been obtained earlier.
Commit Message: Fix race condition in NewSessionTicket

If a NewSessionTicket is received by a multi-threaded client when
attempting to reuse a previous ticket then a race condition can occur
potentially leading to a double free of the ticket data.

CVE-2015-1791

This also fixes RT#3808 where a session ID is changed for a session already
in the client session cache. Since the session ID is the key to the cache
this breaks the cache access.

Parts of this patch were inspired by this Akamai change:
https://github.com/akamai/openssl/commit/c0bf69a791239ceec64509f9f19fcafb2461b0d3

Reviewed-by: Rich Salz <rsalz@openssl.org>