Fix OCSP Status Request extension unbounded memory growth

A malicious client can send an excessively large OCSP Status Request
extension. If that client continually requests renegotiation,
sending a large OCSP Status Request extension each time, then there will
be unbounded memory growth on the server. This will eventually lead to a
Denial Of Service attack through memory exhaustion. Servers with a
default configuration are vulnerable even if they do not support OCSP.
Builds using the "no-ocsp" build time option are not affected.

I have also checked other extensions to see if they suffer from a similar
problem but I could not find any other issues.

CVE-2016-6304

Issue reported by Shi Lei.

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

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index fbcf2e663cb257f5c91bdb94e153be34398df22d..e4b4e27dd7fc3560df7a4ea2c6b13ec3d563aa1f 100644 (file)
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -2316,6 +2316,23 @@ static int ssl_scan_clienthello_tlsext(SSL *s, unsigned char **p,
                 size -= 2;
                 if (dsize > size)
                     goto err;
+
+                /*
+                 * We remove any OCSP_RESPIDs from a previous handshake
+                 * to prevent unbounded memory growth - CVE-2016-6304
+                 */
+                sk_OCSP_RESPID_pop_free(s->tlsext_ocsp_ids,
+                                        OCSP_RESPID_free);
+                if (dsize > 0) {
+                    s->tlsext_ocsp_ids = sk_OCSP_RESPID_new_null();
+                    if (s->tlsext_ocsp_ids == NULL) {
+                        *al = SSL_AD_INTERNAL_ERROR;
+                        return 0;
+                    }
+                } else {
+                    s->tlsext_ocsp_ids = NULL;
+                }
+
                 while (dsize > 0) {
                     OCSP_RESPID *id;
                     int idsize;
@@ -2335,13 +2352,6 @@ static int ssl_scan_clienthello_tlsext(SSL *s, unsigned char **p,
                         OCSP_RESPID_free(id);
                         goto err;
                     }
-                    if (!s->tlsext_ocsp_ids
-                        && !(s->tlsext_ocsp_ids =
-                             sk_OCSP_RESPID_new_null())) {
-                        OCSP_RESPID_free(id);
-                        *al = SSL_AD_INTERNAL_ERROR;
-                        return 0;
-                    }
                     if (!sk_OCSP_RESPID_push(s->tlsext_ocsp_ids, id)) {
                         OCSP_RESPID_free(id);
                         *al = SSL_AD_INTERNAL_ERROR;