diff --git a/ChangeLog b/ChangeLog
index 68b1255b9..57024a043 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -2,6 +2,12 @@ mbed TLS ChangeLog (Sorted per branch, date)
 
 = mbed TLS 2.1.2 released 2015-10-xx
 
+Security
+   * Fix potential heap buffer overflow in servers that perform client
+     authentication against a crafted CA cert. Cannot be triggered remotely
+     unless you allow third parties to pick trust CAs for client auth.
+     Found by Guido Vranken.
+
 Changes
    * Fixed paths for check_config.h in example config files. (Found by bachp)
      (#291)
diff --git a/library/ssl_srv.c b/library/ssl_srv.c
index b48a609ee..1bda53c46 100644
--- a/library/ssl_srv.c
+++ b/library/ssl_srv.c
@@ -2351,6 +2351,7 @@ static int ssl_write_certificate_request( mbedtls_ssl_context *ssl )
     size_t dn_size, total_dn_size; /* excluding length bytes */
     size_t ct_len, sa_len; /* including length bytes */
     unsigned char *buf, *p;
+    const unsigned char * const end = ssl->out_msg + MBEDTLS_SSL_MAX_CONTENT_LEN;
     const mbedtls_x509_crt *crt;
     int authmode;
 
@@ -2471,10 +2472,14 @@ static int ssl_write_certificate_request( mbedtls_ssl_context *ssl )
     total_dn_size = 0;
     while( crt != NULL && crt->version != 0 )
     {
-        if( p - buf > 4096 )
-            break;
-
         dn_size = crt->subject_raw.len;
+
+        if( end < p || (size_t)( end - p ) < 2 + dn_size )
+        {
+            MBEDTLS_SSL_DEBUG_MSG( 1, ( "skipping CAs: buffer too short" ) );
+            break;
+        }
+
         *p++ = (unsigned char)( dn_size >> 8 );
         *p++ = (unsigned char)( dn_size      );
         memcpy( p, crt->subject_raw.p, dn_size );