diff -up mutt-1.5.21/mutt_ssl_gnutls.c.debug mutt-1.5.21/mutt_ssl_gnutls.c
--- mutt-1.5.21/mutt_ssl_gnutls.c.debug	2011-11-03 15:56:29.992235238 +0100
+++ mutt-1.5.21/mutt_ssl_gnutls.c	2011-11-03 16:23:42.960123578 +0100
@@ -434,9 +434,16 @@ static int tls_compare_certificates (con
       return 0;
     }
 
-    ptr = (unsigned char *)strstr((char*)b64_data.data, CERT_SEP) + 1;
-    ptr = (unsigned char *)strstr((char*)ptr, CERT_SEP);
+    ptr = (unsigned char *)memmem((char*)b64_data.data, b64_data.size, CERT_SEP, sizeof(CERT_SEP));
+    if (ptr == NULL)
+    {
+      gnutls_free(cert.data);
+      FREE (&b64_data_data);
+      return 0;
+    }
 
+    ptr += 1;
+    ptr = (unsigned char *)memmem((char*)ptr, b64_data.size - (ptr - b64_data.data), CERT_SEP, sizeof(CERT_SEP));
     b64_data.size = b64_data.size - (ptr - b64_data.data);
     b64_data.data = ptr;