--- pTk/mTk/generic/tkImgGIF.c.orig	2008-02-05 11:38:58.000000000 +0100
+++ pTk/mTk/generic/tkImgGIF.c	2008-02-05 11:42:46.000000000 +0100
@@ -831,6 +831,12 @@
 		Tcl_PosixError(interp), (char *) NULL);
 	return TCL_ERROR;
     }
+
+    if (initialCodeSize > MAX_LWZ_BITS) {
+	Tcl_SetResult(interp, "malformed image", TCL_STATIC);
+	return TCL_ERROR;
+    }
+
     if (transparent != -1) {
 	cmap[transparent][CM_RED] = 0;
 	cmap[transparent][CM_GREEN] = 0;