diff -urNad zoo-2.10~/zooext.c zoo-2.10/zooext.c
--- zoo-2.10~/zooext.c   1993-05-01 05:58:50.000000000 +0200
+++ zoo-2.10/zooext.c  2007-05-23 23:19:10.000000000 +0200
@@ -89,6 +89,7 @@
 #endif
 struct direntry direntry;                 /* directory entry */
 int first_dir = 1;                                                            /* first dir entry seen? */
+ + unsigned long zoo_pointer = 0;            /* Track our position in the file */
 
 static char extract_ver[] = "Zoo %d.%d is needed to extract %s.\n";
 static char no_space[] = "Insufficient disk space to extract %s.\n";
@@ -169,6 +170,9 @@
                exit_status = 1;
    }
    zooseek (zoo_file, zoo_header.zoo_start, 0); /* seek to where data begins */
+
+   /* Begin tracking our position in the file */ 
+   zoo_pointer = zoo_header.zoo_start; 
 }
 
 #ifndef PORTABLE
@@ -597,6 +601,11 @@
    } /* end if */
 
 loop_again:
+   /* Make sure we are not seeking to already processed data */ 
+   if (next_ptr <= zoo_pointer)
+     prterror ('f', "ZOO chain structure is corrupted\n");
+   zoo_pointer = next_ptr; 
+
    zooseek (zoo_file, next_ptr, 0); /* ..seek to next dir entry */
 } /* end while */

diff -urNad zoo-2.10~/zoolist.c zoo-2.10/zoolist.c
--- zoo-2.10~/zoolist.c  1991-07-20 00:57:27.000000000 +0200
+++ zoo-2.10/zoolist.c 2007-05-23 23:21:03.000000000 +0200
@@ -92,6 +92,7 @@
 int show_mode = 0;                             /* show file protection */
 #endif
 int first_dir = 1;                             /* if first direntry -- to adjust dat_ofs */
+unsigned long zoo_pointer = 0; /* Track our position in the file */
 
 while (*option) {
    switch (*option) {
@@ -210,6 +211,9 @@
                void show_acmt PARMS ((struct zoo_header *, ZOOFILE, int));
                show_acmt (&zoo_header, zoo_file, 0);           /* show archive comment */
        }
+                                                                         
+       /* Begin tracking our position in the file */
+       zoo_pointer = zoo_header.zoo_start;
 
    /* Seek to the beginning of the first directory entry */
    if (zooseek (zoo_file, zoo_header.zoo_start, 0) != 0) {
@@ -437,6 +441,11 @@
          if (verb_list && !fast)
             show_comment (&direntry, zoo_file, 0, (char *) NULL);
       } /* end if (lots of conditions) */
+
+      /* Make sure we are not seeking to already processed data */
+      if (direntry.next <= zoo_pointer)
+        prterror ('f', "ZOO chain structure is corrupted\n");
+      zoo_pointer = direntry.next;
    
                /* ..seek to next dir entry */
       zooseek (zoo_file, direntry.next, 0);