When decompressing with -N or -NT, strip any path from header name.

This uses the path of the compressed file combined with the name
from the header as the name of the decompressed output file.  Any
path information in the header name is stripped.  This avoids a
possible vulnerability where absolute or descending paths are put
in the gzip header.

pigz.c

@@ -3520,26 +3520,35 @@ local void process(char *path)
                  " (use -f to force)");
     }
     else {
-        char *to, *repl;
-
-        /* use header name for output when decompressing with -N */
-        to = g.inf;
-        if (g.decode && (g.headis & 1) != 0 && g.hname != NULL) {
-            to = g.hname;
-            len = strlen(g.hname);
+        char *to = g.inf, *sufx = "";
+        size_t pre = 0;
+
+        /* select parts of the output file name */
+        if (g.decode) {
+            /* for -dN or -dNT, use the path from the input file and the name
+               from the header, stripping any path in the header name */
+            if ((g.headis & 1) != 0 && g.hname != NULL) {
+                pre = justname(g.inf) - g.inf;
+                to = justname(g.hname);
+                len = strlen(to);
+            }
+            /* for -d or -dNn, replace abbreviated suffixes */
+            else if (strcmp(to + len, ".tgz") == 0)
+                sufx = ".tar";
         }
-
-        /* replace .tgz with .tar when decoding */
-        repl = g.decode && strcmp(to + len, ".tgz") ? "" : ".tar";
+        else
+            /* add appropriate suffix when compressing */
+            sufx = g.sufx;
 
         /* create output file and open to write */
-        g.outf = MALLOC(len + (g.decode ? strlen(repl) : strlen(g.sufx)) + 1);
+        g.outf = MALLOC(pre + len + strlen(sufx) + 1);
         if (g.outf == NULL)
             bail("not enough memory", "");
-        memcpy(g.outf, to, len);
-        strcpy(g.outf + len, g.decode ? repl : g.sufx);
+        memcpy(g.outf, g.inf, pre);
+        memcpy(g.outf + pre, to, len);
+        strcpy(g.outf + pre + len, sufx);
         g.outd = open(g.outf, O_CREAT | O_TRUNC | O_WRONLY |
-                             (g.force ? 0 : O_EXCL), 0600);
+                              (g.force ? 0 : O_EXCL), 0600);
 
         /* if exists and not -f, give user a chance to overwrite */
         if (g.outd < 0 && errno == EEXIST && isatty(0) && g.verbosity) {