New file.

+/* GNU Mailutils -- a suite of utilities for electronic mail
+   Copyright (C) 2008 Free Software Foundation, Inc.
+
+   GNU Mailutils is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3, or (at your option)
+   any later version.
+
+   GNU Mailutils is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Mailutils; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+   MA 02110-1301 USA */
+
+#ifdef HAVE_CONFIG_H
+# include <config.h>
+#endif
+#include <stdlib.h>
+#include <string.h>
+#include <pwd.h>
+#include <grp.h>
+#include <unistd.h>
+#include <mailutils/assoc.h>
+#include <mailutils/errno.h>
+#include <mailutils/error.h>
+#include <mailutils/errno.h>
+#include <mailutils/nls.h>
+#include <mailutils/list.h>
+#include <mailutils/iterator.h>
+#include <xalloc.h>
+
+/* Switch to the given UID/GID */
+int
+mu_switch_to_privs (uid_t uid, gid_t gid, mu_list_t retain_groups)
+{
+  int rc = 0;
+  gid_t *emptygidset;
+  size_t size = 1, j = 1;
+  mu_iterator_t itr;
+
+  if (uid == 0)
+    return 0;
+
+  /* Create a list of supplementary groups */
+  mu_list_count (retain_groups, &size);
+  size++;
+  emptygidset = xmalloc (size * sizeof emptygidset[0]);
+  emptygidset[0] = gid ? gid : getegid ();
+
+  if (mu_list_get_iterator (retain_groups, &itr) == 0)
+    {
+      for (mu_iterator_first (itr);
+	   !mu_iterator_is_done (itr); mu_iterator_next (itr)) 
+	mu_iterator_current (itr,
+			     (void **)(emptygidset + j++));
+      mu_iterator_destroy (&itr);
+    }
+
+  /* Reset group permissions */
+  if (geteuid () == 0 && setgroups (j, emptygidset))
+    {
+      mu_error(_("setgroups(1, %lu) failed: %s"),
+	       (unsigned long) emptygidset[0], mu_strerror (errno));
+      rc = 1;
+    }
+  free (emptygidset);
+	
+  /* Switch to the user's gid. On some OSes the effective gid must
+     be reset first */
+
+#if defined(HAVE_SETEGID)
+  if ((rc = setegid (gid)) < 0)
+    mu_error (_("setegid(%lu) failed: %s"),
+	      (unsigned long) gid, mu_strerror (errno));
+#elif defined(HAVE_SETREGID)
+  if ((rc = setregid (gid, gid)) < 0)
+    mu_error (_("setregid(%lu,%lu) failed: %s"),
+	      (unsigned long) gid, (unsigned long) gid,
+	      mu_strerror (errno));
+#elif defined(HAVE_SETRESGID)
+  if ((rc = setresgid (gid, gid, gid)) < 0)
+    mu_error (_("setresgid(%lu,%lu,%lu) failed: %s"),
+	      (unsigned long) gid,
+	      (unsigned long) gid,
+	      (unsigned long) gid,
+	      mu_strerror (errno));
+#endif
+
+  if (rc == 0 && gid != 0)
+    {
+      if ((rc = setgid (gid)) < 0 && getegid () != gid) 
+	mu_error (_("setgid(%lu) failed: %s"),
+		  (unsigned long) gid, mu_strerror (errno));
+      if (rc == 0 && getegid () != gid)
+	{
+	  mu_error (_("Cannot set effective gid to %lu"),
+		    (unsigned long) gid);
+	  rc = 1;
+	}
+    }
+
+  /* Now reset uid */
+  if (rc == 0 && uid != 0)
+    {
+      uid_t euid;
+
+      if (setuid (uid) || geteuid () != uid
+	  || (getuid () != uid && (geteuid () == 0 || getuid () == 0)))
+	{
+#if defined(HAVE_SETREUID)
+	  if (geteuid () != uid)
+	    {
+	      if (setreuid (uid, -1) < 0)
+		{
+		  mu_error (_("setreuid(%lu,-1) failed: %s"),
+			    (unsigned long) uid,
+			    mu_strerror (errno));
+		  rc = 1;
+		}
+	      if (setuid (uid) < 0)
+		{
+		  mu_error (_("second setuid(%lu) failed: %s"),
+			    (unsigned long) uid, mu_strerror (errno));
+		  rc = 1;
+		}
+	    } else
+#endif
+	        {
+		  mu_error (_("setuid(%lu) failed: %s"),
+			    (unsigned long) uid,
+			    mu_strerror (errno));
+		  rc = 1;
+		}
+	}
+	
+      euid = geteuid ();
+      if (uid != 0 && setuid (0) == 0)
+	{
+	  mu_error (_("seteuid(0) succeeded when it should not"));
+	  rc = 1;
+	}
+      else if (uid != euid && setuid (euid) == 0)
+	{
+	  mu_error (_("Cannot drop non-root setuid privileges"));
+	  rc = 1;
+	}
+    }
+  return rc;
+}
+
+