It was discovered that sudo's sudoers file parses does not correctly handle group specification in Runas_User.  If group was specified in the list (using syntax %group, to allow some user to run commands as any member of the group) and the user was already member of the group, sudo actually allowed the user to run commands as arbitrary system user.

SuSE and upstream bug report:
https://bugzilla.novell.com/show_bug.cgi?id=468923
http://www.gratisoft.us/bugzilla/show_bug.cgi?id=327

This issue was confirmed on multiple 1.6.9 sudo versions.  Latest upstream 1.7.0 was reported not to be affected, 1.6.8p12 previously shipped with Red Hat Enterprise Linux 5 was not affected as well.  Problem was confirmed on 1.6.9p17 in RHEL5 and Fedora 10.

Upstream patch:

Index: parse.c
===================================================================
RCS file: /home/cvs/courtesan/sudo/parse.c,v
retrieving revision 1.160.2.21
diff -u -r1.160.2.21 parse.c
--- parse.c	2 Nov 2008 14:35:53 -0000	1.160.2.21
+++ parse.c	23 Jan 2009 19:16:55 -0000
@@ -651,9 +651,11 @@
     /*
      * If the user has a supplementary group vector, check it first.
      */
-    for (i = 0; i < user_ngroups; i++) {
-	if (grp->gr_gid == user_groups[i])
-	    return(TRUE);
+    if (strcmp(user, user_name) == 0) {
+	for (i = 0; i < user_ngroups; i++) {
+	    if (grp->gr_gid == user_groups[i])
+		return(TRUE);
+	}
     }
     if (grp->gr_mem != NULL) {
 	for (cur = grp->gr_mem; *cur; cur++) {