Bug 26982 – pkexec information disclosure vulnerability

pkexec is vulnerable to a minor information disclosure vulnerability
that allows an attacker to verify whether or not arbitrary files
exist, violating directory permissions. I reproduced the issue on my
Karmic installation as follows:

 $ mkdir secret
 $ sudo chown root:root secret
 $ sudo chmod 400 secret
 $ sudo touch secret/hidden
 $ pkexec /home/drosenbe/secret/hidden
 (password prompt)
 $ pkexec /home/drosenbe/secret/doesnotexist
 Error getting information about /home/drosenbe/secret/doesnotexist: No such
 file or directory

I've attached my patch for the issue. I replaced the stat() call
entirely with access() using F_OK, so rather than check that the
target exists, pkexec now checks if the user has permission to verify
the existence of the program. There might be another way of doing
this, such as chdir()'ing to the parent directory of the target and
calling lstat(), but this seemed like more code than necessary to
prevent such a minor problem.  I see no reason to allow pkexec to
execute targets that are not accessible to the executing user because
of directory permissions. This is such a limited use case anyway that
this doesn't really affect functionality.

http://bugs.freedesktop.org/show_bug.cgi?id=26982

Signed-off-by: David Zeuthen <davidz@redhat.com>

1 files changed, 2 insertions, 3 deletions

diff --git a/src/programs/pkexec.c b/src/programs/pkexec.c
index 860e665..17c191e 100644
--- a/src/programs/pkexec.c
+++ b/src/programs/pkexec.c
@@ -411,7 +411,6 @@ main (int argc, char *argv[])
   gchar *opt_user;
   pid_t pid_of_caller;
   uid_t uid_of_caller;
-  struct stat statbuf;
 
   ret = 127;
   authority = NULL;
@@ -520,9 +519,9 @@ main (int argc, char *argv[])
       g_free (path);
       argv[n] = path = s;
     }
-  if (stat (path, &statbuf) != 0)
+  if (access (path, F_OK) != 0)
     {
-      g_printerr ("Error getting information about %s: %s\n", path, g_strerror (errno));
+      g_printerr ("Error accessing %s: %s\n", path, g_strerror (errno));
       goto out;
     }
   command_line = g_strjoinv (" ", argv + n);