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Bug 197748 - GDM lets people know if it's a bad password or a bad username when authenticating
Summary: GDM lets people know if it's a bad password or a bad username when authentica...
Alias: None
Product: Fedora
Classification: Fedora
Component: pam
Version: rawhide
Hardware: All
OS: Linux
Target Milestone: ---
Assignee: Tomas Mraz
QA Contact:
: 201787 (view as bug list)
Depends On:
Blocks: FC6Target
TreeView+ depends on / blocked
Reported: 2006-07-05 22:27 UTC by Stewart Adam
Modified: 2007-11-30 22:11 UTC (History)
2 users (show)

Fixed In Version: pam-
Doc Type: Bug Fix
Doc Text:
Clone Of:
Last Closed: 2006-08-10 13:48:57 UTC

Attachments (Terms of Use)
Here's /etc/pam.d/system-auth (deleted)
2006-07-25 13:09 UTC, Stewart Adam
no flags Details
return "user unknown" error instead of "service error" when the user is unknown. (deleted)
2006-08-04 18:57 UTC, Ray Strode [halfline]
no flags Details | Diff

Description Stewart Adam 2006-07-05 22:27:26 UTC
Description of problem:
When authenticating with GDM, I've noticed that one can tell if a it's a bad
user or a bad password from the combo of bad username/password, giving hackers
an edge:
- If it's a good username with a bad password, PAM returns with the 'incorrect
username or password. Please try again' message.
- If it's a bad username (eg, the user isn't added on the system) a popup dialog
comes up saying 'authentication failure'

Version-Release number of selected component (if applicable):
gdm 2.14.x
gdm 2.15.x

How reproducible:

Steps to Reproduce:
1. Start GDM
2. Login with a good username, bad password
3. Try again with a bas username, and the password doesn't matter as the user
isn't on the system anyways...
4. Compare the results of step (2) and (3)
Actual results:
Either a popup dialog or text message is returned, and based on this result a
hacker can possible find out if a user exists on the system.

Expected results:
GDM shows consistent failure results. Could be fixed easily by implementing this
other bug report:
Simply a shake as it does in Mac OS X would not let the hackers or anyone else
know if they have a bad username or a bad password. It's not too big an issue in
terms of worrying about hackers unless you have VNC or some other GUI remote
administration server running, but nevertheless it's still a big security issue.

Additional info:
Could this be a PAM bug? I recall this happening a while back with a SSH
auto-blocker and it was due to PAM.

Comment 1 Stewart Adam 2006-07-24 15:30:37 UTC
Any news on this issue?

Comment 2 Ray Strode [halfline] 2006-07-24 22:35:37 UTC
When you enter an invalid username does it fail immediately or ask for a password?

Comment 3 Stewart Adam 2006-07-25 00:04:33 UTC
No, it accepts the username and password no matter what, but it's the result
that changes:
(valid user, bad password) = dialog resets and white text appears under 'Enter
Username' stating that you entered bad credentials

(invalid user, any password) = a popup dialog comes up with a red X and it says
'Authentication Error'.

Comment 4 Ray Strode [halfline] 2006-07-25 04:39:23 UTC
Can you attach you /etc/pam.d/system-auth file?

Comment 5 Stewart Adam 2006-07-25 13:09:00 UTC
Created attachment 132975 [details]
Here's /etc/pam.d/system-auth

Comment 6 Ray Strode [halfline] 2006-07-25 17:35:10 UTC
does it say "Authentication Error" or "Authentication Failed" ?

Comment 7 Stewart Adam 2006-07-25 19:24:53 UTC
Authentication Failed

Comment 8 Stewart Adam 2006-08-04 14:31:13 UTC
I've changed the version to 'devel' as it's still happening in the development
versions GDM. Do you think this issue will be resolved for FC6?

Comment 9 Ray Strode [halfline] 2006-08-04 15:10:18 UTC
Yes, we should fix this before FC6 is released.

Comment 10 Ray Strode [halfline] 2006-08-04 18:53:43 UTC
So this turns out to be a bug in the pam_succeed_if PAM module.

Comment 11 Ray Strode [halfline] 2006-08-04 18:57:11 UTC
Created attachment 133654 [details]
return "user unknown" error instead of "service error" when the user is unknown.

GDM currently shows the desired error message when the error code is	       
 PAM_AUTH_ERR or PAM_USER_UNKNOWN, and shows the other error message when there
is a problem with the pam configuration.

the module is returning the wrong error code.  The above
patch should fix things up.

Comment 12 Ray Strode [halfline] 2006-08-04 18:59:43 UTC
reassigning to PAM maintainer

Comment 13 Ray Strode [halfline] 2006-08-09 02:05:55 UTC
*** Bug 201787 has been marked as a duplicate of this bug. ***

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