Date: Wed, 2 Jul 2003 07:58:03 -0700
From: "bob" <inboxbob@....net>
To: "Michal Zalewski" <lcamtuf@...ttot.org>, <vulndiscuss@...nwatch.org>,
   <full-disclosure@...sys.com>, <vuln-dev@...urityfocus.com>
Cc: <bugtraq@...urityfocus.com>, <vulnwatch@...nwatch.org>
Subject: Re: Red Hat 9: free tickets


Why you giving out my warez Michal, Nice Name "free tickets" , redhat didnt
respond because they to busy with telentd bug soon to be released by you im
sure.
But i tested on all my redhat boxes with exploit you sent me and non worked
on kerns 2.5* so the world is safe from the evil race you sent to the wild.
I think thats why redhat never responsed to us because they is in the TAPE
with JAF and Bob Hacker.
Yoh Peace !


----- Original Message ----- 
From: "Michal Zalewski" <lcamtuf@...ttot.org>
To: <vulndiscuss@...nwatch.org>; <full-disclosure@...sys.com>;
<vuln-dev@...urityfocus.com>
Cc: <bugtraq@...urityfocus.com>; <vulnwatch@...nwatch.org>
Sent: Wednesday, July 02, 2003 2:36 AM
Subject: Red Hat 9: free tickets


>
> [ This is not strictly a new vulnerability - but a description of
>   a flaw that can be combined with any of the minor vulnerabilities
>   that pop up once a week to turn them into a major vulnerability.
>   I will leave it up to the moderators of BUGTRAQ and VulnWatch to
>   approve or reject it... ]
>
> The reason for this post is that largely under-appreciated file creation
> vulnerabilities can now get a higher profile, I think. Below are some
> hopefully interesting thoughts on turning almost any O_CREAT w/o O_EXCL in
> a world-writable directory - or similar issues, you name it - into an
> instant, generic root compromise scenario on modern Linux boxes (primarily
> Red Hat and derivates)...
>
> Until recently, you could basically exercise three possible active attack
> vectors against such a /tmp vulnerability - none of which is particularly
> tempting in terms of privilege escalation:
>
>   1) Denial of Service:
>
>      Just create an appropriate symlink to create /etc/nologin,
>      truncate /lib/libc.so.6 or do something of a similar nature once
>      followed by a broken application. This is possible in almost all
>      cases when a vulnerable application is run by a privileged user.
>      Fortunately or unfortunately, there's very little benefit for the
>      attacker in this scenario.
>
>   2) Contents manipulation:
>
>      Attack the application itself by securing a write access to the
>      soon-to-be-accessed temporary file (it's usually enough to create
>      it in advance). Then stuff its temporary file with some unexpected
>      data. This could be successful only if the application uses file
>      contents in some interesting way later on. While there are some
>      nice examples (for example, older versions of GCC), this is seldom
>      a feasible scenario, and is very application-specific.
>
>   3) Contents redirection:
>
>      If you can control the data written by the application to a temporary
>      file, you could use a symlink to force writing to a file like
>      /etc/ld.so.preload, /etc/passwd, ~/.ssh/authorized_keys or such. In
>      most cases, however, races occur in boot scripts, cron scripts,
>      non-suid applications run by privileged users, etc - and the attacker
>      can exercise very little control over the contents of such a file.
>
> As far as I know, there was no neat and generic way to exploit an insecure
> /tmp file creation alone - well, until now.
>
> Starting release 9, Red Hat ships and uses pam_timestamp_check.so module
> (accompanied by /sbin/pam_timestamp_check setuid helper), a part of the
> new pam-0.75 (Pluggable Authentication Modules) package. PAM is a generic
> centralized authentication and session management component that is also
> shipped by an increasing number of other distributions, so it is
> reasonable that the code is about to propagate to other distros.
>
> The module mentioned implements a credential caching functionality, very
> closely inspired on a tty ticketing system used in sudo. Most sudo users
> are familiar with the fact they are not prompted for password if a
> subsequent sudo session is opened shortly after a previous one on the same
> terminal - and this is exactly what pam_timestamp_check tries to implement
> for other services.
>
> The system used in sudo is somewhat naive and does have its problems, but
> the impact caused by an eventual ticket stealing attack is fairly minimal
> - the user has to be trusted and listed in /etc/sudoers in the first
> place, and the credentials that are cached are for his own account (sudo
> users enter their own password, as opposed to root's).
>
> The way Red Hat deployed this mechanism is badly broken, since they use it
> to cache root credentials for access to critical components of the system,
> and there are no restrictions as to who can use those components. While in
> sudo, stealing or spoofing a ticket is worth exactly as much as knowing
> the password for the account you already have access to, and the account
> has to be trusted, in Red Hat, it is worth root's password almost all the
> time, and any user can use it. As such, there should me much more caution
> exercised with such a mechanism. But there is not, causing an obvious
> exposure.
>
> The way the module (and sudo) works, in essence, is that it gets current
> pseudo-terminal name A (which can be trivially spoofed, but this is of no
> relevance at the moment), current user name B, and the user for which
> credentials are cached, C (usually root for Red Hat applications, user
> himself for sudo). Then the code checks for /var/run/sudo/B/A:C (or
> /var/run/sudo/B/A if B == C), and if the file is recent, the module
> returns success, and enables the user to skip the usual password
> authentication.
>
> The mechanism is used in Red Hat to make it easier for users to perform
> administrative tasks without having to switch to root via su or sudo,
> granted they know the admin password. There is a number of management
> applications that can be invoked via a single setuid PAM-enabled wrapper,
> /usr/sbin/userhelper, that all have pam_timestamp_check.so included in
their
> PAM configs. From quite harmless ones, such as redhat-config-mouse,
> to pretty much instant root scenarios once the mechanism is compromised
> - say, redhat-config-rootpassword, redhat-install-packages,
up2date-config,
> redhat-config-services, etc.
>
> A noble concept indeed, but there is a nasty issue - since there's no
> check for file origin, it should be more than obvious that suddenly, any
> insecure file creation problem in an application used by a superuser (or
> from privileged scripts, such as boot rc files, crontabs, etc), it is
> possible to spoof a ticket in /var/run and bypass root password prompt and
> other checks, and perform administrative tasks, easily modifying system
> config, installing custom components (say, a rootshell), etc. All this by
> crafting a single symlink that is later opened with O_CREAT with no O_EXCL
> or O_NOFOLLOW.
>
> The simplest workaround for all concerned users is to first remove all
> occurrences of pam_timestamp_check.so from /etc/pam.d, and replace
> /sbin/pam_timestamp_check standalone helper, if possible. Perhaps
> reconsider the necessity of having /usr/sbin/userhelper mechanism
> implemented at all.
>
> For Red Hat, my suggestion is to verify ticket contents. Say, have a
> host-wide random key K, and put user_name, expire_time, MD5(K + user_name
> + expire_time) in every ticket. The check code would verify the MD5
> signature to make sure the origin of the ticket is sane, and the
> originating application performed a specific operation on a not publicly
> readable key.
>
> On a side note, the per-terminal ticketing in pam_timestamp_check the way
> it is has absolutely no significance and adds no protection, since the A
> element of the path can be easily manipulated. Just an example (there are
> other possible ways of accomplishing this):
>
>     ln `tty` /tmp/tty1
>     /usr/sbin/userhelper -w -t redhat-install-packages </tmp/tty1
>
> As such, those tickets effectively become per-user, and an attacker who
> compromised the account can snatch a ticket granted to the legitimate user
> who already authenticated. Consider dropping the honor tty system and
> granting per-user tickets to avoid giving users a false sense of security.
>
> I mailed pam_timestamp_check maintainer at Red Hat (Nalin Dahyabhai) about
> a week ago, but never heard back from him. Since this is not an issue
> alone, I decided to post the information here.
>
> -- 
> ------------------------- bash$ :(){ :|:&};: --
>  Michal Zalewski * [http://lcamtuf.coredump.cx]
>     Did you know that clones never use mirrors?
> --------------------------- 2003-07-02 11:07 --
>
>
>
>

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