lcamtuf wykrył poważną lukę w zabezpieczeniach najnowszej dystrybucji Red Hat Linuksa. Błąd tkwi w wadliwie zaprojektowanym mechaniźmie, ułatwiającym użytkownikom wykonywanie zadań administracyjnych bez przełączania się na konto root’a za pomocą sudo czy su (userhelper). Luka pozwala na uzyskanie dostępu do narzędzi administracyjnych za pomocą odpowiednio spreparowanych plików.

Oryginalna treść wiadomości na BugTraq:

From: lcamtuf@ghettot.org (Michal Zalewski)  To: bugtraq@securityfocus.com  Newsgroups: sierp.bugtraq  Subject: Red Hat 9: free tickets  Date: Wed, 2 Jul 2003 11:36:26 +0200 (CEST)  Organization: SierpNet      [ 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 --

Znamienne jest, że deweloperzy Red Hat’a do tej pory nie zareagowali.

Post na grupę usenetową pl.comp.os.linux (Message-ID: <20030702233939.2bba4bdf.NOspam@sierp.netwytnijcalosc>) nadesłał Sierp.

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