CVE-2021-33909 - Linux LPE Vulnerability

THE THREAT

Security researchers from Qualys have announced a Local Privilege Escalation (LPE) vulnerability impacting the Linux operating system. The vulnerability is tracked as CVE-2021-33909 and has been alternatively named Sequoia. Exploitation would allow a threat actor with previous access to a vulnerable system to run code with root privileges. Proof-of-Concept (PoC) exploit code is publicly available; at this time the exploit code only allows for denial-of-service, but it is expected to be modified for LPE in the near future.

The Sequoia vulnerability has been confirmed to impact Debian, Ubuntu, and Fedora; other Linux distributions are likely vulnerable but have not been confirmed at this point. Organizations are strongly recommended to apply security patches as they are released by individual Linux distributions.

What we’re doing about it

• MVS will automatically add the relevant plugins for this vulnerability once details are made available
• eSentire security teams continue to track this topic for additional details and detection opportunities

What you should do about it

• After performing a business impact review, apply the relevant security patches released by individual Linux distributions

Additional information

Exploitation of CVE-2021-33909 has not been identified in the wild at this time. Exploitation is expected in the near future, as PoC code is publicly available, reducing the barriers of exploitation for threat actors.

Qualys has tested the exploit against installations of Ubuntu 20.04, Ubuntu 20.10, Ubuntu 21.04, Debian 11, and Fedora 34 Workstation. It is highly likely that other Linux distributions are impacted.

Qualys has released two mitigations for CVE-2021-33909. Despite mitigations, the primary recommendation is for organizations to apply the relevant security patches as soon as possible. It should be noted that both of the following mitigations may be bypassed.

• Mitigation 1: “Set /proc/sys/kernel/unprivileged_userns_clone to 0, to prevent an attacker from mounting a long directory in a user namespace”
• Mitigation 2: “Set /proc/sys/kernel/unprivileged_bpf_disabled to 1, to prevent an attacker from loading an eBPF program into the kernel”

References:

[1] https://www.qualys.com/2021/07/20/cve-2021-33909/sequoia-local-privilege-escalation-linux.txt