CVE-2021-31440: Kubernetes container escape using eBPF
In a recent post by ZDI, researchers found an out-of-bounds access flaw (CVE-2021-31440) in the Linux kernel’s (5.11.15) implementation of the eBPF code verifier: an incorrect register bounds calculation occurs while checking unsigned 32-bit instructions in an eBPF program. The flaw can be leveraged to escalate privileges and execute arbitrary code in the context of the kernel.
This vulnerability allows a local privilege escalation, which means an attacker with non-root access to the system can gain higher privileges by exploiting this vulnerability. The non-root access can be a user account without sudo or group privileges, which are usually provided to the application user.
Why you should be worried
In a Kubernetes environment, containers use the host kernel to run themselves. Therefore, the execution of malicious eBPF code as an unprivileged user in the context of the kernel can result in container escape and privilege escalation to the host.
Unprivileged users inside the container need CAP_SYS_ADMIN permission already assigned to the container to run a malicious eBPF program. For Linux kernels 5.8 and above, a new permission, CAP_BPF, is added to allow users to run eBPF programs. CAP_BPF is a subset of CAP_SYS_ADMIN.
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