CVE-2021-47607 is a vulnerability in the Linux kernel's implementation of the BPF (Berkeley Packet Filter) subsystem, specifically in the atomic compare-and-exchange (cmpxchg) operation used by BPF programs. The vulnerability arises because the kernel verifier fails to reject unprivileged BPF programs where the R0 register contains a pointer as the 'old value' parameter in the BPF_CMPXCHG instruction. Normally, BPF_CMPXCHG compares a memory location's current value with an expected old value and, if they match, swaps it with a new value. The R0 register is used both as an input (old value) and output (returning the old value from memory). However, the verifier does not properly check that R0 does not hold a pointer in unprivileged programs, allowing attackers to leak kernel memory addresses by brute forcing. The proof-of-concept (PoC) demonstrates that an attacker can leak kernel pointers by probing addresses and using map slots to detect matches, taking approximately 16 seconds on the author's machine. This kernel address leakage can facilitate further attacks such as kernel address space layout randomization (KASLR) bypass, which is a critical step in privilege escalation exploits. The vulnerability was fixed by adding checks to reject unprivileged programs where R0 contains pointers during the cmpxchg operation. No known exploits are reported in the wild at the time of publication, and no CVSS score has been assigned yet.

For European organizations, this vulnerability poses a significant risk primarily to systems running vulnerable Linux kernel versions that support unprivileged BPF programs. The ability to leak kernel pointers undermines kernel address space layout randomization (KASLR), a key security mechanism designed to prevent attackers from reliably exploiting kernel vulnerabilities. By bypassing KASLR, attackers can more easily develop and execute privilege escalation exploits, potentially gaining root-level access to affected systems. This can lead to full system compromise, data breaches, disruption of critical services, and lateral movement within networks. Organizations relying on Linux servers for critical infrastructure, cloud services, or internal operations could face increased risk of targeted attacks, especially if attackers combine this vulnerability with other kernel bugs. The lack of requirement for user interaction and the relatively fast brute force time (~16 seconds) increase the threat's practicality. Although no exploits are currently known in the wild, the vulnerability's nature and ease of exploitation make it a high-value target for attackers aiming to escalate privileges on Linux hosts.

European organizations should prioritize updating their Linux kernels to versions that include the patch for CVE-2021-47607. Since the vulnerability involves unprivileged BPF programs, organizations should audit and restrict the use of unprivileged BPF where possible, using kernel lockdown features or security modules like SELinux or AppArmor to limit BPF program loading and execution. Additionally, system administrators should monitor for unusual BPF activity or kernel pointer leaks using advanced kernel auditing tools and intrusion detection systems. For environments where immediate patching is not feasible, consider disabling unprivileged BPF program loading via sysctl settings (e.g., setting /proc/sys/kernel/unprivileged_bpf_disabled to 1) to reduce attack surface. Regularly review and harden kernel security configurations and ensure that only trusted users have the capability to load BPF programs. Finally, maintain comprehensive logging and alerting to detect potential exploitation attempts early.