Overview
The Linux kernel, a crucial component that powers millions of servers and devices worldwide, has recently been found to contain a significant vulnerability, designated as CVE-2021-47668. This vulnerability pertains to a use-after-free bug in the Controller Area Network (CAN) device driver, which if exploited, could lead to system compromise or data leakage. This vulnerability is particularly concerning due to the widespread use of the Linux kernel in various devices and systems, from personal computers to enterprise servers, underscoring the urgent need for patching and mitigation.
Vulnerability Summary
CVE ID: CVE-2021-47668
Severity: High (7.8 CVSS score)
Attack Vector: Local
Privileges Required: Low
User Interaction: None
Impact: Potential system compromise or data leakage due to use-after-free bug in the Linux kernel.
Affected Products
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Product | Affected Versions
Linux Kernel | Versions before the patched release
How the Exploit Works
The vulnerability arises from a use-after-free bug in the Linux kernel’s CAN network device driver. This occurs when the kernel attempts to access a data structure (specifically, a socket buffer or skb) after it’s been freed or deallocated. In particular, a can_frame structure (cf), which aliases skb’s memory, is accessed after the netif_rx_ni() function call, resulting in a potentially unsafe dereference. This could lead to several adverse outcomes, including memory corruption, leading to a system crash or, in the worst-case scenario, arbitrary code execution.
Conceptual Example Code
The following pseudocode illustrates the order of operations that can lead to the vulnerability:
struct can_frame *cf;
struct sk_buff *skb;
// Receive data
skb = netif_rx_ni();
// Dereference after free
cf = (struct can_frame *)skb->data;
// This is the problematic line - accessing cf after netif_rx_ni
stats->rx_bytes += cf->len;To mitigate the vulnerability, the code should be reordered as follows:
struct can_frame *cf;
struct sk_buff *skb;
// Dereference before free
cf = (struct can_frame *)skb->data;
stats->rx_bytes += cf->len;
// Then receive data
skb = netif_rx_ni();To protect against this vulnerability, users are advised to apply the latest vendor patches. In cases where immediate patching is not possible, deploying a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can provide temporary mitigation.