Overview
The Linux kernel, core to the operating system and responsible for managing system resources, has recently been identified with a significant vulnerability, CVE-2021-47669. This flaw resides in the Virtual eXtended CAN (vxcan) transmission process and is classified as a ‘use after free’ bug. This vulnerability is of particular concern to Linux users and system administrators due to the far-reaching implications of any potential exploitation. Given the Linux kernel’s ubiquity in various devices ranging from servers, personal computers, to embedded systems, this vulnerability could potentially impact a vast number of systems globally.
Vulnerability Summary
CVE ID: CVE-2021-47669
Severity: High (7.8 CVSS Score)
Attack Vector: Network
Privileges Required: Low
User Interaction: None
Impact: Potential system compromise or data leakage
Affected Products
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Product | Affected Versions
Linux Kernel | [All versions prior to the patch]
How the Exploit Works
The vulnerability occurs in the vxcan_xmit function of the Linux kernel’s Virtual eXtended CAN. The issue arises when the function calls netif_rx_ni(skb), after which dereferencing skb becomes unsafe. Particularly, the canfd_frame cfd, which aliases skb memory, is accessed after the netif_rx_ni call. Successful exploitation of this bug could lead to unauthorized information disclosure, modification, or disruption of service.
Conceptual Example Code
While the exact exploitation method is dependent on the specific configuration and use case of the vulnerable system, conceptually a malicious actor might exploit this vulnerability through network packets that cause the kernel to dereference a freed skb object, leading to unexpected behavior or even system compromise. This could be conceptually illustrated with the following pseudocode:
// Create malicious packet
struct sk_buff *skb = alloc_skb(...);
...
// Fill skb with malicious data
...
// Send packet to vulnerable system
send_to_vulnerable_system(skb);
// Free skb, but it will be used later in vxcan_xmit
kfree_skb(skb);Mitigation Guidance
The most reliable way to mitigate this vulnerability is by applying the vendor patch. Until the patch can be applied, using a Web Application Firewall (WAF) or Intrusion Detection System (IDS) can offer temporary protection by monitoring and potentially blocking malicious network traffic that attempts to exploit this vulnerability.