CVE-2021-34693 Overview
A vulnerability exists in the Linux kernel's Controller Area Network (CAN) Broadcast Manager (BCM) implementation within net/can/bcm.c. The vulnerability allows local users to obtain sensitive information from kernel stack memory due to uninitialized data structure members. This information disclosure vulnerability affects Linux kernel versions through 5.12.10 and has been addressed in subsequent kernel releases.
Critical Impact
Local attackers can leverage this vulnerability to read sensitive kernel memory contents, potentially exposing cryptographic keys, memory addresses useful for bypassing ASLR, or other security-sensitive data stored in kernel stack memory.
Affected Products
- Linux Kernel through version 5.12.10
- Debian Linux 9.0
- Debian Linux 10.0
Discovery Timeline
- 2021-06-14 - CVE-2021-34693 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-34693
Vulnerability Analysis
The vulnerability resides in the CAN BCM (Broadcast Manager) subsystem of the Linux kernel. The CAN protocol is commonly used in automotive and industrial applications for communication between microcontrollers and devices. The BCM module provides advanced message filtering and management capabilities for CAN networks.
The core issue stems from incomplete initialization of data structures within net/can/bcm.c. When certain structures are allocated and used without proper initialization, portions of the kernel stack memory remain in an undefined state. When these structures are subsequently copied to user space or processed in ways that expose their contents, local users can read this uninitialized memory, leading to information disclosure.
This type of vulnerability (CWE-909: Missing Initialization of Resource) is particularly concerning in kernel space because the exposed memory may contain sensitive information from previous kernel operations, including memory addresses that could defeat kernel address space layout randomization (KASLR) or other security mechanisms.
Root Cause
The root cause is the failure to properly initialize all members of data structures in the CAN BCM implementation before use. When structures containing uninitialized fields are copied to userspace or otherwise exposed, the garbage data present in those fields leaks kernel stack contents. The fix involves ensuring proper initialization of all structure members using memset() or explicit field initialization before the structure is used.
Attack Vector
This vulnerability requires local access to exploit. An attacker with local user privileges can interact with the CAN BCM subsystem through socket operations to trigger the information disclosure. The attack does not require user interaction and can be performed programmatically.
The exploitation mechanism involves:
- Opening a CAN BCM socket from userspace
- Performing operations that cause the kernel to allocate and use the vulnerable data structures
- Reading back data that contains uninitialized portions of kernel stack memory
- Analyzing the leaked data to extract sensitive kernel information
The technical details of this vulnerability are documented in the Linux Kernel Commit which contains the fix. Additional discussion can be found in the Kernel Network Development Thread.
Detection Methods for CVE-2021-34693
Indicators of Compromise
- Unusual or unexpected CAN socket operations from non-privileged processes
- Processes repeatedly opening and closing CAN BCM sockets in rapid succession
- Memory access patterns suggesting kernel memory disclosure attempts
- Applications accessing CAN subsystem that don't typically require CAN functionality
Detection Strategies
- Monitor for processes opening AF_CAN sockets with CAN_BCM protocol that don't have legitimate CAN-related purposes
- Implement kernel auditing to log CAN BCM socket operations
- Use SentinelOne's behavioral AI to detect anomalous kernel subsystem access patterns
- Deploy kernel integrity monitoring to identify exploitation attempts targeting kernel memory
Monitoring Recommendations
- Enable kernel audit logging for socket operations involving the CAN subsystem
- Monitor system logs for unusual CAN-related kernel messages
- Implement runtime kernel memory integrity checks where possible
- Review process behavior analytics for applications interacting with CAN interfaces unexpectedly
How to Mitigate CVE-2021-34693
Immediate Actions Required
- Update the Linux kernel to a patched version that addresses CVE-2021-34693
- For Debian systems, apply the security updates referenced in DSA-4941
- Restrict access to CAN interfaces to only trusted users and applications
- Review and audit applications that utilize CAN BCM functionality
Patch Information
The vulnerability has been fixed in the upstream Linux kernel. The fix ensures proper initialization of all data structure members in the CAN BCM implementation. The patch is available in kernel commit 5e87ddbe3942e27e939bdc02deb8579b0cbd8ecc.
For Debian-based systems, security updates are available as documented in:
Workarounds
- If CAN functionality is not required, consider blacklisting the can-bcm kernel module to prevent loading
- Restrict access to the CAN subsystem using kernel module parameters or SELinux/AppArmor policies
- Limit local user access on systems where CAN functionality is critical but patches cannot be immediately applied
- Implement network segmentation to isolate systems with CAN interfaces from general-purpose computing environments
# Blacklist CAN BCM module if not required
echo "blacklist can-bcm" >> /etc/modprobe.d/blacklist-can.conf
echo "install can-bcm /bin/false" >> /etc/modprobe.d/blacklist-can.conf
# Update module dependencies
depmod -a
# Verify module is not loaded
lsmod | grep can
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
