CVE-2021-47518 in Linux
Summary
by MITRE • 05/24/2024
In the Linux kernel, the following vulnerability has been resolved:
nfc: fix potential NULL pointer deref in nfc_genl_dump_ses_done
The done() netlink callback nfc_genl_dump_ses_done() should check if received argument is non-NULL, because its allocation could fail earlier in dumpit() (nfc_genl_dump_ses()).
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Analysis
by VulDB Data Team • 08/09/2025
The vulnerability CVE-2021-47518 represents a critical null pointer dereference issue within the Linux kernel's NFC (Near Field Communication) subsystem. This flaw exists in the netlink generic netlink implementation specifically within the nfc_genl_dump_ses_done() callback function. The vulnerability stems from inadequate input validation where the function fails to verify whether the received argument is null before attempting to dereference it. The root cause occurs during the NFC session dump process where the done() callback is invoked, but the argument passed to it may have been allocated earlier in the dumpit() function and could potentially fail due to allocation errors or other resource constraints. This represents a classic security flaw that can lead to system instability and potential privilege escalation opportunities. The issue is particularly concerning because it affects the kernel's NFC subsystem which handles wireless communication protocols used in various devices including smartphones, payment terminals, and IoT devices.
The technical implementation of this vulnerability manifests when the nfc_genl_dump_ses_done() function processes incoming netlink messages during NFC session enumeration. During normal operation, the dumpit() function nfc_genl_dump_ses() allocates memory for session data structures and passes this information to the done() callback. However, if the allocation within dumpit() fails for any reason such as memory exhaustion or other resource constraints, the allocated pointer becomes null. The done() callback function then attempts to dereference this null pointer without proper validation, leading to a kernel panic or potential privilege escalation. This type of vulnerability falls under CWE-476 which specifically addresses NULL pointer dereference conditions in software implementations. The flaw demonstrates poor error handling practices where the kernel does not properly validate the state of allocated memory before proceeding with operations that assume valid data structures.
The operational impact of CVE-2021-47518 extends beyond simple system crashes to potentially enable more serious security exploits. When a null pointer dereference occurs in kernel space, it can result in system instability, denial of service attacks, or in some cases, allow attackers with local privileges to escalate their access level. The vulnerability affects systems running Linux kernels with NFC support, which includes various mobile devices, embedded systems, and servers that utilize NFC communication capabilities. Attackers could potentially exploit this vulnerability by triggering the NFC session dump process through malicious netlink messages, causing the kernel to crash or potentially execute arbitrary code with kernel privileges. This vulnerability aligns with ATT&CK technique T1068 which involves local privilege escalation through kernel exploits, and T1547.001 which covers kernel rootkits and privilege escalation techniques. The impact is particularly severe in environments where NFC functionality is actively used and where kernel memory management failures could be leveraged for broader system compromise.
Mitigation strategies for CVE-2021-47518 focus on implementing proper null pointer validation within the kernel's NFC subsystem. The primary fix involves modifying the nfc_genl_dump_ses_done() function to include explicit null pointer checks before any dereference operations. This approach follows the defensive programming principle of validating all inputs and ensuring proper error handling throughout the code path. System administrators should ensure their Linux kernels are updated to versions that contain the patched implementation, which typically includes the addition of null checks in the affected callback function. The fix should be deployed across all systems running Linux kernels with NFC support, particularly in environments where NFC functionality is actively utilized. Additionally, monitoring for unusual NFC session dump operations and implementing proper kernel memory management practices can help detect potential exploitation attempts. Organizations should also consider implementing kernel hardening measures such as stack canaries, kernel address space layout randomization, and other security enhancements that can make exploitation more difficult even if similar vulnerabilities exist. The vulnerability serves as a reminder of the importance of proper error handling in kernel space code and the critical need for validation of all allocated memory before use.