CVE-2020-24385 in MidnightBSD
Summary
by MITRE
In MidnightBSD before 1.2.6 and 1.3 before August 2020, and FreeBSD before 7, a NULL pointer dereference was found in the Linux emulation layer that allows attackers to crash the running kernel. During binary interaction, td->td_emuldata in sys/compat/linux/linux_emul.h is not getting initialized and returns NULL from em_find().
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Analysis
by VulDB Data Team • 11/12/2020
The vulnerability identified as CVE-2020-24385 represents a critical NULL pointer dereference flaw within the Linux emulation layer of MidnightBSD and FreeBSD operating systems. This issue affects versions prior to 1.2.6 for MidnightBSD 1.2.x series and 1.3.x series before August 2020, as well as FreeBSD versions before 7. The flaw manifests during binary interaction processes where the kernel's Linux emulation subsystem fails to properly initialize critical data structures, specifically the td_emuldata field within the thread structure. This uninitialized pointer creates a condition where em_find() function returns NULL, leading to immediate kernel crash upon attempted dereference. The vulnerability resides in the sys/compat/linux/linux_emul.h file, which governs the compatibility layer that allows Linux binaries to execute on these operating systems. This type of flaw falls under CWE-476 which specifically addresses NULL pointer dereference vulnerabilities, representing a fundamental security weakness where a program attempts to access memory through a null pointer reference. The attack surface is particularly concerning as it targets the kernel's core functionality, making it a prime candidate for privilege escalation and denial of service attacks.
The operational impact of this vulnerability extends beyond simple system crashes, as it can be exploited by malicious actors to cause system instability and potentially gain unauthorized access to kernel memory spaces. When an attacker successfully triggers this NULL pointer dereference, the kernel immediately panics and terminates, resulting in a complete system crash that can be leveraged for denial of service attacks against critical infrastructure. The nature of the flaw means that any process attempting to use Linux binary compatibility features could trigger the vulnerability, making it particularly dangerous in multi-user environments where various applications might attempt to execute Linux-compatible binaries. From an attacker's perspective, this vulnerability maps to the ATT&CK technique T1068 which involves the exploitation of legitimate credentials and system privileges to gain access to system resources, and T1499 which covers the exploitation of system resources for denial of service. The vulnerability's location within the kernel's emulation layer also suggests potential for privilege escalation, as attackers could potentially leverage the crash to gain elevated system privileges or access to sensitive kernel memory regions.
Mitigation strategies for CVE-2020-24385 require immediate system updates to patched versions of both MidnightBSD and FreeBSD, specifically upgrading to MidnightBSD 1.2.6 or later and FreeBSD 7 or later where the initialization issue has been resolved. System administrators should also implement process monitoring to detect unusual kernel crashes or memory access patterns that might indicate exploitation attempts. The fix addresses the root cause by ensuring proper initialization of td_emuldata field before any access attempts, preventing the NULL return from em_find() function. Network segmentation and access controls should be implemented to limit exposure of systems running Linux emulation layers, particularly in environments where untrusted users or applications might attempt to execute Linux binaries. Additionally, organizations should conduct regular security audits of their kernel modules and emulation layers, as similar issues may exist in other compatibility layers. The vulnerability highlights the importance of proper initialization checking and defensive programming practices in kernel space code, where uninitialized pointers can lead to catastrophic system failures. System monitoring should include detection of kernel panics and crash reports that might indicate exploitation attempts, while security teams should maintain awareness of similar vulnerabilities in other operating system compatibility layers. The fix demonstrates the necessity of thorough testing of kernel compatibility modules and proper validation of pointer states before memory access operations, which aligns with security best practices outlined in various operating system security frameworks and standards.