CVE-2026-49425 in FreeBSD
Summary
by MITRE • 08/19/2026
The compat32 kevent() handler translates a 64-bit kevent struct into a stack- declared 32-bit struct. It did not first zero the stack struct.
An unprivileged user may observe a small amount of uninitialized kernel stack data, which may contain sensitive information.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability identified in this context stems from an improper initialization practice within the Linux kernel's compatibility layer for handling kevent system calls on 32-bit architectures running atop 64-bit kernels. The compat32 subsystem is designed to allow legacy 32-bit applications to interact with a 64-bit operating system by translating data structures between different word sizes. In this specific instance, the handler responsible for processing the kevent structure allocates a stack-based buffer intended to hold the translated 32-bit version of the event data. However, prior to copying or populating fields from the user-supplied input into this local variable, the code fails to explicitly zero out the memory region allocated on the kernel stack. This omission creates a classic uninitialized memory exposure scenario where residual data previously used by other kernel functions remains in those specific bytes of the stack frame.
From a technical perspective, this flaw represents an information leak vulnerability because the kevent structure contains multiple fields that are not always overwritten during normal operation depending on which events are being registered or monitored. When the system call completes and returns control to user space via the compat32 translation layer, any portions of the 32-bit struct that were not explicitly set by the application's input will retain whatever values happened to reside in those stack locations from prior kernel activity. These residual values can include pointers, file descriptors, process identifiers, or other sensitive state information relevant to the current execution context of the kernel thread handling the request. Although the volume of leaked data is described as small, even partial exposure of internal kernel structures can provide attackers with valuable intelligence regarding memory layout and active processes within the system.
The operational impact of this vulnerability allows an unprivileged user to potentially extract sensitive information from the kernel stack through repeated invocations of the affected kevent interface. By carefully crafting requests that trigger different code paths or vary in size, a local attacker can attempt to isolate specific bytes containing uninitialized data and analyze them for recognizable patterns indicative of internal state. This aligns with CWE-200, which classifies such issues as Information Exposure through Unintentional Read of Memory. The ability to read kernel stack memory violates the fundamental security principle that user-space processes must not have access to privileged memory regions or sensitive data belonging to other contexts. While direct exploitation for arbitrary code execution is unlikely given the nature of the leak, the information gained could facilitate further attacks such as address space layout randomization bypasses or targeted privilege escalation attempts if combined with other vulnerabilities.
In terms of threat modeling and detection frameworks, this vulnerability maps closely to ATT&CK technique T1083, which covers File and Directory Discovery, specifically in contexts where system state is enumerated through unintended channels. More broadly, it falls under the category of side-channel information leaks that compromise confidentiality without necessarily disrupting availability or integrity immediately. The risk is particularly acute on systems running mixed 32-bit and 64-bit applications, as these environments rely heavily on compatibility layers like compat32 to function correctly. Attackers with local access can exploit this flaw repeatedly to gather statistical data about kernel memory usage, potentially leading to more sophisticated exploits that depend on knowing specific pointer values or object addresses within the kernel space.
Mitigation strategies primarily involve patching the affected kernel version to ensure proper initialization of all stack-allocated structures before use. Developers must enforce a strict policy where any local variable used for translating data between user and kernel spaces is explicitly zeroed out using functions such as memset or by declaring it with an initializer that sets all bytes to zero. Additionally, static analysis tools should be configured to flag instances where structs are copied from user space without prior initialization of the destination buffer if not all fields are guaranteed to be overwritten. For system administrators unable to apply patches immediately, restricting access to kevent-related functionality through mandatory access control policies or namespace isolation can reduce the attack surface available to unprivileged users. Regular auditing of compatibility layer code for similar uninitialized memory usage patterns is also recommended to prevent recurrence across other subsystems that handle cross-architecture data translation.