CVE-2026-45256 in FreeBSD
Summary
by MITRE • 06/26/2026
When used to deliver a signal to a specific thread, thr_kill2(2) called p_cansignal() to determine whether the operation was permitted but did not check the result before delivering the signal. The signal was sent even when the permission check failed. The system call returned the resulting error to the caller, but by then the signal had already been delivered.
The missing check allows an unprivileged local user who knows or can guess a target's process and thread IDs to send any signal to a process they would not normally be permitted to signal, including processes owned by other users or by root. The same check enforces jail boundaries, so a jailed process can signal processes on the host or in other jails. Thread IDs are allocated globally and sequentially, and so can be discovered by brute force with no visibility into the target.
An attacker can stop or terminate arbitrary processes, including critical system daemons, resulting in a Denial of Service (DoS).
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Analysis
by VulDB Data Team • 06/26/2026
This vulnerability exists in the kernel-level signal delivery mechanism where the thr_kill2 system call fails to properly validate permission before executing signal delivery operations. The flaw occurs during the execution flow when p_cansignal() is invoked to determine if a signal operation should be permitted, yet the system does not verify this permission check result prior to sending the actual signal. This represents a classic race condition and authorization bypass scenario where the system reports an error code indicating failure but still performs the unauthorized action. The vulnerability stems from improper control flow management in kernel space operations and can be classified under CWE-691 as insufficient control flow management, while also manifesting as CWE-284 which deals with inadequate access control mechanisms.
The technical implementation of this flaw demonstrates a fundamental security oversight in signal handling within the operating system kernel. When an unprivileged user attempts to deliver a signal to a target thread using thr_kill2, the system should first validate that the requesting process has adequate permissions according to the established security model. However, the missing validation check allows attackers to bypass these security controls entirely, as the permission verification occurs but its result is ignored. This creates an arbitrary signal delivery capability that extends beyond normal privilege boundaries and can be exploited across different user contexts, including system daemons running with elevated privileges or processes owned by root.
The operational impact of this vulnerability is severe and encompasses multiple attack vectors that can compromise system integrity and availability. An attacker with knowledge of process and thread identifiers can effectively target any process on the system regardless of ownership or privilege level, enabling them to send signals such as SIGTERM, SIGKILL, or SIGSTOP that can terminate critical system services. This capability directly enables denial of service attacks against essential system components including network daemons, authentication services, or system monitoring tools. The global allocation and sequential nature of thread identifiers makes this attack surface particularly dangerous as attackers can systematically brute force valid thread IDs without requiring additional reconnaissance or visibility into target processes.
The security implications extend beyond simple DoS attacks to include potential privilege escalation opportunities within containerized environments or jail configurations. Since the same permission check enforces boundaries between different system contexts, a jailed process could potentially signal processes running on the host system or within other security domains, effectively breaking out of isolation boundaries. This represents a critical compromise of the security model's integrity and can undermine the fundamental assumptions underlying containerization technologies and virtualized environments. The vulnerability affects both local privilege escalation scenarios where users can target arbitrary processes and broader system compromise cases where attackers can disrupt essential services.
Mitigation strategies should focus on implementing proper input validation and access control enforcement within kernel space operations, ensuring that permission checks are not only performed but also properly evaluated before any action is taken. System administrators should consider applying kernel patches that correct the control flow logic in thr_kill2 implementations to ensure that permission failures halt signal delivery operations rather than allowing them to proceed. Additionally, monitoring solutions should be implemented to detect unusual signal delivery patterns from unprivileged processes, particularly those targeting system-critical services or processes owned by privileged users. The broader security community should consider this vulnerability as a critical issue requiring immediate attention in all affected kernel versions and implementations.