EUVD-2026-39776
GHSA-hvc9-hvw8-q2fx
Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Jail boundary bypass constitutes a scope change (S:C); exploitation requires only a local low-privileged account; impact is availability-only from signal delivery.
Primary rating from Vendor (freebsd).
CVSS VectorVendor: freebsd
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
Lifecycle Timeline
3DescriptionCVE.org
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).
AnalysisAI
Signal delivery bypass in FreeBSD's thr_kill2(2) system call allows an unprivileged local user - or a jailed process - to send arbitrary signals to any process on the system, including root-owned processes and critical daemons, causing Denial of Service. The flaw stems from a missing check on the return value of p_cansignal(), the kernel's permission enforcement primitive: the signal is unconditionally delivered before the error is propagated to the caller. …
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Attack ChainAIDerived
Hypothetical attack flow derived from CVE metadata
Vulnerability AssessmentAI
| Exploitation | Requires a local unprivileged account on the affected FreeBSD system (AV:L, PR:L) - remote exploitation is not possible without a pre-existing local foothold. … Additional conditions and limiting factors are described in the full assessment. |
| Risk Assessment | The official CVSS 3.1 score of 5.5 (AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H) accurately reflects the local, low-privilege exploitation vector and availability-only impact, but assigns Unchanged scope despite the advisory explicitly describing jail boundary bypass - this is a meaningful understatement of risk for jail-based multi-tenant FreeBSD deployments where cross-jail and host signaling represent a scope violation. … Full risk analysis with EPSS, KEV, and SSVC signal comparison available after sign-in. |
| Exploit Scenario | A low-privileged local user or a process running inside a FreeBSD jail iterates through globally sequential thread IDs - requiring no prior knowledge of the target - and repeatedly calls thr_kill2() with candidate PID/TID pairs targeting a root-owned system daemon such as syslogd, cron, or ntpd. The kernel delivers SIGKILL to each matched thread regardless of the p_cansignal() result, terminating the daemon. … |
| Remediation | Apply the vendor-released patches documented in FreeBSD Security Advisory FreeBSD-SA-26:25.thr.asc (https://security.freebsd.org/advisories/FreeBSD-SA-26:25.thr.asc): upgrade FreeBSD 15.0-RELEASE to p10 or later, FreeBSD 14.3-RELEASE to p15 or later, and FreeBSD 14.4-RELEASE to p6 or later using freebsd-update or from source. … Detailed patch versions, workarounds, and compensating controls in full report. |
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Same weakness CWE-269 – Improper Privilege Management
View allSame technique Privilege Escalation
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External POC / Exploit Code
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