EUVD-2026-39963Severity by source
AV:L/AC:L/PR:L/UI:N/S:C/C:N/I:N/A:H
Local-only attack requiring existing sandbox presence (AV:L, PR:L); scope changes because impact extends to external processes (S:C); only availability impacted via SIGKILL/SIGSTOP (A:H, C:N, I:N).
Primary rating from Vendor (freebsd).
CVSS VectorVendor: freebsd
Lifecycle Timeline
3DescriptionCVE.org
sigqueue(2) was marked as permitted in capability mode with the introduction of Capsicum in 2011, but the implementation of kern_sigqueue did not include a capability mode check restricting signal delivery to the calling process's own PID.
A process in capability mode can use sigqueue(2) to send signals to any process it could signal following standard Unix permissions, bypassing the Capsicum sandbox restriction. A compromised sandboxed process could interfere with other processes, for example by sending SIGKILL or SIGSTOP. This could be any process running as the same user, or any process, for a superuser sandboxed process.
AnalysisAI
Capsicum sandbox escape in FreeBSD allows a compromised capability-mode process to send arbitrary signals - including SIGKILL and SIGSTOP - to processes outside its sandbox, undermining Capsicum's core isolation guarantee. Affected are FreeBSD 14.3-RELEASE through 15.0-RELEASE in versions prior to their respective patch levels; the root cause is a 2011-era omission in kern_sigqueue that never enforced a PID self-restriction when a process operates in capability mode. EPSS is 0.14% (4th percentile), SSVC marks exploitation as none and non-automatable, and no public exploit has been identified at time of analysis.
Technical ContextAI
Capsicum is FreeBSD's capability-based security framework (introduced 2011) that restricts a process to a minimal set of permitted system calls once cap_enter(2) is invoked. The policy designates sigqueue(2) as permitted in capability mode, but the kernel implementation in kern_sigqueue never added a corresponding check requiring that signal delivery be restricted to the calling process's own PID. CWE-266 (Incorrect Privilege Assignment) captures this precisely: a capability was granted in the policy layer without enforcing the intended restriction at the implementation layer. Standard Unix signal permissions then govern which external processes can be reached - same-UID processes for non-root sandboxed processes, or any process system-wide for a sandboxed superuser. Affected CPE: cpe:2.3:a:freebsd:freebsd:*:*:*:*:*:*:*:*.
RemediationAI
Apply the vendor-released patches by upgrading to FreeBSD 15.0-RELEASE-p10, FreeBSD 14.4-RELEASE-p6, or FreeBSD 14.3-RELEASE-p15 as specified in FreeBSD-SA-26:28.capsicum.asc (https://security.freebsd.org/advisories/FreeBSD-SA-26:28.capsicum.asc). For systems where patching is not immediately feasible, reduce blast radius by ensuring Capsicum-sandboxed processes run under dedicated, unprivileged user accounts with no other processes sharing that UID - this limits signal delivery to only the sandboxed process itself under Unix permissions. Avoid running sandboxed workloads as root, as a sandboxed superuser can signal any system process. Note that there is no runtime configuration toggle to disable the flawed sigqueue(2) behavior without patching; these compensating controls reduce scope but do not eliminate the vulnerability.
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Same weakness CWE-266 – Incorrect Privilege Assignment
View allSame technique Authentication Bypass
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External POC / Exploit Code
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