EUVD-2026-39965Severity by source
AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:H
Local unprivileged user (AV:L, PR:L) reliably preloads code into a setuid-root binary, yielding full C/I/A control, so C:H rather than the reported C:N.
Primary rating from Vendor (freebsd).
CVSS VectorVendor: freebsd
Lifecycle Timeline
3DescriptionCVE.org
The Linuxulator determined whether a binary was set-user-ID or set-group-ID by checking the P_SUGID process flag. During execve(2), this flag is not yet set at the point where the auxiliary vector is constructed, so AT_SECURE was incorrectly set to zero for set-user-ID and set-group-ID executables.
An unprivileged local user can inject a shared library via LD_PRELOAD into a set-user-ID or set-group-ID Linux binary, gaining the privileges of that binary.
AnalysisAI
Local privilege escalation in FreeBSD's Linux binary compatibility layer (Linuxulator) allows an unprivileged user to preload a malicious shared library into set-user-ID or set-group-ID Linux binaries. Because the P_SUGID flag is not yet set when execve(2) builds the auxiliary vector, AT_SECURE is written as zero, so the Linux runtime loader fails to disable LD_PRELOAD and other unsafe behaviors for privileged binaries. Affects FreeBSD 14.3, 14.4, and 15.0 release branches; no public exploit identified at time of analysis, and EPSS risk is low (0.14%, 4th percentile).
Technical ContextAI
The Linuxulator is FreeBSD's kernel-level ABI compatibility subsystem that lets native Linux ELF binaries run on FreeBSD. The Linux dynamic loader (ld.so) relies on the AT_SECURE entry in the ELF auxiliary vector to decide whether a process is running in 'secure mode' - when AT_SECURE is 1, the loader ignores attacker-influenced environment variables such as LD_PRELOAD and LD_LIBRARY_PATH. FreeBSD's implementation derived AT_SECURE from the P_SUGID process flag, but during execve(2) that flag is set only after the auxiliary vector has already been constructed, producing a time-of-construction ordering bug in which AT_SECURE is always emitted as zero for setuid/setgid Linux binaries. This maps to CWE-266 (Incorrect Privilege Assignment): a privileged execution context is not correctly recognized as privileged, so the security hardening that should apply to setuid/setgid programs is silently skipped.
RemediationAI
Vendor-released patch: update to FreeBSD 14.4-RELEASE-p6, 14.3-RELEASE-p15, or 15.0-RELEASE-p10 (or later) as applicable to your branch, following FreeBSD-SA-26:30.linux (https://security.freebsd.org/advisories/FreeBSD-SA-26:30.linux.asc); apply via freebsd-update or by rebuilding the kernel from patched sources, then reboot so the corrected execve/auxiliary-vector code path is loaded. If patching must be deferred, the most effective compensating control is to unload/disable the Linux compatibility layer entirely by removing the linux kernel module (kldunload linux linux64) and preventing linux_enable in rc.conf - this eliminates the vulnerable code path but breaks any workloads that depend on running Linux binaries. Where the Linuxulator must remain enabled, reduce exposure by removing the setuid/setgid bit from Linux binaries that do not strictly require it and by restricting untrusted local/interactive access to hosts that run privileged Linux executables; note these controls only shrink the attack surface and are not a substitute for the fix.
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Same weakness CWE-266 – Incorrect Privilege Assignment
View allSame technique Code Injection
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External POC / Exploit Code
Leaving vuln.today