CVE-2026-45250 Overview
CVE-2026-45250 is a kernel stack buffer overflow in the FreeBSD setcred(2) system call. The vulnerability stems from the kernel copying a user-supplied supplementary groups list into a fixed-size stack buffer before validating its length. Although setcred(2) is intended for privileged callers, the privilege check executes after the unsafe copy. An unprivileged local user can therefore trigger the overflow without holding elevated rights. The flaw is classified under [CWE-121] (Stack-based Buffer Overflow).
Critical Impact
Successful exploitation allows an unprivileged local user to execute arbitrary code in kernel context and obtain root privileges on the affected FreeBSD system.
Affected Products
- FreeBSD operating system kernel implementing the setcred(2) system call
- Refer to the FreeBSD Security Advisory for specific affected releases and patch levels
Discovery Timeline
- 2026-05-21 - CVE-2026-45250 published to NVD
- 2026-05-21 - Last updated in NVD database
Technical Details for CVE-2026-45250
Vulnerability Analysis
The setcred(2) system call lets a process replace its credentials, including the supplementary group list. The kernel implementation accepts a user-supplied groups array along with its declared count. Inside the syscall handler, the kernel copies that array into a fixed-size buffer allocated on the kernel stack. The copy proceeds before the length of the supplied list is validated against the buffer capacity. When an attacker supplies a group count larger than the buffer can hold, the copy writes past the buffer boundary into adjacent kernel stack memory.
Overwriting the kernel stack corrupts saved registers, return addresses, and stack canaries depending on the build configuration. An attacker with control over the overflowed bytes can redirect kernel control flow and execute arbitrary code at ring 0.
Root Cause
The defect is an ordering bug. The privilege check that gates setcred(2) to privileged callers, and the length check on the supplementary groups list, both run after the kernel has already copied attacker-controlled data into the stack buffer. The validation is therefore unreachable before memory corruption occurs. This is a textbook [CWE-121] stack buffer overflow caused by missing input length validation prior to a bounded copy.
Attack Vector
Exploitation requires local access. An unprivileged user invokes setcred(2) with a crafted struct setcred whose supplementary groups field references an oversized array. Because the syscall returns an error only after the unsafe copy, the attacker does not need to bypass the privilege gate to reach the vulnerable code path. Refer to the FreeBSD Security Advisory for the precise field layout and reproduction details.
Detection Methods for CVE-2026-45250
Indicators of Compromise
- Unexpected kernel panics or page faults referencing the setcred syscall path in /var/log/messages or core dumps
- Unprivileged processes that transition to UID 0 without invoking su, sudo, or a setuid binary
- Userland binaries invoking setcred(2) from non-root contexts, which is unusual outside of privileged daemons
Detection Strategies
- Audit syscall telemetry for setcred(2) calls originating from unprivileged UIDs and correlate with subsequent privilege changes
- Monitor dtrace or auditd records for abnormally large sc_supp_groups_nb values passed to setcred
- Hunt for kernel crash artifacts that include the setcred symbol in the backtrace, which may indicate failed exploitation attempts
Monitoring Recommendations
- Forward FreeBSD audit logs and kernel crash dumps to a centralized log platform for retention and correlation
- Alert on new local binaries that issue setcred(2) and have not been previously baselined
- Track post-exploitation behavior such as new root shells, modifications to /etc/master.passwd, and loading of unsigned kernel modules
How to Mitigate CVE-2026-45250
Immediate Actions Required
- Apply the FreeBSD kernel patch referenced in the FreeBSD Security Advisory and reboot affected hosts
- Restrict local shell access on multi-user FreeBSD systems until patching is complete
- Inventory all FreeBSD assets, including jails and appliances built on FreeBSD, to confirm patch coverage
Patch Information
FreeBSD has published fixes through the FreeBSD Security Advisory FreeBSD-SA-26:18.setcred. Administrators should apply the vendor-supplied source patch or upgrade to a corrected release using freebsd-update fetch install followed by a reboot. Consult the advisory for the exact branch-to-patch mapping and supported upgrade paths.
Workarounds
- No supported configuration workaround removes the vulnerable code path; patching is required
- Reduce exposure by limiting interactive and SSH access to the system to trusted administrative users only
- Where feasible, run untrusted workloads inside jails with reduced syscall surface and monitor those jails for kernel anomalies
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
