CVE-2026-49414: FreeBSD ASLR Bypass in Setuid Binaries via Timing Flaw

Technical summary

The vulnerability stems from incorrect sequencing in the ELF image activator code path. When executing a setuid binary marked as Position Independent Executable (PIE), the kernel should clear per-process ASLR disable flags before computing the PIE base address. Instead, the code computes the base address first, then clears the flags. An unprivileged local process can exploit this by calling procctl(2) with PR_ASLR to disable ASLR for itself, then execve(2) into a setuid PIE binary. The kernel will honor the user-requested disable and compute a predictable, non-randomized base address, effectively nullifying ASLR for that execution. This is classified under CWE-179 (Improper Neutralization of Value Set in Shared Repository), reflecting the failure to properly enforce the ASLR control before critical calculations.

Business impact

Organizations running FreeBSD in multi-user or containerized environments face elevated risk to privilege escalation attacks. Any memory corruption vulnerability (buffer overflow, use-after-free, etc.) in a setuid binary becomes substantially more exploitable because ASLR can be reliably disabled by an unprivileged attacker. This is particularly concerning for services or daemons that rely on setuid mechanics for privilege separation and assume ASLR as a baseline defense. Affected systems may experience unauthorized privilege escalation, unauthorized data access, and potential system compromise.

Affected systems

FreeBSD is the sole affected vendor and product. The vulnerability impacts all standard FreeBSD releases running the vulnerable ELF image activator code. Verification of specific affected versions should be confirmed against the official FreeBSD security advisory, as the provided data lists multiple FreeBSD entries without explicit version granularity.

Exploitability

This vulnerability is highly exploitable in its preconditions but requires local access. An unprivileged local user with shell access can reliably disable ASLR on setuid binaries with no special tools or elevated privileges. The attack is deterministic—procctl(2) and execve(2) are standard syscalls available to any user process. However, exploitation of the resulting ASLR bypass requires a separate memory corruption vulnerability in the target setuid binary. The CVSS 3.1 vector (AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H, score 7.8 HIGH) reflects the low attack complexity and local-only access requirement, combined with potential for confidentiality, integrity, and availability impact once chained with another defect.

Remediation

Patches must reorder the code in the ELF image activator so that per-process ASLR disable flags are cleared before the PIE base address is computed. This ensures that user-requested ASLR disables cannot override the security policy for setuid binaries. Administrators should monitor for official FreeBSD security patches addressing this timing issue and apply them to production systems as soon as they become available. Until patched, compensating controls such as restricting shell access to untrusted users or reducing reliance on setuid binaries in sensitive contexts may reduce attack surface.

Patch guidance

Consult the FreeBSD security advisory corresponding to this CVE for specific patch versions and installation instructions. Patches should be tested in a non-production environment first to verify compatibility with local FreeBSD configurations and running services. Apply patches to all FreeBSD systems in scope, prioritizing those hosting multi-user environments, containerized workloads, or services that rely on setuid privilege separation. Verify patch application by checking the kernel version and, if available, running regression tests on setuid binaries to confirm ASLR enforcement.

Detection guidance

Monitor system logs for unusual patterns of procctl(2) calls followed by execve(2) into setuid binaries, particularly from unprivileged processes. Observe for failed exploitation attempts (segmentation faults or crashes in setuid programs) that might indicate an attacker testing for memory corruption vulnerabilities now that ASLR has been disabled. Endpoint detection and response (EDR) tools can flag processes that attempt to disable ASLR via procctl(2) immediately before executing setuid code. Post-patch, enable audit logging of procctl(2) calls to establish a baseline of legitimate usage and identify anomalies.

Why prioritize this

This vulnerability merits high priority because it systematically weakens a critical exploit mitigation (ASLR) for privileged binaries. Any organization running FreeBSD in a multi-user context should treat it as an urgent patch candidate. The attack requires only local access and no special privileges, making it a realistic threat in shared hosting, containerized, or lab environments. Chaining this flaw with any existing memory corruption vulnerability in a setuid binary could lead to privilege escalation; therefore, prompt remediation directly reduces exposure to a broad class of secondary exploits.

Risk score, explained

The CVSS 3.1 score of 7.8 (HIGH) reflects a low-barrier local attack (AV:L, AC:L, PR:L) with no user interaction required, and potential for complete system compromise (C:H, I:H, A:H, S:U). The score appropriately captures the severity of the vulnerability as a privilege-escalation enabler, though the actual impact depends on the presence of exploitable memory corruption in setuid binaries on the target system. Organizations with strong input validation and bounds checking in setuid code, or those restricting shell access, may experience lower practical risk.

Frequently asked questions