CVE-2026-49415: SUID Binary Privilege Escalation Flaw

CVE-2026-49415 Overview

CVE-2026-49415 is a race condition vulnerability in the FreeBSD kernel's execve(2) system call. During execution of a SUID binary, the kernel installs the new virtual address space before updating the process credentials. This creates a time-of-check to time-of-use (TOCTOU) window where a co-user process can access the target's memory through procfs or linprocfs. The kernel's debugging permission check still evaluates against the original credentials during this window. An unprivileged local user can exploit this race to modify the address space of a SUID binary before its credentials are elevated. Successful exploitation grants full control of the affected system.

Critical Impact
Local privilege escalation to root through a race condition against SUID binary execution, enabling full system compromise.

Affected Products

FreeBSD operating system (see FreeBSD Security Advisory)
Systems with procfs mounted
Systems with linprocfs mounted for Linux compatibility

Discovery Timeline

2026-08-19 - CVE-2026-49415 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-49415

Vulnerability Analysis

The vulnerability is a time-of-check to time-of-use race condition classified as [CWE-367]. The FreeBSD kernel performs SUID binary execution in a specific sequence during execve(2). It installs the new virtual address space, containing the SUID target's code and data, before it updates the process credentials to reflect the elevated privileges.

Between these two operations, the process still carries the calling user's credentials. Any other process running under that same user can attach to the target through procfs or linprocfs. The kernel's debugger access check evaluates against the pre-execve credentials, granting access to what will imminently become a privileged address space.

An attacker who wins this race can write to the target's memory pages before credential elevation completes. Modifying executable pages or stack contents redirects execution once the SUID binary begins running with root privileges.

Root Cause

The root cause is incorrect ordering of security-critical operations inside execve(2). Credential updates must complete before the new address space becomes reachable through debugging interfaces. FreeBSD's implementation installs the address space first, leaving a window where the old credentials govern access checks against the new memory.

Attack Vector

Exploitation requires local access as an unprivileged user. The attacker spawns a helper process, then invokes a SUID binary in a sibling process. The helper opens the target's memory through /proc/<pid>/mem or the linprocfs equivalent during the race window. Writing crafted bytes to the address space hijacks control flow once credentials transition to root. The FreeBSD Security Advisory provides authoritative technical details on the vulnerable code path.

Detection Methods for CVE-2026-49415

Indicators of Compromise

Unexpected procfs or linprocfs access to memory files (/proc/*/mem) of SUID processes by unprivileged users
Repeated execve(2) invocations of SUID binaries from the same user session within tight timing windows
New root-owned processes spawned from unprivileged user shells without a legitimate sudo or su audit trail
Modifications to SUID binary behavior at runtime that diverge from binary contents on disk

Detection Strategies

Audit execve system calls invoking SUID binaries and correlate with concurrent open calls against the same PID's /proc memory entries
Monitor procfs and linprocfs mounts for cross-process memory access patterns typical of TOCTOU exploitation
Alert on unprivileged processes writing to /proc/<pid>/mem where the target PID is executing a SUID binary

Monitoring Recommendations

Enable FreeBSD audit framework (auditd) rules covering execve and procfs file operations
Forward host telemetry to a central data lake for correlation of race-condition signatures across the fleet
Track privilege transitions and flag processes that gain root without an authenticated escalation path

How to Mitigate CVE-2026-49415

Immediate Actions Required

Apply the FreeBSD kernel patch referenced in FreeBSD-SA-26:39.execve on all supported releases
Reboot affected systems after patching to load the corrected kernel
Inventory all SUID binaries and remove the SUID bit from any binary that does not require it
Restrict local user access on multi-tenant FreeBSD hosts until patches are deployed

Patch Information

FreeBSD has released a kernel update that corrects the ordering of credential updates and address space installation within execve(2). Refer to the FreeBSD Security Advisory for supported release versions, patch commit hashes, and installation procedures using freebsd-update or source builds.

Workarounds

Unmount procfs and linprocfs filesystems where they are not operationally required
Set security.bsd.unprivileged_proc_debug=0 via sysctl to restrict cross-process debugging access
Reduce the SUID binary footprint through find / -perm -4000 audits and privilege minimization

bash

# Configuration example - restrict unprivileged debugging and audit SUID binaries
sysctl security.bsd.unprivileged_proc_debug=0
echo 'security.bsd.unprivileged_proc_debug=0' >> /etc/sysctl.conf
umount /proc 2>/dev/null
umount /compat/linux/proc 2>/dev/null
find / -xdev -type f -perm -4000 -exec ls -l {} \;