CVE-2026-58088: ELF Core Dump Privilege Escalation Flaw

CVE-2026-58088 Overview

CVE-2026-58088 is a race condition vulnerability in the FreeBSD kernel's ELF core dump code. The kernel counts dumpable Virtual Memory (VM) map entries, allocates a buffer for the corresponding program headers, then iterates the map a second time to populate the buffer. A process sharing the address space through rfork(2) can mutate the map between these two passes. The second pass then writes program headers past the end of the allocated buffer, producing an out-of-bounds write on the kernel heap. An unprivileged local user sharing an address space with a process that dumps core can trigger the condition and potentially escalate privileges. The flaw is classified under [CWE-362] (Concurrent Execution using Shared Resource with Improper Synchronization).

Critical Impact
An unprivileged local user can trigger a kernel heap out-of-bounds write, potentially leading to privilege escalation on affected FreeBSD systems.

Affected Products

FreeBSD operating system (kernel ELF core dump subsystem)
Systems where processes use rfork(2) to share address space
Refer to the FreeBSD Security Advisory for specific affected releases

Discovery Timeline

2026-08-19 - CVE-2026-58088 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-58088

Vulnerability Analysis

The vulnerability resides in the kernel routine that produces ELF core dumps for crashed or signaled processes. The routine performs a two-pass algorithm over the VM map. The first pass counts eligible entries and sizes a program header buffer accordingly. The second pass walks the map again and writes one program header per entry into that buffer.

Between the two passes, the VM map is not held in a state that prevents concurrent modification by other threads sharing the same address space. When rfork(2) creates a process that shares the parent's VM space, that sibling process can add new mappings after the count completes but before the second pass finishes. The second pass then observes more entries than were counted and writes past the buffer boundary into adjacent kernel heap memory.

Root Cause

The root cause is a Time-of-Check to Time-of-Use (TOCTOU) race between the counting pass and the population pass. The buffer size is derived from stale data, so any concurrent growth of the VM map turns the second pass into a linear heap overflow. The lack of a consistent lock across both passes allows the mutation window to exist.

Attack Vector

Exploitation requires local access and a process pair sharing an address space via rfork(2). The attacker triggers a core dump in one process while the sibling adds VM map entries in a tight loop. The resulting out-of-bounds write can corrupt adjacent kernel heap objects. Skilled attackers can shape the heap to overwrite function pointers or credential structures, enabling privilege escalation from an unprivileged user to root.

No public proof-of-concept exploit is currently available. See the FreeBSD Security Advisory for authoritative technical detail.

Detection Methods for CVE-2026-58088

Indicators of Compromise

Unexpected kernel panics or memory corruption faults following core dump events on multiuser FreeBSD hosts
Processes invoking rfork(2) with RFMEM immediately before or during core-dumping activity of a sibling process
Anomalous privilege transitions from unprivileged UIDs to UID 0 without a corresponding su, sudo, or setuid binary execution

Detection Strategies

Audit process creation telemetry for rfork usage combined with signals that generate core dumps (SIGSEGV, SIGABRT, SIGQUIT) targeting the sibling
Correlate kernel log entries referencing core dump activity with subsequent kernel memory allocator warnings or UMA sanity check failures
Monitor /var/log/messages and dmesg for kernel diagnostic output indicating heap metadata corruption

Monitoring Recommendations

Enable process accounting and audit ELF core dump generation events across production hosts
Track privilege escalation events using host-based telemetry that captures effective UID changes per process
Alert on repeated core dumps from the same user within short time windows, which may indicate exploitation attempts

How to Mitigate CVE-2026-58088

Immediate Actions Required

Apply the vendor patch published in the FreeBSD Security Advisory as soon as it is available for your release
Restrict local shell access on multiuser FreeBSD systems until patches are applied
Review workloads that rely on rfork(2) with shared memory semantics and treat those hosts as higher priority for patching

Patch Information

FreeBSD has issued security advisory FreeBSD-SA-26:55.elf addressing the ELF core dump race condition. Administrators should follow the advisory's freebsd-update or source patch instructions for supported releases and reboot the affected systems to load the corrected kernel.

Workarounds

Disable core dump generation for untrusted users by setting kern.coredump=0 via sysctl where core dumps are not operationally required
Set the resource limit RLIMIT_CORE to zero for interactive user sessions using login.conf class defaults
Restrict rfork(2) usage through mandatory access control policies such as mac_bsdextended on hosts that expose local shell access to untrusted users

bash

# Configuration example: disable core dumps system-wide until patched
sysctl kern.coredump=0
echo 'kern.coredump=0' >> /etc/sysctl.conf

# Enforce zero core file size for default login class
# Edit /etc/login.conf and set:
#   :coredumpsize=0:
cap_mkdb /etc/login.conf