CVE-2026-18453: 389 Directory Server DOS Vulnerability

CVE-2026-18453 Overview

A null pointer dereference vulnerability affects the 389 Directory Server, an enterprise Lightweight Directory Access Protocol (LDAP) implementation used in identity management deployments. The flaw resides in the paged results handling logic of the op_shared_search function. A missing NULL pointer check allows an unauthenticated remote attacker to crash the LDAP daemon by sending a crafted sequence of search requests that leverage the USE_ONE_BACKEND control. Successful exploitation results in denial of service against the directory server, disrupting authentication and directory lookup services across dependent systems. The vulnerability is tracked as [CWE-476: NULL Pointer Dereference].

Critical Impact
Unauthenticated remote attackers can crash 389 Directory Server instances over the network without any user interaction, disrupting authentication services enterprise-wide.

Affected Products

389 Directory Server (upstream project)
Red Hat Directory Server
Red Hat Enterprise Linux Identity Management (IdM) components using 389-ds-base

Discovery Timeline

2026-09-07 - CVE-2026-18453 published to the National Vulnerability Database
2026-09-09 - Last updated in NVD database

Technical Details for CVE-2026-18453

Vulnerability Analysis

The defect lives in the op_shared_search function of 389 Directory Server, which orchestrates LDAP search operations. When a client submits a paged search request together with the USE_ONE_BACKEND control, the code path responsible for tracking paged results state fails to validate a pointer before dereferencing it.

An attacker can trigger the fault by issuing a specific sequence of search operations that place the internal paged results structure into an unexpected state. When the server later processes the follow-up request, it dereferences a NULL pointer and the ns-slapd process crashes. Because the LDAP bind requirement is not enforced for the vulnerable code path, no credentials are required to trigger the crash.

The impact is limited to availability. The directory server terminates, blocking authentication, group lookups, and policy retrieval for every downstream application that depends on it.

Root Cause

The root cause is a missing NULL pointer check in the paged results handling logic. The code assumes a data structure related to the backend selection made by USE_ONE_BACKEND will always be populated when the paged results code path executes. Under the crafted request sequence, that assumption does not hold, and the unchecked dereference triggers a segmentation fault.

Attack Vector

Exploitation requires only network reachability to the LDAP port (typically TCP 389 or 636). An attacker sends a crafted LDAP search request that uses simple paged results together with the USE_ONE_BACKEND control, followed by additional operations that drive the server into the vulnerable state. No authentication and no user interaction are required.

The vulnerability affects the availability of directory services. Confidentiality and integrity are not directly impacted, but cascading effects on authentication systems can extend the outage across dependent workloads.

Refer to the Red Hat CVE-2026-18453 advisory and Red Hat Bug Report #2509696 for detailed technical analysis.

Detection Methods for CVE-2026-18453

Indicators of Compromise

Unexpected termination or repeated restarts of the ns-slapd process on directory server hosts.
Segmentation fault entries or core dumps referencing op_shared_search in system logs.
LDAP client applications reporting connection resets or bind failures across multiple services simultaneously.

Detection Strategies

Parse 389 Directory Server access logs for search operations that include the paged results control combined with the USE_ONE_BACKEND control from unauthenticated or anonymous binds.
Alert on abnormal spikes in LDAP search request volume from a single source followed by service unavailability.
Correlate ns-slapd process crashes with preceding LDAP request patterns to identify probing attempts.

Monitoring Recommendations

Enable systemd or process supervisor alerting on ns-slapd service failures and restart events.
Forward 389 Directory Server access, error, and audit logs to a centralized logging platform for retention and correlation.
Monitor authentication success rates from downstream applications as a leading indicator of directory service outages.

How to Mitigate CVE-2026-18453

Immediate Actions Required

Apply the Red Hat security updates released under advisories RHSA-2026:64771 through RHSA-2026:64804 and RHSA-2026:65119 for affected Red Hat Enterprise Linux and Directory Server versions.
Restrict network access to LDAP ports (389/TCP and 636/TCP) using host-based or perimeter firewalls to limit exposure to trusted subnets only.
Restart ns-slapd after patching and validate that paged search operations continue to function correctly.

Patch Information

Red Hat has released fixes across multiple product streams. See Red Hat Security Advisory RHSA-2026:64771, RHSA-2026:64776, RHSA-2026:64778, RHSA-2026:64779, RHSA-2026:64780, RHSA-2026:64781, RHSA-2026:64783, RHSA-2026:64784, RHSA-2026:64785, RHSA-2026:64789, RHSA-2026:64790, RHSA-2026:64791, RHSA-2026:64792, RHSA-2026:64793, RHSA-2026:64804, and RHSA-2026:65119. Update 389-ds-base to the patched version provided for your distribution.

Workarounds

Limit LDAP exposure to trusted management networks and identity consumers using firewall rules until patches are applied.
Require authenticated binds for search operations where operationally feasible, and monitor for anonymous search traffic.
Deploy a load-balanced or replicated directory topology so that a crash of one instance does not eliminate authentication capability.

bash

# Example: restrict LDAP access to trusted subnets using firewalld
sudo firewall-cmd --permanent --zone=public --remove-service=ldap
sudo firewall-cmd --permanent --zone=public --remove-service=ldaps
sudo firewall-cmd --permanent --new-zone=ldap-trusted
sudo firewall-cmd --permanent --zone=ldap-trusted --add-source=10.0.0.0/8
sudo firewall-cmd --permanent --zone=ldap-trusted --add-service=ldap
sudo firewall-cmd --permanent --zone=ldap-trusted --add-service=ldaps
sudo firewall-cmd --reload

# Update 389-ds-base on Red Hat Enterprise Linux
sudo dnf update 389-ds-base
sudo systemctl restart dirsrv@<instance-name>