CVE-2026-76259: Splunk Enterprise Auth Bypass Vulnerability

CVE-2026-76259 Overview

CVE-2026-76259 affects Splunk Enterprise for Windows versions below 10.4.2, 10.2.6, 10.0.9, 9.4.13, and 9.3.14. A local user with access to the Windows host can bind to the Splunk management port before the Splunk Enterprise service starts. Once Splunk starts, the attacker's rogue listener intercepts authentication tokens sent by child processes. The attacker then uses those tokens to compromise data and system integrity accessible to the account running Splunk Enterprise. The flaw exists because the Windows management-port listener does not apply exclusive address binding protections before the service starts. This is a local privilege escalation issue classified under [CWE-269: Improper Privilege Management].

Critical Impact
Local attackers can hijack Splunk authentication tokens and inherit the privileges of the Splunk Enterprise service account, exposing all data and operations available to that account.

Affected Products

Splunk Enterprise for Windows versions below 10.4.2 on the 10.4 branch
Splunk Enterprise for Windows versions below 10.2.6 and 10.0.9
Splunk Enterprise for Windows versions below 9.4.13 and 9.3.14

Discovery Timeline

2026-08-19 - CVE-2026-76259 published to NVD
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-76259

Vulnerability Analysis

The vulnerability enables a local, authenticated user to pre-empt the Splunk management port on Windows. Because the listener does not set exclusive address binding before the Splunk Enterprise service initializes, an attacker-controlled process can bind first and receive traffic intended for the legitimate service. Splunk child processes then forward authentication tokens to the rogue listener during startup. With these tokens, the attacker impersonates Splunk internal callers and issues privileged management operations. Impact scales with the privileges of the account running the Splunk service, which in typical deployments has broad access to indexed data, configuration, and search operations.

Root Cause

The Windows management-port listener in Splunk Enterprise omits the SO_EXCLUSIVEADDRUSE binding protection during service initialization. Windows allows multiple sockets to bind the same port unless explicit exclusivity is requested, so a lower-privileged process that binds first retains the socket. When Splunk Enterprise starts, the operating system routes connections to the earlier binder. This design gap is a privilege management failure classified under [CWE-269].

Attack Vector

Exploitation requires local access and low privileges on the Windows host. The attacker binds to the Splunk management port before the Splunk service starts, for example after a reboot or a scheduled restart. The attacker then waits for Splunk child processes to connect and submit authentication tokens. Captured tokens are replayed against the legitimate Splunk management interface to execute operations as the Splunk service account. No user interaction is required. See the Splunk Security Advisory SVD-2026-0801 for vendor technical details.

Detection Methods for CVE-2026-76259

Indicators of Compromise

Unexpected non-Splunk processes owning the Splunk management port (default 8089) prior to splunkd.exe startup.
Splunk startup logs showing failed binds, port conflicts, or unusual authentication token issuance to unknown child processes.
Windows Event Log entries recording new listeners on the management port from user-context processes rather than the Splunk service account.

Detection Strategies

Baseline which process owns TCP 8089 immediately after boot and alert on deviations using netstat -ano or Get-NetTCPConnection telemetry.
Correlate process creation events with subsequent socket bind operations on Splunk management ports to identify pre-service squatting.
Monitor Splunk internal logs for anomalous authentication token requests originating from unexpected local endpoints.

Monitoring Recommendations

Ingest Windows Sysmon Event ID 3 (network connection) and process creation events into a centralized analytics platform for continuous review.
Track service start ordering to ensure SplunkForwarder/Splunkd services are the first to bind their management ports on host boot.
Alert on any local user account interacting with TCP 8089 outside of documented administrative workflows.

How to Mitigate CVE-2026-76259

Immediate Actions Required

Upgrade Splunk Enterprise for Windows to 10.4.2, 10.2.6, 10.0.9, 9.4.13, 9.3.14, or later per the vendor advisory.
Restrict interactive and remote logon rights on Windows hosts running Splunk Enterprise to trusted administrators only.
Audit all local accounts on Splunk hosts and remove unnecessary users, particularly those with the ability to run arbitrary processes.

Patch Information

Splunk released fixed versions in Splunk Security Advisory SVD-2026-0801. Upgrade to 10.4.2 on the 10.4 branch, 10.2.6 on the 10.2 branch, 10.0.9 on the 10.0 branch, 9.4.13 on the 9.4 branch, or 9.3.14 on the 9.3 branch. The fix applies exclusive address binding to the Windows management-port listener, preventing rogue processes from claiming the port before service start.

Workarounds

If patching is not immediately possible, restrict local logon on Splunk Enterprise hosts to a minimal set of administrators.
Configure Windows host firewall rules to limit access to the management port to loopback and required management sources.
Ensure Splunk services are set to start automatically at boot before any interactive user sessions become available.

bash

# Verify Splunk management port ownership on Windows
Get-NetTCPConnection -LocalPort 8089 | Select-Object LocalAddress,LocalPort,OwningProcess,State
Get-Process -Id (Get-NetTCPConnection -LocalPort 8089).OwningProcess