CVE-2026-76258: Splunk Enterprise Auth Bypass Vulnerability

CVE-2026-76258 Overview

CVE-2026-76258 affects Splunk Enterprise and Splunk Secure Gateway, allowing a low-privileged authenticated user to register an arbitrary companion app. The Splunk Secure Gateway then forwards mobile user requests, including authentication tokens, to an attacker-controlled Uniform Resource Locator (URL). The vulnerability stems from a hard-coded cryptographic key [CWE-321] in the companion app registration handler that permits unrestricted callback URL registration. Attackers who exploit this flaw can compromise all data accessible to the affected mobile user.

Critical Impact
A user without admin or power Splunk roles can hijack mobile session tokens by redirecting Splunk Secure Gateway traffic to an attacker-controlled URL.

Affected Products

Splunk Enterprise versions below 10.4.2, 10.2.6, 10.0.9, and 9.4.14
Splunk Secure Gateway versions below 3.10.10, 3.9.24, and 3.8.71
Deployments exposing Splunk Secure Gateway mobile companion app registration

Discovery Timeline

2026-08-19 - CVE-2026-76258 published to the National Vulnerability Database (NVD)
2026-08-20 - Last updated in NVD database

Technical Details for CVE-2026-76258

Vulnerability Analysis

CVE-2026-76258 resides in the Splunk Secure Gateway companion app registration handler. The handler relies on a hard-coded cryptographic key to validate companion app registrations. Because the key is static and embedded in the application, any authenticated user who obtains it can forge valid registration requests. The vulnerability affects the confidentiality of mobile user tokens because Splunk Secure Gateway subsequently forwards mobile requests, including bearer tokens, to the registered callback URL.

Root Cause

The root cause is the use of a hard-coded cryptographic key [CWE-321] inside the companion app registration flow. Splunk did not enforce role-based restrictions on which callback Uniform Resource Locator (URL) values could be registered. As a result, users without admin or power roles can register callback endpoints that they control. The trust boundary between low-privileged users and mobile session data collapses when the shared key is discoverable.

Attack Vector

An attacker begins with valid Splunk credentials at any role level below admin or power. The attacker crafts a companion app registration request signed with the hard-coded key. The request specifies an attacker-controlled URL as the callback destination. When mobile users interact with Splunk Secure Gateway, their requests and tokens flow to the attacker endpoint. The attacker replays the captured tokens to access data the mobile user can view.

No public proof-of-concept exploit is available at the time of publication. See the Splunk Security Advisory SVD-2026-0801 for vendor-provided technical details.

Detection Methods for CVE-2026-76258

Indicators of Compromise

Splunk Secure Gateway companion app registrations pointing to unfamiliar or external domains
Outbound HTTPS traffic from Splunk hosts to Uniform Resource Locators (URLs) not associated with approved mobile gateway infrastructure
Companion app registration events initiated by accounts without admin or power role assignments
Anomalous token usage patterns for mobile users, including access from unexpected source IP addresses

Detection Strategies

Audit the Splunk Secure Gateway configuration for registered companion apps and validate each callback URL against an allowlist
Search Splunk internal logs (index=_internal) for splunk_secure_gateway registration events performed by non-privileged accounts
Correlate mobile session token issuance with subsequent authentication events from unusual geolocations or user agents

Monitoring Recommendations

Monitor egress traffic from Splunk search heads for connections to previously unseen external domains
Alert on any modification to Splunk Secure Gateway registration state performed by users without administrative capabilities
Track role assignments and capability grants to detect attempts to escalate privileges after initial exploitation

How to Mitigate CVE-2026-76258

Immediate Actions Required

Upgrade Splunk Enterprise to version 10.4.2, 10.2.6, 10.0.9, or 9.4.14, depending on the deployed release line
Upgrade Splunk Secure Gateway to version 3.10.10, 3.9.24, or 3.8.71
Review all existing companion app registrations and remove any callback URLs that cannot be attributed to approved mobile infrastructure
Rotate any mobile user tokens that may have been exposed prior to patching

Patch Information

Splunk released fixed versions addressing CVE-2026-76258. Refer to the Splunk Security Advisory SVD-2026-0801 for the complete patch matrix and upgrade guidance. Administrators should also review Define roles on the Splunk platform with capabilities to align role assignments with least-privilege principles.

Workarounds

Restrict Splunk account creation and limit low-privileged user access to instances running Splunk Secure Gateway until patches are applied
Disable the Splunk Secure Gateway app if mobile access is not required in the environment
Enforce network egress controls that block outbound traffic from Splunk hosts to unapproved destinations

bash

# Configuration example: disable Splunk Secure Gateway when mobile access is not required
$SPLUNK_HOME/bin/splunk disable app splunk_secure_gateway -auth admin:<password>
$SPLUNK_HOME/bin/splunk restart