CVE-2026-20139: Splunk Enterprise DoS Vulnerability

CVE-2026-20139 Overview

A client-side denial-of-service vulnerability exists in Splunk Enterprise and Splunk Cloud Platform that allows low-privileged users to inject malicious payloads through the password change REST API endpoint. The vulnerability resides in the /splunkd/__raw/services/authentication/users/username endpoint where the realname, tz, or email parameters can be manipulated to cause resource exhaustion in the client browser, resulting in degraded page performance or temporary unresponsiveness of Splunk Web.

Critical Impact

Low-privileged attackers without "admin" or "power" roles can craft malicious payloads that significantly slow page load times or render Splunk Web temporarily unresponsive, impacting security operations and monitoring capabilities.

Affected Products

• Splunk Enterprise versions below 10.2.0, 10.0.2, 9.4.8, 9.3.9, and 9.2.12
• Splunk Cloud Platform versions below 10.2.2510.3, 10.1.2507.8, 10.0.2503.9, and 9.3.2411.121

Discovery Timeline

• 2026-02-18 - CVE CVE-2026-20139 published to NVD
• 2026-02-19 - Last updated in NVD database

Technical Details for CVE-2026-20139

Vulnerability Analysis

This vulnerability is classified as CWE-400 (Uncontrolled Resource Consumption), enabling client-side denial-of-service attacks through improperly validated user input parameters. The flaw allows authenticated users with minimal privileges to inject specially crafted payloads into user profile fields during password change operations.

The attack exploits insufficient input validation in the REST API endpoint responsible for handling user authentication data modifications. When a low-privileged user submits a password change request, they can include malicious content in the realname, tz (timezone), or email parameters that, when rendered by the Splunk Web interface, causes excessive resource consumption in the client's browser.

This vulnerability is particularly concerning in enterprise environments where Splunk serves as a critical security information and event management (SIEM) platform. A successful attack could disrupt security analysts' ability to monitor and respond to security events during the denial-of-service condition.

Root Cause

The root cause of this vulnerability is improper input validation and sanitization of user-controllable parameters in the authentication REST API. The realname, tz, and email fields do not adequately restrict the size or complexity of input values, allowing attackers to inject payloads designed to exhaust client-side resources when the affected user profile data is rendered in the Splunk Web interface.

Attack Vector

The attack is network-based and requires only low-privileged authentication. An attacker with basic Splunk user credentials (without "admin" or "power" roles) can exploit this vulnerability by:

1. Authenticating to Splunk with minimal privileges
2. Initiating a password change request to the vulnerable REST API endpoint
3. Injecting a malicious payload into the realname, tz, or email parameters
4. When other users or administrators view the affected user profile or related pages, the malicious payload causes browser resource exhaustion

The vulnerability mechanism involves crafting payloads that, when processed by the browser during page rendering, trigger excessive computation or memory allocation. This could include deeply nested structures, extremely long strings, or other constructs that exploit client-side parsing behavior. See the Splunk Security Advisory SVD-2026-0204 for complete technical details.

Detection Methods for CVE-2026-20139

Indicators of Compromise

• Unusual or excessively large values in user profile fields (realname, tz, email) within Splunk user accounts
• Abnormal REST API requests to /splunkd/__raw/services/authentication/users/username with unusually large parameter payloads
• User complaints of slow Splunk Web interface performance or browser unresponsiveness when viewing user management pages
• Repeated password change requests from low-privileged accounts with suspicious parameter values

Detection Strategies

• Monitor Splunk internal logs for REST API calls to the /splunkd/__raw/services/authentication/users/ endpoint with parameter sizes exceeding normal thresholds
• Implement alerting on user profile modifications that contain unusual patterns or excessive data lengths
• Configure Splunk to audit authentication-related API endpoint activity and flag anomalous password change requests
• Use SentinelOne Singularity to detect browser-based resource exhaustion patterns indicative of client-side DoS attacks

Monitoring Recommendations

• Enable detailed audit logging for all Splunk REST API authentication endpoints
• Set up dashboards to monitor user profile field modifications and identify outliers
• Configure browser performance monitoring for Splunk Web administrators to detect degradation patterns
• Implement automated alerts when REST API parameters exceed defined size thresholds

How to Mitigate CVE-2026-20139

Immediate Actions Required

• Upgrade Splunk Enterprise to version 10.2.0, 10.0.2, 9.4.8, 9.3.9, 9.2.12 or later depending on your version branch
• Upgrade Splunk Cloud Platform to version 10.2.2510.3, 10.1.2507.8, 10.0.2503.9, 9.3.2411.121 or later
• Review user profile fields for any existing malicious or suspicious content and remediate affected accounts
• Implement additional input validation at the network perimeter for Splunk REST API endpoints

Patch Information

Splunk has released security updates addressing this vulnerability. Refer to the Splunk Security Advisory SVD-2026-0204 for official patch information and download links. Organizations should prioritize patching based on their Splunk deployment's role in security operations and the potential impact of service degradation.

Workarounds

• Implement web application firewall (WAF) rules to limit parameter sizes for the affected REST API endpoint
• Restrict access to the /splunkd/__raw/services/authentication/users/ endpoint to only necessary users and networks
• Audit existing low-privileged user accounts and remove unnecessary access to password change functionality
• Consider implementing additional client-side resource limits or timeouts in browsers accessing Splunk Web

Organizations should refer to the official Splunk security advisory for specific configuration guidance and recommended mitigation steps until patches can be applied.