Brief Summary: CVE-2026-20253 — Unauthenticated File Operations in Splunk Enterprise PostgreSQL Sidecar

Introduction

The platform that many organizations trust to detect threats just became a threat vector itself. CVE-2026-20253 is a critical (CVSS 9.8) unauthenticated vulnerability in Splunk Enterprise and Splunk Cloud Platform. Any attacker who can reach the network can create or truncate arbitrary files through an exposed PostgreSQL sidecar service endpoint, with no credentials required.

Splunk holds the number one position in IDC's SIEM market share rankings for the fifth consecutive year and has been named a Gartner Magic Quadrant Leader for SIEM eleven consecutive times. A vulnerability this severe in the data platform layer that Splunk Enterprise Security runs on means a substantial number of enterprise Security Operations Centers may be exposed. Ironically, the system designed to be the eyes and ears of an organization's security posture can be blinded by an unauthenticated attacker.

Technical Information

Root Cause: CWE-306, Missing Authentication for Critical Function

The vulnerability is classified under CWE-306: Missing Authentication for Critical Function, which MITRE defines as occurring when "the product does not perform any authentication for functionality that requires a provable user identity or consumes a significant amount of resources." CWE-306 appears on MITRE's CWE Top 25 Most Dangerous Software Weaknesses, and its common consequences list "Gain Privileges or Assume Identity" as the primary technical impact.

In this case, the PostgreSQL sidecar service endpoint in Splunk Enterprise performs file creation and file truncation on the host system without any authentication. The endpoint is network-accessible, so anyone who can route packets to it can invoke these file operations.

The PostgreSQL Sidecar Component

The attack surface comes from Splunk's sidecar architecture. According to Splunk documentation, "Sidecars are processes that run alongside splunkd to fulfill specific functions. They are long-running which means that they take a significant" amount of resources and operate continuously. The documentation also notes that "Sidecars can occupy network ports," and that is the mechanism that creates the attack surface for CVE-2026-20253.

The PostgreSQL sidecar handles internal database management tasks for the Splunk platform. Because it exposes a network-accessible endpoint for file operations without authentication, it opens a direct path from the network to the host filesystem.

CVSS v3.1 Vector Analysis

The assigned CVSS vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, scoring 9.8 out of 10. The metrics break down as follows:

This is the most dangerous combination in the CVSS v3.1 framework: a trivially exploitable, unauthenticated, network-accessible vulnerability with maximum impact on all three security properties.

Attack Flow

The exploitation path is direct and requires little sophistication:

1. Reconnaissance: The attacker identifies a network-reachable Splunk Enterprise instance. The PostgreSQL sidecar occupies a network port that standard port scanning can find.

2. Unauthenticated Access: The attacker connects to the PostgreSQL sidecar service endpoint. No credentials, tokens, or session cookies are required, and the endpoint accepts requests from any network-reachable source.

3. File Operations: Through the endpoint, the attacker invokes file creation or file truncation. The advisory does not say how these operations are invoked (HTTP API, direct socket communication, or another protocol). Either way, the attacker can create new files or truncate existing files at arbitrary paths on the host filesystem.

4. Impact Realization: The consequences depend on what the attacker wants.

   Arbitrary File Creation lets the attacker write files to sensitive locations. That could mean planting malicious configurations in Splunk's configuration directories, introducing backdoor accounts, establishing persistence mechanisms, or overwriting critical system files. On a SIEM platform, modifying security policies or detection rules through file creation could silently disable threat detection.

   Arbitrary File Truncation destroys file contents by zeroing them out. Targets could include Splunk configuration files, authentication databases, audit logs, or operating system files. Truncating audit logs is particularly concerning because it destroys forensic evidence of the attack itself. Truncating critical configuration files causes denial of service and could take the SIEM offline during an active intrusion.

On a security monitoring platform, these two primitives together let an attacker both establish a foothold and erase evidence of doing so.

Disclosure and Tracking

Alex Hordijk (hordalex) discovered and responsibly disclosed the vulnerability. Splunk tracks it internally as Bug ID VULN-67169 and externally as SVD-2026-0603. The vulnerability source is listed as [email protected], which reflects how Splunk's security response was folded into Cisco's Product Security Incident Response Team after the $28 billion acquisition completed on March 18, 2024.

Affected Systems and Versions

Splunk Enterprise 10.2.4 was released on May 29, 2026, according to end-of-life tracking data. Version 10.0 was released around July 2025, so the vulnerable window spans nearly a year for organizations on the 10.0 branch.

The vendor is actively patching Splunk Cloud Platform instances. On-premises Splunk Enterprise deployments require manual intervention.

The Splunk advisory explicitly states that no mitigations or workarounds are available for this vulnerability, so patching is the only remediation path. Organizations that cannot upgrade immediately should consider network segmentation and firewall rules that restrict access to the PostgreSQL sidecar service port. These are compensating controls, and the vendor has not endorsed them.

Vendor Security History

Splunk's 2026 advisories show a pattern of recurring access control and authentication issues in the on-premises Enterprise product:

Other notable CVEs from 2026 include CVE-2026-20203 (improper access control in Data Model Acceleration), CVE-2026-20166 (information disclosure via improper access control), and CVE-2026-20140 (local privilege escalation via DLL hijacking on Windows).

The PostgreSQL component in particular has been a recurring concern. Community discussions on Reddit have noted that "Postgres 17.4 is affected by various vulnerabilities on the Postgres list. Previous versions of Splunk also shipped vulnerable Postgres," a sign that this component has been under scrutiny for some time.

Organizations running Splunk should audit their patch compliance against all 2026 advisories, not only CVE-2026-20253.

References

• NVD: CVE-2026-20253
• SVD-2026-0603: Splunk Vendor Advisory
• Splunk Security Advisories Archive
• CWE-306: Missing Authentication for Critical Function
• About Splunk Sidecars (Splunk Enterprise Documentation)
• Sidecar Configuration Settings (Splunk Cloud Platform)
• CVSS v3.1 Calculator
• CISA Known Exploited Vulnerabilities Catalog
• VulnCheck: Quantifying 2026 Routinely Targeted Vulnerabilities
• Splunk End of Life Tracking
• IDC SIEM Market Share Report (Splunk)
• Cisco Completes Acquisition of Splunk
• CVE-2026-20203 (NVD)
• CVE-2026-20166 (SentinelOne)
• CVE-2026-20140: Splunk Privilege Escalation via DLL Hijacking
• Reddit: Splunk Update Ships New Postgres Vulnerability