Vulnerability Analysis

CVE-2026-20253: Splunk Enterprise Unauthenticated RCE via PostgreSQL Sidecar — What It Is & How to Fix It

Executive Summary

CVE-2026-20253 is a CVSS 9.8 critical vulnerability in Splunk Enterprise that allows completely unauthenticated remote attackers to create or truncate arbitrary files on the host system — and chain that primitive into full remote code execution — by sending crafted requests to an internally exposed PostgreSQL sidecar service with no authentication controls. CISA added this flaw to its Known Exploited Vulnerabilities (KEV) catalog on June 18, 2026, with confirmed active exploitation observed in the wild. Organizations running Splunk Enterprise must patch immediately: no workaround exists, and the affected service is Splunk's own SIEM platform, meaning a compromise here puts your entire security monitoring infrastructure at risk.

1. What Is This Vulnerability?

Root Cause

Splunk Enterprise ships with a PostgreSQL Sidecar Service — a companion process that manages a local PostgreSQL database used for features such as the Splunk Secure Gateway and audit logs. This sidecar exposes two internal HTTP recovery endpoints:

POST /v1/postgres/recovery/backup
POST /v1/postgres/recovery/restore

Neither endpoint enforces any authentication. Any network-accessible client — including an unauthenticated attacker on a corporate LAN or an internet-exposed Splunk instance — can invoke these endpoints freely. The vulnerability is classified as CWE-306: Missing Authentication for Critical Function.

The core bug is straightforward: developers exposed powerful database backup/restore functionality over an internal HTTP API and simply forgot (or chose not) to gate it behind credentials.

Attack Vector

The exploitation chain has three stages:

Stage 1 — Arbitrary File Write via lo_export

PostgreSQL's lo_export() function extracts a Large Object (BLOB) from the database and writes it to a file on the underlying filesystem. An attacker can:

  1. Connect to the unauthenticated /backup endpoint to trigger a database dump to the local filesystem.
  2. Craft a malicious PostgreSQL dump that includes a custom function using lo_export to write attacker-controlled content to an arbitrary path.
  3. Send the crafted dump to the /restore endpoint, causing Splunk's PostgreSQL instance to import and execute the function — resulting in an arbitrary file written to the host as the Splunk service user.

Stage 2 — Python Script Overwrite

Splunk regularly and automatically executes Python scripts within its installed apps. Researchers identified a particularly useful target:

/opt/splunk/etc/apps/splunk_secure_gateway/bin/ssg_enable_modular_input.py

This script runs on a scheduled basis with Splunk process privileges. By using the Stage 1 file-write primitive to overwrite it with attacker-controlled content, the attacker's code is executed automatically the next time Splunk invokes the script.

Stage 3 — Remote Code Execution

Once the malicious Python script is in place, it executes as the splunk user when the Splunk scheduler triggers it — typically within minutes. The attacker has achieved full RCE without ever authenticating to any Splunk interface.

# Conceptual attack flow (NOT exploit code — illustrative only):

# 1. Trigger backup to get database dump on Splunk filesystem
curl -X POST http://splunk-host:8088/v1/postgres/recovery/backup

# 2. Craft malicious pg_dump with lo_export payload -> overwrite Python script

# 3. Trigger restore to execute malicious dump
curl -X POST http://splunk-host:8088/v1/postgres/recovery/restore \
     --data-binary @malicious.dump

# 4. Wait for Splunk scheduler to execute the now-malicious Python script -> RCE

Real-World Impact

Active exploitation was confirmed by Splunk's PSIRT and by CISA in June 2026. Observed attacker behavior includes:

  • Writing "proof of exploit" files to the filesystem (e.g., watchTowr.txt in exposed web directories, consistent with security research PoC behavior)
  • Establishing reverse shells from compromised Splunk servers
  • Pivoting laterally using Splunk's privileged network position — Splunk Enterprise often receives log data from across an organization's entire infrastructure, giving an attacker on the Splunk host access to sensitive telemetry and potentially credentials

2. Who Is Affected?

Splunk Enterprise Branch Vulnerable Versions Fixed Version
10.2.x 10.2.0 – 10.2.3 10.2.4
10.0.x 10.0.0 – 10.0.6 10.0.7
10.4.x None 10.4.0 (safe)

Risk amplifiers:

  • Splunk instances accessible from the internet (or from broad internal segments) are at highest risk.
  • The /v1/postgres/recovery/ endpoints do not require the standard Splunk management port (8089) — verify your actual endpoint exposure.
  • Splunk Cloud customers are not affected; Splunk has confirmed the cloud service was patched directly.
  • The vulnerability does NOT require any Splunk user accounts — zero authentication, zero user interaction needed.

3. How to Detect It (Testing)

Manual Testing Steps

Step 1: Identify your Splunk version

Log into your Splunk Web interface and navigate to:

Settings → About → Version

Or check via CLI:

/opt/splunk/bin/splunk version

Step 2: Check if the sidecar endpoint is reachable

From a test host (NOT from the Splunk server itself), attempt to reach the recovery API:

curl -sk -o /dev/null -w "%{http_code}" \
  http://<splunk-host>:<port>/v1/postgres/recovery/backup

A 200, 400, or 500 response (rather than a connection refused or 404 from a reverse proxy) indicates the endpoint is accessible. On unpatched systems, a 200 or 400 with a JSON body confirms the endpoint exists and is unauthenticated.

Step 3: Check filesystem for indicators of prior compromise

# Check for unexpected files in temp/run directories
ls -la /tmp/ | grep -v "^total"
ls -la /opt/splunk/var/run/supervisor/pkg-run/

# Check modification times on the known-targeted Python script
stat /opt/splunk/etc/apps/splunk_secure_gateway/bin/ssg_enable_modular_input.py

# Look for proof-of-exploit files left by researchers/attackers
find /opt/splunk -name "*.txt" -newer /opt/splunk/bin/splunk -ls 2>/dev/null

Step 4: Review Splunk internal logs for anomalous restore/backup calls

grep -i "postgres/recovery" /opt/splunk/var/log/splunk/*.log

Automated Scanning

Using Nuclei:

nuclei -u https://your-splunk-host -t cves/2026/CVE-2026-20253.yaml

(Community templates for this CVE are already available in the nuclei-templates repository.)

Using Nessus / Tenable: Tenable plugin coverage for CVE-2026-20253 was released in June 2026. Run a credentialed scan with "Splunk" plugins enabled — the plugin will check the installed version against the affected range.

Using Splunk itself (ironic but valid): If you have a secondary Splunk instance, you can search your Splunk logs for hits on the recovery endpoint:

index=_internal sourcetype=splunkd path="*/postgres/recovery/*" 
| stats count by clientip, method, uri_path, _time
| where count > 0

Code Review Checklist

  • Confirm Splunk Enterprise version is NOT in the 10.0.0–10.0.6 or 10.2.0–10.2.3 range
  • Verify the /v1/postgres/recovery/ endpoints are not reachable from untrusted network segments
  • Confirm ssg_enable_modular_input.py hash matches known-good from Splunk's official package
  • Check Splunk logs for any backup or restore calls you didn't initiate
  • Verify no unexpected user accounts exist in Splunk's local PostgreSQL instance

4. How to Fix It (Mitigation)

Step-by-Step Remediation

Splunk has confirmed: there is no workaround. Upgrading is the only fix. The sidecar service cannot be safely disabled while keeping Splunk functional.

  1. Back up your Splunk configuration and indexes before upgrading:

    /opt/splunk/bin/splunk stop
tar -czf splunk-backup-$(date +%F).tar.gz /opt/splunk/etc/

  2. Download the patched version from Splunk's official download portal:

  3. Verify the package checksum before installing:

    sha256sum splunk-10.2.4-*.tgz  # compare against Splunk's published hash

  4. Install the upgrade (Linux example):

    tar -xzf splunk-10.2.4-*.tgz -C /opt
/opt/splunk/bin/splunk start --accept-license
/opt/splunk/bin/splunk version  # confirm: 10.2.4

  5. Restart and validate that Splunk services are running correctly and all forwarders reconnect.

  6. For distributed deployments, prioritize patching in this order:

    • Search Heads (internet-facing and most data-rich)
    • Indexers
    • Heavy Forwarders
    • Deployment Servers

Configuration Hardening (Defense-in-Depth)

Even after patching, apply these network-level controls to reduce the blast radius of any future Splunk vulnerabilities:

# Example: block the sidecar port at the host firewall (adjust port as needed)
# Note: this is a belt-and-suspenders measure; patching is the real fix
iptables -A INPUT -p tcp --dport 8088 -s 0.0.0.0/0 -j DROP
iptables -A INPUT -p tcp --dport 8088 -s <trusted-mgmt-subnet> -j ACCEPT

Restrict Splunk management access to dedicated management networks via server.conf:

[httpServer]
acceptFrom = 10.0.0.0/8 172.16.0.0/12 192.168.0.0/16

5. How to Test the Fix (Validation)

Regression Test Scenarios

  • Scenario A: After patching, re-run the curl probe from Step 2 of detection — the endpoint should return 404 or connection refused, no longer exposing the recovery API.
  • Scenario B: Verify the Splunk version string reports 10.2.4, 10.0.7, or 10.4.0 (or later).
  • Scenario C: Confirm all Splunk dashboards, alerts, and forwarder connections still function normally post-upgrade.

Security Test Cases

Test Case 1: Verify the unauthenticated endpoint is gone

  • Precondition: Upgrade to patched version and restart Splunk
  • Steps: 
    curl -v http://<splunk-host>:<port>/v1/postgres/recovery/backup
  • Expected Result: HTTP 404 or connection refused — the endpoint no longer exists on patched builds

Test Case 2: Verify the Python script integrity

  • Precondition: Patch applied
  • Steps: 
    sha256sum /opt/splunk/etc/apps/splunk_secure_gateway/bin/ssg_enable_modular_input.py
# Compare against the hash from the official Splunk package
rpm -V splunk   # or dpkg --verify splunk  (if package-managed)
  • Expected Result: Hash matches official distribution; no tampered files

Test Case 3: Verify no anomalous PostgreSQL processes

  • Precondition: Patch applied
  • Steps: 
    ps aux | grep postgres
netstat -tlnp | grep 5432
  • Expected Result: PostgreSQL is only listening on localhost (127.0.0.1), not on external interfaces

Automated Tests

Add the following to your infrastructure scanning pipeline (Python/requests):

import requests
import sys

def test_cve_2026_20253(host, port=8088):
    """
    Test whether the unauthenticated PostgreSQL sidecar endpoint is reachable.
    A 404 response on a patched system; any other HTTP response = still vulnerable.
    """
    url = f"http://{host}:{port}/v1/postgres/recovery/backup"
    try:
        resp = requests.post(url, timeout=5, verify=False)
        if resp.status_code == 404:
            print(f"[PASS] {host}: Endpoint returns 404 — likely patched")
            return True
        else:
            print(f"[FAIL] {host}: Endpoint returned HTTP {resp.status_code} — MAY BE VULNERABLE")
            return False
    except requests.exceptions.ConnectionError:
        print(f"[PASS] {host}: Connection refused — endpoint not reachable")
        return True

if __name__ == "__main__":
    host = sys.argv[1] if len(sys.argv) > 1 else "localhost"
    test_cve_2026_20253(host)

6. Prevention & Hardening

Best Practices

Segment Splunk from untrusted networks. Splunk Enterprise should never be directly internet-accessible. Place it behind a firewall that restricts access to the Splunk web port (8000), management port (8089), and any other exposed ports to known management subnets only.

Run Splunk as a non-root, low-privilege service user. If Splunk runs as root, an RCE achieves immediate full host compromise. Running as a dedicated splunk user limits the blast radius:

# Confirm Splunk runs as a dedicated user (not root)
ps aux | grep splunk | head -1

Subscribe to Splunk security advisories. Add the Splunk SVD RSS feed to your vulnerability management workflow: https://advisory.splunk.com/advisories. Splunk publishes advisories on the first Wednesday of each month.

Implement integrity monitoring on Splunk application directories. Use a file integrity monitoring (FIM) tool (OSSEC, Wazuh, Tripwire) to alert on unexpected changes to:

/opt/splunk/etc/apps/*/bin/*.py
/opt/splunk/etc/apps/*/appserver/controllers/

Apply Splunk patches within 24–72 hours for Critical findings. CVE-2026-20253 demonstrates that Splunk vulnerabilities can be weaponized within days of disclosure. Treat Splunk upgrades as on-par in urgency with OS-level patches.

Monitoring & Detection

Add the following Splunk search as a saved alert (ideally running on a separate Splunk instance or SIEM):

index=_internal sourcetype=splunkd 
  (uri_path="*/postgres/recovery/*" OR uri_path="*/v1/postgres/*")
| stats count, values(clientip) AS sources, values(method) AS methods BY uri_path
| where count > 0
| eval severity=if(count > 5, "HIGH", "MEDIUM")
| table _time, uri_path, sources, methods, count, severity

Alert on any process spawned by the Splunk user that establishes outbound network connections on unexpected ports — a common sign of a reverse shell established via RCE:

# auditd rule to detect Splunk user spawning network connections
auditctl -a always,exit -F arch=b64 -S connect -F uid=$(id -u splunk) \
  -k splunk_network_anomaly

References

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