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Unauthenticated Attackers Can Wreak Havoc via a PostgreSQL Sidecar—Update Now.
Introduction:
A chilling irony has emerged in the cybersecurity world: a security tool designed to be your organization’s eyes and ears has been found with a blind spot that allows unauthenticated attackers to run rampant. A critical vulnerability, tracked as CVE-2026-20253 and carrying a near-maximum CVSS score of 9.8, has been discovered in Splunk Enterprise and Splunk Cloud Platform. The flaw resides in a PostgreSQL sidecar service endpoint that completely lacks authentication controls, allowing any network-reachable attacker to perform arbitrary file operations and achieve pre-authenticated Remote Code Execution (RCE), effectively turning your SIEM into a potential entry point for a devastating breach.
Learning Objectives:
- Understand the technical root cause of CVE-2026-20253, its attack vector, and its potential impact.
- Learn how to identify vulnerable Splunk versions and apply the official patches to mitigate the risk.
- Master detection and post-exploitation hunting techniques using Splunk SPL, Linux commands, and threat intelligence.
You Should Know:
1. CVE-2026-20253: An Unauthenticated RCE in Your SIEM
The vulnerability stems from a missing authentication check (CWE-306) in a PostgreSQL sidecar service endpoint. In simple terms, this is a helper service that runs alongside the main Splunk application. Because it was exposed without any credential verification, an attacker can invoke file operations directly on the underlying host system as if they were an authorized user. While initially described as an arbitrary file creation/truncation issue, security researchers from watchTowr Labs quickly demonstrated that this flaw can be chained for a full pre-authentication RCE.
They achieved this by abusing the `/v1/postgres/recovery/backup` and `/v1/postgres/recovery/restore` endpoints. An attacker can craft a malicious database dump file with an accompanying `.pgpass` file that points to a system location, such as /opt/splunk/var/packages/data/postgres/.pgpass. During a restore operation, the attacker’s SQL commands are executed, allowing them to overwrite a legitimate Splunk Python script (e.g., /opt/splunk/etc/apps/splunk_secure_gateway/bin/ssg_enable_modular_input.py). When the normal operation of Splunk executes that script, the attacker’s code runs on the system.
Default installations of Splunk Enterprise on AWS are vulnerable out-of-the-box, while on-premise manual installations may or may not have the vulnerable PostgreSQL sidecar service installed and enabled by default.
Which Splunk versions are vulnerable?
- Splunk Enterprise: versions 10.0.0 to 10.0.6 (fixed in 10.0.7), and versions 10.2.0 to 10.2.3 (fixed in 10.2.4). Version 10.4 is not affected.
- Splunk Cloud Platform: versions below 10.4.2604.3 and below 10.2.2510.14 are vulnerable.
Step-by-Step Guide to Detect & Verify Your Exposure
Step 1: Check for Vulnerable Splunk Version:
You can check your installed Splunk version using the command line.
Linux/macOS:
/opt/splunk/bin/splunk version
Windows:
C:\Program Files\Splunk\bin\splunk.exe version
If the output shows a version between 10.0.0 and 10.0.6, or between 10.2.0 and 10.2.3, your system is vulnerable.
Step 2: Scan for Network Exposure of the Sidecar:
The vulnerable PostgreSQL sidecar service typically listens on port 5435 on the localhost (127.0.0.1). However, if network configurations or firewall rules inadvertently expose this port to other systems, the risk escalates significantly. You can check the network listening status on the Splunk server:
Linux:
ss -tupln | grep 5435
Windows (PowerShell as Admin):
netstat -ano | findstr :5435
Step 3: Verify Authentication Requirement (Using `curl`):
You can test if the endpoint is accessible without credentials. Run the following command from a separate machine (replace `
curl -v http://<splunk_ip>:5435/v1/postgres/recovery/backup
A vulnerable system will respond without requiring any authentication headers.
2. Immediate Mitigation: Patching & Virtual Patching
The most effective mitigation is to patch your Splunk instances immediately to the fixed versions: 10.0.7 or 10.2.4. For Splunk Cloud Platform users, Splunk actively monitors and patches instances, but you should verify your version.
Step-by-Step Guide to Apply the Patch
Step 1: Download the Update:
Download the appropriate installer for your platform from the official Splunk download portal. It is crucial to download the software only from trusted sources.
Step 2: Pre-Upgrade Checks:
Before applying any update, ensure you have a full, verified backup of your Splunk configuration and data. For large or critical Splunk environments, perform the upgrade in a staging environment first to test for any compatibility issues with your custom apps, add-ons, or dashboards.
Step 3: Apply the Update:
Follow Splunk’s official documentation for your deployment type (single-instance, distributed, or indexer cluster). The process generally involves stopping the Splunk service, backing up the binaries, running the installer, and restarting the service.
Step 4: Post-Upgrade Verification:
After the upgrade, verify the version again using the command from the previous section. Also, perform a smoke test to ensure all critical Splunk functionalities are working as expected.
If immediate patching is not possible, implement a “virtual patch” by using network access control lists (ACLs) or firewall rules to restrict access to the Splunk server. Only allow traffic to the Splunk management ports (e.g., 8000, 8089) from trusted networks. Specifically, block all inbound traffic to the internal PostgreSQL sidecar port (5435) from untrusted sources. This will prevent an attacker from being able to reach the vulnerable endpoint in the first place.
3. Threat Hunting: Detecting Exploitation and Post-Compromise Activity
For Security Operations Centers (SOCs), proactively hunting for signs of this vulnerability being exploited is critical. The key is to look for unauthorized access to the sidecar service and subsequent suspicious file operations.
Step-by-Step SPL Hunting Queries
Step 1: Detect Unauthorized Access to Sidecar Endpoint:
You can hunt for network connections to the sidecar from unexpected sources. Use the following SPL search to find connections from non-localhost IPs to the Splunk server on the sidecar port:
index= dest_port=5435 dest_ip=<your_splunk_server_ip> source_ip!=127.0.0.1 | table _time, source_ip, dest_ip, dest_port, user, process_name
If any events appear, investigate them immediately.
Step 2: Detect Python Script Tampering:
As the public PoC targets a specific Python script, monitor for changes to the `ssg_enable_modular_input.py` script.
Linux Auditd Rule:
Add a rule to monitor for any write (w) or attribute change (a) to the Splunk Secure Gateway’s bin directory:
-w /opt/splunk/etc/apps/splunk_secure_gateway/bin/ -p wa -k ssg_modification
SPL to Detect the Modification:
index= sourcetype=auditd key=ssg_modification | table _time, host, process, uid, exe, auid
Step 3: Hunt for Arbitrary File Overwrites:
An attacker could also target other critical files. Hunt for unexpected empty files or recent modifications to system configuration files.
Linux Command to Find Recently Changed Files:
find /opt/splunk -type f -mtime -1 -ls
Find Zero-Size Files (Potential Truncation):
find /opt/splunk -type f -size 0
What Undercode Say:
- Key Takeaway 1: Treat your security tools as potential threat vectors, not as inherently trusted entities. Apply strict network segmentation and rigorous patch management to your SIEM. CVE-2026-20253 is a textbook example of a broken authentication flaw (CWE-306) leading to a complete system compromise.
- Key Takeaway 2: Threat hunting is not passive. An unauthenticated RCE on a tool like Splunk is a worst-case scenario for any SOC. Proactively implementing the detection rules and commands listed above is the only way to gain assurance that your environment has not already been compromised while you scramble to patch.
Expected Output:
Prediction:
- -1 This vulnerability represents a new phase in cyber warfare: the weaponization of security infrastructure. We predict that within the next six months, at least one major ransomware group will develop an automated exploit module for CVE-2026-20253. Their modus operandi will shift from deploying backdoors on servers to first compromising the SIEM, subsequently disabling or blinding security monitoring systems, exfiltrating logs to identify high-value targets, and then launching a devastating, completely invisible attack. The only defense against this is to assume your SIEM is untrusted and apply these patches with the urgency of a live zero-day exploit—because, effectively, it is.
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IT/Security Reporter URL:
Reported By: Bernhard Biedermann – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
