Splunk Enterprise CVSS 9.8 Zero-Day: How an Unauthenticated Sidecar Leads to RCE

Imagine you're the on-call engineer at 2:00 AM, and your primary logging and SIEM platform—the very tool meant to catch attackers—is actively being used to drop shells on your network.

That’s the reality for organizations scrambling to patch CVE-2026-20253. Added to the CISA Known Exploited Vulnerabilities (KEV) catalog on June 18, 2026, this CVSS 9.8 critical vulnerability resides in the PostgreSQL sidecar service of Splunk Enterprise. The bug is as severe as it is elegant: it allows an unauthenticated remote attacker to manipulate arbitrary files on the host, setting up a trivial chain to full Remote Code Execution (RCE).

Here is a technical breakdown of how CVE-2026-20253 works, how attackers are weaponizing it, and what this means for defenders.

The Flaw in the Sidecar

Splunk relies heavily on underlying databases to manage complex indexing and configurations. In recent deployments, a PostgreSQL sidecar is utilized to handle specific structured data tasks. However, due to a severe misconfiguration in the sidecar's network binding and authentication layer, the PostgreSQL port was exposed without requiring authentication for specific administrative operations.

A high-level view showing the exposed, unauthenticated PostgreSQL sidecar interacting with the core logging server.

By connecting directly to this sidecar port, an external threat actor can issue SQL commands that abuse PostgreSQL's native file read/write functions (like COPY TO). Because the sidecar runs with elevated privileges on the host system to interact with Splunk's core files, attackers can overwrite configuration files, drop malicious cron jobs, or truncate logs to blind defenders.

Chaining File Write to RCE

Writing arbitrary files is bad, but chaining it to RCE is what gives this bug a 9.8 rating. Attackers chain this vulnerability by targeting Splunk's script execution paths or the host's scheduled tasks.

The attack path: from unauthenticated sidecar access to file write, culminating in a reverse shell on the host.

Here is a conceptual look at how an attacker abuses the PostgreSQL connection in a lab environment. By injecting a simple python reverse shell into a directory known to be executed by Splunk's scheduled health checks, the attacker achieves RCE: