Every security team has that one internal service nobody talks about — the helper process, the sidecar, the “it’s only listening on localhost, right?” component that gets bolted onto a production stack and quietly forgotten. This week, Splunk Enterprise users learned what happens when that forgotten helper turns out to be a PostgreSQL sidecar with no authentication and a file-write primitive bolted to the front door.
On June 13, 2026, Splunk (now part of Cisco) shipped emergency patches for CVE-2026-20253, a 9.8 CVSS flaw that lets an unauthenticated, network-reachable attacker create or truncate arbitrary files through an exposed PostgreSQL sidecar endpoint. By the end of the week, watchTowr Labs had published the full chain — and “arbitrary file write” turned out to be a polite way of saying “pre-auth remote code execution.”
Why an Unauthenticated Database Sidecar Is the Worst Possible Trust Boundary
According to Splunk’s advisory, the vulnerability exists because the PostgreSQL sidecar service endpoint “lacks authentication controls, allowing any network-reachable user to invoke file operations without credentials.” The two endpoints in question — /v1/postgres/recovery/backup and /v1/postgres/recovery/restore — were apparently designed under the assumption that the database itself would handle auth. That assumption is exactly what watchTowr’s writeup skewers in its title: “Why use app-level auth when every database has auth?”
Sidecars sit inside the trust boundary of the main application. When a developer wires a helper process to an HTTP route, they often defer authn to “whatever is downstream” — the database, the queue, the storage backend. The problem is that downstream auth assumes the caller can already reach the downstream system. An HTTP endpoint flips that assumption inside out: anyone who can hit the port can drive the backend without ever proving who they are.
If you run Splunk Enterprise 10.0.0 through 10.0.6 or 10.2.0 through 10.2.3 on-prem, you are the practical example here. An attacker who can reach your indexer’s management port can, with no credentials, walk straight through the sidecar and start writing files into /opt/splunk. The take: the era of treating “internal” HTTP endpoints as implicitly trusted is over, and any team building custom REST APIs and service integrations needs to assume every endpoint will eventually be reachable from somewhere it shouldn’t be.
How watchTowr Turned a File Write Into Full Code Execution
The chain that Piotr Bazydlo and Yordan Ganchev documented is elegant in the way that good exploit research usually is — each step is mundane on its own, and the danger only emerges when they’re composed.
First, the attacker stands up their own PostgreSQL instance configured to allow passwordless authentication and grants its user permission to call lo_export, the Postgres function that writes a large object out to the filesystem. Next, they hit the unauthenticated /backup endpoint to drop a dump of that attacker-controlled database onto the Splunk host. Then they call /restore with a passfile argument pointing at /opt/splunk/var/packages/data/postgres/.pgpass — the local file that already contains the postgres_admin credentials. Splunk’s own PostgreSQL instance dutifully reads its own password, authenticates, and executes the attacker’s SQL during the restore.
From there, the SQL defines a function that uses lo_export to write attacker-controlled content to disk, and the attacker overwrites a Python script Splunk runs frequently — the researchers used /opt/splunk/etc/apps/splunk_secure_gateway/bin/ssg_enable_modular_input.py. The next time Splunk fires that script, the payload runs.
This is not a memory corruption bug, a deserialization gadget, or a sandbox escape. It’s three legitimate features — backup, restore, and lo_export — chained through an unauthenticated HTTP wrapper. Detection rules that look for “weird” payloads won’t catch it. The traffic looks like backup traffic, because it is backup traffic.
If you’re running Splunk Enterprise as the SIEM for a regulated environment — say, a hospital network that also relies on HIPAA-aligned healthcare platforms for patient records — the same box you depend on to detect intrusions is now the easiest pre-auth foothold in your environment. The take: expect a public Metasploit or Nuclei template within weeks, because the steps are too well-documented to stay theoretical.
What Defenders Should Do Before the Opportunistic Scans Start
Splunk’s advisory is clear: upgrade to 10.0.7 or 10.2.4. Splunk Enterprise 10.4 is not affected, and Splunk Cloud customers are out of scope because the Postgres sidecar isn’t deployed there. The article from The Hacker News notes there is no evidence of exploitation in the wild, but adds that “the availability of the exploit specifics can be enough to drive threat actors to trigger opportunistic attempts.”
Translation: the clock started on Friday. CVSS 9.8, pre-auth, and a published attack chain is the exact profile that botnets and access brokers love. Patching is step one, but defenders should also consider segmenting Splunk management ports so they aren’t reachable from general workstation VLANs, auditing any other internal sidecars for the same anti-pattern, and treating the on-prem Splunk host as potentially compromised if its management interface has been exposed to untrusted networks at any point in the last week.
Teams investing in AI-driven workflow automation and other event-driven backends are spinning up exactly these kinds of sidecar services every day — vector DB helpers, model proxies, queue bridges. Every one of them is a CVE-2026-20253 waiting to happen if the team treats “internal” as a synonym for “safe.”
FAQ
Q: What is CVE-2026-20253 and how severe is it? A: CVE-2026-20253 is a critical vulnerability in Splunk Enterprise versions below 10.2.4 and 10.0.7, rated 9.8 on the CVSS scale. An unauthenticated, network-reachable attacker can perform arbitrary file operations through an exposed PostgreSQL sidecar — which watchTowr Labs chained into pre-auth RCE.
Q: Is Splunk Cloud affected by this PostgreSQL sidecar flaw? A: No. Splunk explicitly stated that Splunk Cloud is not impacted because the Postgres sidecar service is not deployed in the cloud product. The vulnerability only affects on-premises Splunk Enterprise in those version ranges; Splunk Enterprise 10.4 is also not affected.
Q: How does the attack chain achieve remote code execution?
A: According to watchTowr’s writeup, an attacker connects the vulnerable Splunk instance to an attacker-controlled PostgreSQL database via the unauthenticated /v1/postgres/recovery/backup and /restore endpoints, uses Splunk’s own .pgpass file for local authentication, and then runs malicious SQL that calls lo_export to overwrite a Python script Splunk regularly executes — turning a file write into full RCE.
Key Takeaways
- Patch Splunk Enterprise to 10.0.7 or 10.2.4 immediately if you run any on-prem instance — the public attack chain makes opportunistic scanning a near-certainty in the coming weeks.
- Audit every internal HTTP-fronted sidecar in your stack for the “downstream auth is enough” anti-pattern; if a service speaks HTTP, it needs its own authn, regardless of what it proxies to.
- Segment Splunk management interfaces away from general-purpose VLANs so that even unpatched windows don’t expose the management port to every workstation on the network.
- Treat any Splunk Enterprise host whose management port was reachable from untrusted networks during the disclosure window as potentially compromised, and hunt for unexpected modifications under
/opt/splunk/etc/apps/. - Expect this exact pattern — unauthenticated sidecars wrapping authenticated backends — to drive a wave of similar CVEs across SIEM, observability, and AI infrastructure products over the next year.