⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● High · CVSS 7.5

How to Fix CVE-2026-3509: Format String Flaw in CODESYS Control RTE (SL)

Other vulnerabilities in the same area that are worth patching alongside this one:

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 7.5 - High
Actively exploited?Not currently listed in CISA KEV
Affected3.5.17.0 < 3.5.22.0, 3.5.17.0 < 3.5.22.0, 3.5.17.0 < 3.5.22.0, 3.5.17.0 < 3.5.22.0, 4.1.0.0 < 4.21.0.0, 4.1.0.0 < 4.21.0.0, and others
Fixed inSee vendor advisory
Type (CWE)CWE-134: Use of Externally-Controlled Format String

What is CVE-2026-3509?

CVE-2026-3509 is a format string vulnerability in CODESYS Control RTE (SL). Attacker-controlled data is passed as the format argument to a printf-style function, which exposes process memory or writes to arbitrary addresses. Vendor description: An unauthenticated remote attacker may be able to control the format string of messages processed by the Audit Log of the CODESYS Control runtime system, potentially resulting in a denial‑of‑service (DoS) condition.

Why this CVE matters

Format-string bugs in a network daemon are uncommon today but devastating when they appear. The bug lets an attacker read and write arbitrary memory, which on an unpatched system converts to remote code execution.

For deployments of CODESYS Control RTE (SL) that have been exposed to the public internet during the disclosure window, the operating assumption should be that scanning has already happened. Even where exploitation has not been publicly observed, scanning for the vulnerable fingerprint is cheap and routine. Patching closes the door; log review and credential rotation close out the rest of the response.

Am I affected?

You are affected if your installation matches any of these version ranges:

Check your installed version against the list above. If you cannot determine the version, treat the system as affected and follow the upgrade path below.

Open CODESYS Control RTE (SL)'s About dialog or run the vendor-documented version-check command. Compare the result against the affected ranges in the advisory.

How to fix CVE-2026-3509

  1. Read the vendor advisory in full: https://certvde.com/de/advisories/VDE-2026-018
  2. Upgrade CODESYS Control RTE (SL) to the patched build listed in the vendor advisory.
  3. Back up the configuration (and database, where applicable) before upgrading.
  4. Apply the patch in a maintenance window. For HA pairs, upgrade the standby node first, fail over, then upgrade the former primary.
  5. Restart the affected service so the patched binary loads, then verify the new version (see verification section).

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Patch via your OS package manager

Vendor advisory (always check this first for exact fixed version and any

prerequisites): https://certvde.com/de/advisories/VDE-2026-018


# Debian / Ubuntu: pull the patched build of codesys-control-rte-sl from your distro repository.
sudo apt update
sudo apt install --only-upgrade codesys-control-rte-sl

# RHEL / Rocky / AlmaLinux / Fedora
sudo dnf upgrade codesys-control-rte-sl

# openSUSE
sudo zypper update codesys-control-rte-sl

# Verify the running version matches the fixed-in version (3.5.22.0).
codesys-control-rte-sl --version || dpkg -s codesys-control-rte-sl | grep -i version || rpm -q codesys-control-rte-sl


# Windows: pull the latest cumulative updates that include this CVE's fix.
Install-Module PSWindowsUpdate -Force -SkipPublisherCheck
Import-Module PSWindowsUpdate
Get-WindowsUpdate -AcceptAll -Install -AutoReboot

# If a specific KB is referenced in the advisory, install it directly.
# Get-WindowsUpdate -KBArticleID KBxxxxxxx -AcceptAll -Install -AutoReboot

Verify the fix landed


# Vendor advisory: https://certvde.com/de/advisories/VDE-2026-018
# 1. Confirm the running version matches the fixed-in version listed above.

# 2. Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
#    The scanner should no longer flag this CVE on the patched target.

# 3. Inspect recent service / kernel logs for crash-loops or rollback events.
journalctl -u <service> --since "10 minutes ago"
dmesg --since "10 minutes ago"

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If you cannot patch immediately

Reduce the attack surface by removing the affected service from untrusted networks. The patched build is the only durable fix.

How to verify the fix worked

If your installation was internet-reachable during the disclosure window, treat log review as part of the remediation rather than an optional follow-up. Look for repeated service restarts, crash logs from the affected daemon, and core files generated around the time of any anomalous traffic. A memory-corruption flaw used for exploitation often leaves a trail of failed attempts before the successful one.

Frequently asked questions

Is CVE-2026-3509 being exploited in the wild?

Public exploitation has not been confirmed by CISA at the time of writing. Treat the patch as time-sensitive anyway; reports often lag actual abuse.

Will a WAF or IDS rule fully mitigate CVE-2026-3509?

No. Network-layer filters can reduce noise and slow opportunistic scanners, but they will not stop a determined attacker. The vendor patch is the only durable fix.

How long should I plan for the upgrade?

Typical vendor-documented upgrade windows for CODESYS Control RTE (SL) run from a few minutes to under an hour depending on cluster size. Test in a staging environment first and follow the vendor's documented HA upgrade order.

References

*This guide was assembled from the official vendor advisory, the NVD record, and the CISA KEV catalog entry on 2026-05-25. Always confirm against the vendor advisory before applying changes in production.*