⚠ Reference material — not professional advice. Test in staging, back up first, verify against your specific version. Use your own judgment for your environment.
● Medium · CVSS 4

How to Fix CVE-2026-25690: Critical Vulnerability in FortiDeceptor

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

*By Sai Kiran Pandrala*

⚡ At a glance
SeverityCVSS 4 - Medium
Actively exploited?Not currently listed in CISA KEV
Affected6.0.0 <= 6.0.2, 5.3.0 <= 5.3.3, 5.2.0 <= 5.2.1, 5.1.0, 5.0.0
Fixed inSee vendor advisory
Type (CWE)CWE-88: Information disclosure

What is CVE-2026-25690?

CVE-2026-25690 is a security flaw in FortiDeceptor. An improper neutralization of argument delimiters in a command ('argument injection') vulnerability in Fortinet FortiDeceptor 6.0.0 through 6.0.2, FortiDeceptor 5.3.0 through 5.3.3, FortiDeceptor 5.2.0 through 5.2.1, FortiDeceptor 5.1 all versions, FortiDeceptor 5.0 all versions may allow an authenticated attacker with at least read-only admin permission to read log files via HTTP crafted requests.

Why this CVE matters

Unpatched network-facing software is the leading initial-access vector in public breach reporting. Treat any CVSS-9 class flaw on an internet-reachable system as urgent, regardless of whether public exploit code has been observed yet.

For deployments of FortiDeceptor 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.

On FortiGate / FortiOS systems, run get system status from the CLI and compare the Version line against the affected ranges above.

How to fix CVE-2026-25690

  1. Read the vendor advisory in full: https://fortiguard.fortinet.com/psirt/FG-IR-26-138
  2. Upgrade FortiDeceptor 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).

Patched-version commands

Vendor advisory: https://fortiguard.fortinet.com/psirt/FG-IR-26-138

Affected: FortiDeceptor: 6.0.0 <= 6.0.2

Patched in: 6.0.2


# Vendor advisory: https://fortiguard.fortinet.com/psirt/FG-IR-26-138
# Verify the running build.
get system status | grep -i version

# Upload the patched image (FortiOS .out) via TFTP.
execute restore image tftp fortideceptor-6.0.2.out 10.0.0.10

# After auto-reboot, re-check the version string.
get system status | grep -i version


# Fleet check via FortiManager REST API.
$Headers = @{ "Authorization" = "Bearer $env:FMG_TOKEN" }
Invoke-RestMethod -Uri "https://<fmg>/api/v2/monitor/system/status" -Headers $Headers |
  Select-Object -ExpandProperty results | Select-Object hostname, version, build

Verify the fix landed


# Vendor advisory: https://fortiguard.fortinet.com/psirt/FG-IR-26-138
# Post-patch verification (replace <service> with the real service unit).
journalctl -u <service> --since "10 minutes ago"
dmesg --since "10 minutes ago"

# Re-scan with your vulnerability scanner (Nessus, Qualys, Tenable, OpenVAS).
# It should no longer flag CVE-2026-25690 on the patched target.

If you cannot patch immediately

No official workaround exists beyond restricting network exposure to the affected component. Apply the vendor patch as the primary remediation.

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 log entries that do not match your normal request patterns, especially repeated requests to the same uncommon endpoint, and any administrative changes you cannot tie back to a known operator.

Frequently asked questions

Is CVE-2026-25690 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-2026-25690?

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 FortiDeceptor 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.*