CVE-2018-13383

Think of the SSL VPN web portal as the “front door” that lets people access your network from a browser. This bug lets a low-privileged person who is already logged in feed the portal specially crafted web content so it can crash. The main outcome is a sudden outage for other users who are logged in through the same service.

This matters if you run Fortinet FortiOS or FortiProxy and you use the SSL VPN web portal for remote access. It requires the attacker to already be a logged-in user of the SSL VPN web portal, so it’s not a “random internet scan” issue, but it becomes serious if VPN accounts are compromised or weak. Treat it as a risk when the SSL VPN web portal is reachable and attacker-controlled access to a VPN web user account is possible.

This is RED because CVE-2018-13383 is confirmed in the CISA KEV and has been used in ransomware campaigns. Once an attacker has a foothold in the SSL VPN web portal with a logged-in account, they can trigger a crash/termination that disrupts VPN access for other logged-in users.

1. Inventory and confirm exposure

   • Identify whether your environment includes FortiOS (Fortinet FortiOS) and/or FortiProxy.
   • Determine the exact installed versions and compare to the affected ranges from the advisory:
     • FortiOS: 6.0.0 through 6.0.4, 5.6.0 through 5.6.10, 5.4.0 through 5.4.12, 5.2.14 and earlier.
     • FortiProxy: 2.0.0, 1.2.8 and earlier.

2. Reachability and access check (to validate real risk)

   • Verify the SSL VPN web portal is enabled and reachable for your business users.
   • Review whether SSL VPN accounts could plausibly be compromised (for example: shared credentials, recently changed passwords, accounts that were inactive or overused).

3. Apply the fixed versions (primary remediation)

   • FortiProxy: upgrade to 1.2.9 or later.
   • FortiOS: upgrade to 5.2.15 or later.
   • Use the vendor instructions referenced by FortiGuard advisories FG-IR-18-388 and FG-IR-20-229.

4. If patching is delayed (temporary workaround)

   • Apply the vendor workaround guidance from the FortiGuard advisory FG-IR-18-388.
   • Treat the workaround as temporary—prioritize upgrading to the fixed versions as the permanent mitigation.

5. Validation after changes

   • After upgrading, re-check the FortiOS/FortiProxy version numbers and confirm SSL VPN web portal functionality for logged-in users.
   • Confirm monitoring/alerting is in place for unexpected SSL VPN web service terminations.

6. KEV timing note

   • CISA KEV remediation due date for this issue was 2022-07-10; if you have not already remediated, prioritize immediately.

7. What to monitor

   • Service crashes/terminations of the SSL VPN web service.
   • Signs of abnormal activity by VPN users (especially unusual attempts involving web content submission via the portal).

Severity and type: CWE-787 heap buffer overflow leading to SSL VPN web portal service termination (availability impact). Attack vector is network-based, but exploitation requires the attacker to be a logged-in user of the SSL VPN web portal; no user interaction is required. The weakness involves improper handling of JavaScript href data when proxying webpages.

Exploitation status: This CVE is listed in CISA KEV and was used in ransomware campaigns (confirmed real-world exploitation per the findings). There is no public exploit code on record, which may affect tooling availability, but it does not reduce the risk given KEV confirmation.

Likelihood indicators: A rising EPSS trend is noted in the findings, but KEV confirmation is the primary driver for urgency.

What “KEV” means here: CISA lists it when they assess it as actively exploited and provides remediation timelines; the required action is to apply vendor updates/mitigations.