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Introduction:
The Shadowserver Foundation has issued an urgent warning to FortiClient Enterprise Management Server (EMS) administrators after identifying over 2,000 publicly accessible instances globally, two of which are now confirmed to be actively exploited through critical unauthenticated remote code execution (RCE) vulnerabilities[reference:0]. These flaws, CVE-2026-35616 and CVE-2026-21643, allow unauthenticated attackers to remotely execute arbitrary code, potentially gaining full control over the affected server and all endpoints it manages[reference:1].
Learning Objectives:
- Understand the technical details and exploitation vectors of CVE-2026-35616 and CVE-2026-21643.
- Learn how to detect potential compromise on FortiClient EMS servers using log analysis and scanning tools.
- Implement immediate mitigation steps, including patching, access restrictions, and network hardening.
You Should Know:
- Understanding the Dual RCE Threats: CVE-2026-35616 & CVE-2026-21643
The current crisis stems from two distinct unauthenticated RCE vulnerabilities being exploited in the wild. CVE-2026-21643, a critical SQL injection flaw affecting FortiClient EMS version 7.4.4, was the first to be exploited[reference:2]. This vulnerability allows an attacker to send specially crafted HTTP requests to the EMS web interface to execute arbitrary commands[reference:3]. Research has shown that attackers can abuse the publicly accessible `/api/v1/init_consts` endpoint to trigger the injection[reference:4]. More recently, CVE-2026-35616, a newly disclosed improper access control issue in versions 7.4.5 and 7.4.6, has also been confirmed as exploited[reference:5]. This flaw allows attackers to bypass authentication through an API and escalate privileges[reference:6].
To determine if your server is vulnerable, you can use a simple `curl` command to check the version exposed in the HTTP response headers or a specific API endpoint:
curl -k -I https://<EMS_IP>:<PORT>/api/v1/status | grep -i "server"
A vulnerable server might return a header indicating a version like `7.4.4` or 7.4.5. For a more thorough check, use a vulnerability scanner like `nmap` with its `http-vuln-` scripts, or a dedicated tool like `searchsploit` to find and test a proof-of-concept (PoC) exploit for CVE-2026-21643:
Search for a PoC in Exploit-DB searchsploit FortiClient EMS Example: Using a hypothetical Metasploit module (if available) msf6 > use exploit/linux/http/forticlient_ems_sqli_rce msf6 > set RHOSTS <target_IP> msf6 > set RPORT 443 msf6 > set SSL true msf6 > exploit
On Windows, you could use PowerShell to test the endpoint:
Invoke-WebRequest -Uri "https://<EMS_IP>/api/v1/init_consts" -Method Get
A successful exploitation would be indicated by a delayed response or an error message containing SQL syntax.
- Step-by-Step Detection: Hunting for Indicators of Compromise (IoCs)
If you cannot patch immediately, you must actively hunt for signs of compromise. Check the EMS server’s web and API logs for suspicious patterns. On a Linux-based EMS server, use `grep` to search for SQL injection attempts:Search for common SQL injection patterns in access logs grep -E "(union|select|sleep|xp_cmdshell)" /var/log/forticlient-ems/web-access.log
On Windows, you can use `findstr`:
findstr /i "union select sleep xp_cmdshell" C:\ProgramData\Fortinet\EMS\log\web-access.log
Key IoCs include:
Unusual outbound connections from the EMS server to unknown IPs, which could indicate a reverse shell.
Creation of new administrative users on the EMS server or the underlying OS.
Unexpected processes being spawned, such as `cmd.exe` or `powershell.exe` from the EMS service account.
Anomalies in the FortiClient telemetry data, such as configuration pushes to non-existent endpoints.
Use Sysmon or OSQuery to monitor process creation:
Windows: Query Sysmon Event ID 1 (Process Creation)
Get-WinEvent -FilterHashtable @{LogName='Microsoft-Windows-Sysmon/Operational'; ID=1} | Where-Object {$_.Message -match "cmd.exe|powershell.exe"} | Format-List
For Linux, auditd can be configured to track command executions:
Add a rule to audit all command executions by the 'ems' user auditctl -a always,exit -F uid=ems -S execve Then search the audit log for suspicious commands ausearch -ui ems -x "bash" -x "nc" -x "wget"
3. Immediate Mitigation: Network Hardening and Access Control
The most critical step is to immediately restrict internet-facing access to the EMS management interface. This can be achieved by implementing firewall rules that only allow access from trusted internal IP ranges or, preferably, by requiring a VPN connection. On a Linux host running iptables, you can restrict access to TCP port 443 (HTTPS) as follows:
Allow only a specific management subnet (e.g., 192.168.1.0/24) to access the EMS web UI iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT Drop all other traffic to port 443 iptables -A INPUT -p tcp --dport 443 -j DROP
For a Windows-based EMS server, use the `New-NetFirewallRule` PowerShell cmdlet:
Remove any existing broad 'Allow' rule for port 443 Remove-NetFirewallRule -DisplayName "Allow HTTPS Any" Add a new rule allowing only a specific source IP New-NetFirewallRule -DisplayName "Allow HTTPS from Mgmt Subnet" -Direction Inbound -LocalPort 443 -Protocol TCP -RemoteAddress 192.168.1.0/24 -Action Allow Create a block rule for all other traffic New-NetFirewallRule -DisplayName "Block HTTPS All Others" -Direction Inbound -LocalPort 443 -Protocol TCP -Action Block
Additionally, if you are using a FortiGate firewall, create a local-in policy to restrict management access to the EMS server’s IP.
4. Patching and Remediation: The Only Long-Term Solution
While network restrictions are a critical temporary measure, patching is the only definitive solution. Fortinet has released out-of-band patches for both vulnerabilities[reference:7]. Administrators should immediately upgrade to a patched version (7.4.5 or higher for CVE-2026-21643, and the specific hotfix versions for CVE-2026-35616). The exact patch versions are detailed in Fortinet’s official PSIRT advisory (FG-IR-25-1142)[reference:8]. The patching process typically involves downloading the update from the Fortinet support portal and running the installer on the EMS server.
If an immediate patch is not possible, consider implementing a Web Application Firewall (WAF) or an Intrusion Prevention System (IPS) rule to block the malicious SQL injection patterns. For example, a Snort rule to detect the CVE-2026-21643 exploitation might look like this:
alert tcp $EXTERNAL_NET any -> $HOME_NET $HTTP_PORTS (msg:"SQL Injection - FortiClient EMS"; flow:to_server,established; content:"/api/v1/init_consts"; http_uri; pcre:"/(union|select|sleep)/iU"; sid:1000001; rev:1;)
- Incident Response: What to Do If You Are Compromised
If you suspect your EMS server has been compromised, you must act as if the entire network is at risk. Immediately disconnect the EMS server from the network to prevent further attacker actions, such as pushing malicious policies to endpoints or exfiltrating VPN credentials. Preserve forensic evidence by creating a memory dump and a full disk image of the server. On a Linux system, you can use `dd` to create a disk image:dd if=/dev/sda of=/mnt/forensics/ems_image.dd bs=4096 status=progress
For a Windows system, use a tool like `FTK Imager` to acquire a memory and disk image. After the evidence is secured, perform a full reset of all FortiClient endpoints. Force a reinstallation of the FortiClient software and manually rotate all VPN credentials that were managed by the compromised EMS. Finally, restore the EMS server from a known, clean backup that predates the exploitation.
6. Proactive Hardening and Future Defense
To prevent future compromises, adopt a “least privilege” and “defense in depth” approach. Ensure the EMS server is not running any unnecessary services. Apply the principle of least privilege to the EMS service account, ensuring it has minimal rights on the underlying OS and database. Regularly review and rotate all service accounts and API keys. Implement a patch management policy that prioritizes security updates for critical infrastructure like the EMS server. Use a configuration management tool like Ansible to automate the enforcement of security baselines:
- name: Harden FortiClient EMS Server hosts: ems_servers tasks: - name: Ensure firewall is running and enabled service: name: firewalld state: started enabled: yes - name: Allow only management subnet to port 443 firewalld: port: 443/tcp permanent: yes state: enabled source: 192.168.1.0/24 zone: public - name: Block all other access to port 443 firewalld: port: 443/tcp permanent: yes state: disabled
Finally, integrate your EMS server logs with a SIEM solution to enable real-time alerting for the IoCs discussed earlier.
What Undercode Say:
- Key Takeaway 1: The window between vulnerability disclosure and mass exploitation is now measured in days, not months. Organizations must adopt a proactive, zero-trust approach to patching and network exposure for all management interfaces.
- Key Takeaway 2: Relying solely on perimeter defenses is insufficient. Continuous monitoring, robust logging, and automated incident response playbooks are essential to detect and contain post-exploitation activity before attackers can pivot across the network.
Prediction:
This incident marks a significant escalation in the targeting of centralized endpoint management platforms. In the coming months, we expect to see a surge in sophisticated attacks leveraging similar RCE flaws in comparable systems from other vendors. This will force a fundamental shift in security architecture, accelerating the adoption of agent-based, decentralized management models and immutable infrastructure to mitigate the impact of a single point of failure like an EMS server.
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