CVE-2022-42475

CVE-2022-42475: FortiOS/FortiProxy SSL-VPN Heap-Based Buffer Overflow

AI-generated analysis based on the vulnerability data on this page.

Summary

A heap-based buffer overflow vulnerability exists in the SSL-VPN component of FortiOS and FortiProxy. The flaw may allow a remote unauthenticated attacker to execute arbitrary code or commands by sending specifically crafted requests to the SSL-VPN endpoint.

Background

Fortinet FortiOS is the operating system powering FortiGate next-generation firewalls and other security appliances. The SSL-VPN feature provides remote-access connectivity for users and is frequently exposed to the internet. FortiProxy is a secure web gateway that also offers SSL-VPN capabilities. Because SSL-VPN endpoints are often internet-facing, vulnerabilities in this component are attractive to threat actors seeking initial access to enterprise networks.

Root Cause

The vulnerability is classified as CWE-787: Out-of-bounds Write (the source summary also references CWE-122: Heap-based Buffer Overflow). A heap-based buffer overflow occurs when an application writes data beyond the allocated heap buffer boundary. In the SSL-VPN context, this likely stems from insufficient input validation when processing client requests, allowing an attacker to corrupt adjacent memory structures and hijack control flow. The exact function or parser is not disclosed in the source data, but the attack vector is network-facing and does not require authentication.

Impact

The CVSS 3.1 vector is CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H, yielding a base score of 9.8 (Critical).

• Attack Vector (AV:N): Exploitable over the network without local access.
• Attack Complexity (AC:L): Low complexity indicates reliable exploitation is possible.
• Privileges Required (PR:N): No account or credentials are needed.
• User Interaction (UI:N): No victim action is required.
• Confidentiality, Integrity, Availability (C:H/I:H/A:H): Successful exploitation can result in complete system compromise, including data exfiltration, configuration tampering, and service disruption.

Exploitation Walkthrough (Defensive Perspective)

Ethics caveat: This section describes the vulnerability mechanics from a defender's viewpoint only. No weaponized exploit code or step-by-step attack instructions are provided.

From a defensive standpoint, exploitation proceeds as follows:

1. Reconnaissance: The attacker identifies an internet-facing FortiOS or FortiProxy SSL-VPN endpoint (commonly on TCP/443 or a dedicated SSL-VPN port).
2. Crafted Request Delivery: The attacker sends a malformed request to the SSL-VPN service. The malformed input triggers the heap-based buffer overflow by writing beyond the bounds of an allocated buffer during request processing.
3. Memory Corruption: The out-of-bounds write corrupts adjacent heap metadata or function pointers, potentially allowing the attacker to redirect execution to attacker-controlled code.
4. Post-Exploitation: If successful, the attacker gains code execution in the context of the SSL-VPN process, which typically runs with elevated privileges on the appliance, leading to full network access.

Defenders should monitor SSL-VPN access logs for anomalous request patterns, unusually large or malformed payloads, and connections from unexpected geographies or anonymization services.

Affected and Patched Versions

Based on the source data:

• FortiOS SSL-VPN: 7.2.0 through 7.2.2, 7.0.0 through 7.0.8, 6.4.0 through 6.4.10, 6.2.0 through 6.2.11, and 6.0.15 and earlier versions.
• FortiProxy SSL-VPN: 7.2.0 through 7.2.1, and 7.0.7 and earlier versions.

Specific patch release numbers are not provided in the source data. Administrators should consult the FortiGuard PSIRT advisory FG-IR-22-398 for the latest patched firmware releases.

Remediation

1. Upgrade Firmware: Apply the latest patched firmware version from Fortinet as specified in FG-IR-22-398. Prioritize internet-facing appliances.
2. Compensating Controls:
   • If SSL-VPN is not required, disable the feature entirely until patching is complete.
   • Restrict SSL-VPN access to trusted IP ranges using firewall policies or external edge controls.
   • Implement network segmentation to limit lateral movement if an appliance is compromised.
   • Ensure SSL-VPN authentication uses multi-factor authentication (MFA) to raise the bar for follow-on access, though this does not block the unauthenticated exploit itself.
3. Verify Configuration: After patching, confirm the appliance is running the expected version and review SSL-VPN settings for unnecessary exposure.

Detection

• Network Monitoring: Inspect inbound SSL-VPN traffic for malformed payloads, oversized headers, or non-standard request structures that do not match legitimate client behavior.
• Log Analysis: Review FortiOS/FortiProxy event logs and SIEM alerts for SSL-VPN process crashes, memory errors, or repeated authentication failures from a single source.
• Endpoint & Appliance Integrity: Monitor for unexpected configuration changes, new administrator accounts, or unauthorized tunnel establishments after the SSL-VPN stage.
• Threat Intelligence: Correlate SSL-VPN source IPs with known malicious infrastructure and CISA KEV indicators.

Assessment

This vulnerability carries an EPSS score of 0.99474 and sits in the 99.94th percentile, indicating an extremely high probability of active exploitation. Its presence on the CISA Known Exploited Vulnerabilities (KEV) Catalog since 13 December 2022 confirms that threat actors have weaponized it in the wild. The combination of a 9.8 CVSS score, network reachability, no authentication requirement, and confirmed exploitation makes this a patching emergency.

Key lessons:

• Edge appliances are prime targets. Internet-facing VPN concentrators and firewalls should be at the top of any vulnerability management priority list.
• Telemetry beats obscurity. Organizations that log and monitor SSL-VPN sessions are better positioned to detect exploitation attempts even before patching.

References

• https://fortiguard.com/psirt/FG-IR-22-398
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2022-42475