CVE-2023-36639: Fortinet FortiProxy RCE Vulnerability

CVE-2023-36639 Overview

CVE-2023-36639 is a format string vulnerability affecting multiple Fortinet products including FortiProxy, FortiOS, and FortiPAM. This vulnerability allows attackers with low-privilege access to execute unauthorized code or commands via specially crafted API requests. Format string vulnerabilities occur when user-controlled input is improperly used as a format string argument in functions like printf(), enabling attackers to read from or write to arbitrary memory locations.

Critical Impact

Authenticated attackers can exploit this format string vulnerability to achieve remote code execution on affected Fortinet devices, potentially compromising network security infrastructure and gaining unauthorized access to protected networks.

Affected Products

• Fortinet FortiProxy versions 7.2.0 through 7.2.4 and 7.0.0 through 7.0.10
• Fortinet FortiOS versions 7.4.0, 7.2.0 through 7.2.4, 7.0.0 through 7.0.11, 6.4.0 through 6.4.12, 6.2.0 through 6.2.15, and 6.0.0 through 6.0.17
• Fortinet FortiPAM versions 1.0.0 through 1.0.3 and 1.1.0

Discovery Timeline

• December 13, 2023 - CVE-2023-36639 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-36639

Vulnerability Analysis

This vulnerability is classified as CWE-134 (Use of Externally-Controlled Format String). The flaw exists in the API request handling mechanism of the affected Fortinet products. When processing certain API requests, user-supplied input is passed directly to format string functions without proper sanitization, allowing attackers to inject format specifiers.

Format string vulnerabilities are particularly dangerous because they can be leveraged for multiple attack primitives. By injecting format specifiers such as %x, %n, or %s, an attacker can read stack memory contents, crash the application, or write arbitrary values to memory locations. This can lead to information disclosure, denial of service, or full remote code execution.

The attack requires network access and low-privilege authentication, meaning an attacker needs valid credentials to access the API endpoints. However, once authenticated, exploitation can be performed without user interaction, making this a significant threat for organizations with compromised or weak credentials.

Root Cause

The root cause is the improper handling of externally-controlled input in format string functions within the Fortinet API processing code. The vulnerable code paths accept user input and pass it directly to formatting functions without proper validation or sanitization. This is a classic format string vulnerability pattern where untrusted data is used as a format argument rather than as a data argument.

Attack Vector

The attack is executed over the network through specially crafted API requests to vulnerable Fortinet devices. An attacker with valid low-level credentials can send malicious API requests containing format string specifiers. The exploitation flow involves:

1. Authenticating to the target Fortinet device with low-privilege credentials
2. Identifying vulnerable API endpoints that process user-controlled format strings
3. Crafting malicious requests with format string specifiers to manipulate memory
4. Achieving code execution by overwriting function pointers or return addresses

The vulnerability mechanism involves sending API requests with format string payloads embedded in request parameters. When the vulnerable code processes these requests, the format specifiers are interpreted by underlying formatting functions, allowing memory read/write operations. See the FortiGuard Security Advisory for specific technical details and affected API endpoints.

Detection Methods for CVE-2023-36639

Indicators of Compromise

• Unusual or malformed API requests containing format string specifiers (e.g., %x, %n, %s, %p) in request parameters
• Unexpected crashes or service restarts of FortiProxy, FortiOS, or FortiPAM services
• Anomalous authentication patterns followed by API access to uncommon endpoints
• Memory corruption signatures or core dumps on affected Fortinet devices

Detection Strategies

• Monitor API logs for requests containing suspicious format string patterns such as repeated % characters or common format specifiers
• Implement intrusion detection rules to identify format string attack patterns in network traffic to Fortinet management interfaces
• Enable verbose logging on Fortinet devices and forward logs to SIEM for correlation analysis
• Deploy network-based detection for anomalous API request patterns targeting Fortinet management ports

Monitoring Recommendations

• Continuously monitor authentication logs for suspicious access patterns, especially failed attempts followed by successful logins
• Implement alerting for any API requests that trigger application errors or unexpected behavior
• Review access control lists to ensure Fortinet management interfaces are not exposed to untrusted networks
• Establish baseline normal API usage patterns to detect deviations that may indicate exploitation attempts

How to Mitigate CVE-2023-36639

Immediate Actions Required

• Verify the firmware versions of all FortiProxy, FortiOS, and FortiPAM devices in your environment against the affected version ranges
• Apply vendor patches immediately for all affected devices following Fortinet's security advisory guidance
• Restrict network access to Fortinet management interfaces to trusted administrative networks only
• Review and audit user accounts with API access privileges, removing unnecessary permissions

Patch Information

Fortinet has released security updates to address this vulnerability. Organizations should upgrade to the patched versions as specified in the FortiGuard Security Advisory FG-IR-23-138. The advisory provides detailed information about fixed versions for each affected product line. Priority should be given to internet-facing and business-critical Fortinet infrastructure.

Workarounds

• Implement strict access control lists (ACLs) to limit API access to trusted IP addresses and networks only
• Disable or restrict unnecessary API endpoints until patches can be applied
• Deploy web application firewall (WAF) rules to filter requests containing format string patterns
• Enable multi-factor authentication for all administrative and API access to reduce credential-based exploitation risk

# Example: Restrict management interface access (FortiOS CLI)
config system interface
    edit "mgmt"
        set allowaccess ping https ssh
        set trust-ip 10.0.0.0/8
    next
end