CVE-2021-26108: Fortinet FortiOS Information Disclosure

CVE-2021-26108 Overview

CVE-2021-26108 is a hard-coded cryptographic key vulnerability affecting the SSLVPN component of Fortinet FortiOS. This weakness allows attackers to retrieve sensitive cryptographic keys through reverse engineering of the affected software, potentially compromising the security of VPN communications and encrypted data.

Critical Impact

Attackers can extract hard-coded cryptographic keys from FortiOS SSLVPN, enabling unauthorized decryption of sensitive communications and potential compromise of VPN security.

Affected Products

• Fortinet FortiOS versions prior to 7.0.1
• Fortinet FortiOS 7.0.0
• Fortinet FortiOS SSLVPN component across affected versions

Discovery Timeline

• 2021-12-08 - CVE-2021-26108 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-26108

Vulnerability Analysis

This vulnerability stems from the use of hard-coded cryptographic keys within the FortiOS SSLVPN implementation (CWE-798). Hard-coded credentials represent a significant security weakness because they cannot be changed without modifying the underlying code, and once discovered, they affect all deployments using the vulnerable software version.

The vulnerability is network-accessible, requiring no authentication or user interaction to exploit. An attacker with access to the FortiOS binary can perform reverse engineering to extract the embedded cryptographic key material. Once obtained, these keys could be used to decrypt intercepted VPN traffic or forge authentication credentials.

The impact is primarily on confidentiality, as successful exploitation allows attackers to compromise the cryptographic protections intended to secure VPN communications.

Root Cause

The root cause is the embedding of static cryptographic key material directly within the FortiOS SSLVPN code. This violates secure coding best practices, which mandate that cryptographic keys should be dynamically generated, securely stored, and unique per installation. The use of hard-coded keys creates a single point of failure where discovery of one key compromises all installations using the same vulnerable version.

Attack Vector

The attack vector is network-based and exploits the static nature of the embedded cryptographic key. An attacker can obtain the FortiOS firmware or binary through legitimate means (such as downloading from support portals) or by compromising a device. Using reverse engineering tools and techniques, the attacker can analyze the binary to locate and extract the hard-coded cryptographic key material.

Once the key is extracted, it can be used to:

• Decrypt previously captured SSLVPN traffic
• Potentially impersonate the VPN gateway
• Compromise the confidentiality of all communications protected by the affected key

The vulnerability does not require active interaction with a target system for key extraction—only access to the vulnerable software binary.

Detection Methods for CVE-2021-26108

Indicators of Compromise

• Unusual traffic patterns or decryption attempts against SSLVPN sessions
• Evidence of firmware extraction or unauthorized binary downloads from FortiGate devices
• Detection of reverse engineering tools targeting FortiOS binaries in the environment
• Anomalous authentication events or session establishment attempts on SSLVPN services

Detection Strategies

• Monitor for unauthorized access to FortiOS firmware files or configuration backups
• Implement network traffic analysis to detect potential man-in-the-middle attempts against SSLVPN connections
• Deploy file integrity monitoring on FortiGate devices to detect tampering
• Review access logs for any unusual firmware download or extraction activities

Monitoring Recommendations

• Enable comprehensive logging on FortiGate SSLVPN services and forward logs to a centralized SIEM
• Monitor for indicators of cryptographic protocol downgrade attacks or session anomalies
• Track firmware version information across all FortiGate deployments to ensure vulnerable versions are identified
• Implement network segmentation to limit exposure of FortiGate management interfaces

How to Mitigate CVE-2021-26108

Immediate Actions Required

• Upgrade all FortiOS installations to version 7.0.1 or later immediately
• Review VPN access logs for any suspicious activity during the exposure window
• Consider rotating any credentials or certificates associated with SSLVPN configurations
• Restrict access to FortiGate management interfaces and firmware repositories

Patch Information

Fortinet has addressed this vulnerability in FortiOS version 7.0.1 and later releases. Administrators should consult the FortiGuard Security Advisory FG-IR-21-051 for detailed upgrade instructions and version-specific guidance. The patch replaces the hard-coded cryptographic key with properly generated, unique key material.

Workarounds

• Limit network exposure of SSLVPN services to only required users and source IP ranges
• Implement additional network-layer encryption (such as IPsec tunnels) for critical VPN traffic as a defense-in-depth measure
• Enable multi-factor authentication for all SSLVPN access to reduce impact of potential credential compromise
• Monitor SSLVPN sessions closely and implement session timeout policies to limit exposure windows

# Configuration example - Restrict SSLVPN access to specific source addresses
config firewall address
    edit "SSLVPN_Allowed_Sources"
        set type iprange
        set start-ip 10.0.0.1
        set end-ip 10.0.0.254
    next
end

config vpn ssl settings
    set source-address "SSLVPN_Allowed_Sources"
end