CVE-2020-15936: Fortinet FortiOS Information Disclosure

CVE-2020-15936 Overview

CVE-2020-15936 is an improper input validation vulnerability affecting Fortinet FortiGate appliances running FortiOS. The vulnerability exists in the handling of SNI (Server Name Indication) Client Hello TLS packets, allowing an attacker with adjacent network access to disclose sensitive information from the affected device. This flaw stems from insufficient validation of input data during TLS handshake processing.

Critical Impact

Attackers with privileged access on an adjacent network can exploit improper input validation in FortiGate appliances to extract sensitive information via crafted SNI Client Hello TLS packets, potentially exposing confidential data and network configurations.

Affected Products

• Fortinet FortiOS version 6.4.3 and below
• Fortinet FortiOS version 6.2.5 and below
• Fortinet FortiOS version 6.0.11 and below
• Fortinet FortiOS version 5.6.13 and below

Discovery Timeline

• 2022-03-01 - CVE-2020-15936 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-15936

Vulnerability Analysis

This vulnerability is classified as CWE-20 (Improper Input Validation). The flaw occurs when FortiOS processes TLS Client Hello packets containing specially crafted SNI (Server Name Indication) extensions. The SNI extension is part of the TLS handshake protocol, designed to allow a client to indicate which hostname it is attempting to connect to during the initial handshake.

When FortiOS receives a malformed or specially crafted SNI value, the improper validation allows an attacker to extract sensitive information from the FortiGate appliance. This information disclosure vulnerability requires adjacent network access and high privileges, limiting the attack surface but still presenting a significant risk in environments where internal network segmentation is compromised.

Root Cause

The root cause of CVE-2020-15936 lies in insufficient input validation within the TLS packet processing component of FortiOS. The SNI field in TLS Client Hello messages is not properly sanitized or bounds-checked before being processed by the FortiGate appliance. This allows crafted input to bypass expected validation routines and trigger information disclosure through error conditions or improper memory handling during the TLS handshake process.

Attack Vector

The attack requires adjacent network access (AV:A), meaning the attacker must be on the same network segment as the target FortiGate appliance. Additionally, high privileges (PR:H) are required to execute the attack. The attacker sends specially crafted TLS Client Hello packets with malicious SNI values to the target FortiGate device. When processed, these packets exploit the input validation flaw to disclose sensitive information that may include internal configurations, memory contents, or other confidential data.

The attack does not require user interaction and can be executed once the attacker has established the necessary network position and privileges. The confidentiality impact is rated as high, while integrity and availability remain unaffected, indicating this is primarily an information disclosure vulnerability.

Detection Methods for CVE-2020-15936

Indicators of Compromise

• Unusual volume of TLS handshake failures or errors in FortiGate logs
• Abnormal SNI values in TLS Client Hello packets captured in network traffic
• Unexpected memory access patterns or error messages related to TLS processing
• Network traffic from adjacent segments containing malformed TLS packets targeting FortiGate management interfaces

Detection Strategies

• Monitor FortiGate system logs for TLS processing errors and anomalies
• Implement network intrusion detection rules to identify malformed SNI extensions in TLS Client Hello packets
• Deploy packet capture on network segments adjacent to FortiGate appliances to analyze TLS traffic patterns
• Use behavioral analysis to detect privilege abuse patterns from administrative accounts

Monitoring Recommendations

• Enable detailed logging on FortiGate appliances for TLS/SSL inspection features
• Configure alerts for repeated TLS handshake failures from specific source addresses
• Review administrative access logs for unusual patterns indicating potential privilege abuse
• Implement network segmentation monitoring to detect lateral movement attempts toward FortiGate devices

How to Mitigate CVE-2020-15936

Immediate Actions Required

• Upgrade FortiOS to patched versions: 6.4.4 or higher, 6.2.6 or higher, 6.0.12 or higher, or 5.6.14 or higher
• Restrict administrative access to FortiGate appliances from trusted networks only
• Implement network segmentation to limit adjacent network access to FortiGate devices
• Review and audit administrative privileges to ensure least-privilege principles are enforced

Patch Information

Fortinet has released security updates addressing this vulnerability. Organizations should consult the FortiGuard Security Advisory FG-IR-20-091 for detailed patch information and upgrade guidance. The advisory provides specific version information and upgrade paths for affected FortiOS installations.

Affected organizations should prioritize upgrading to the following minimum versions:

• FortiOS 6.4.4 for the 6.4.x branch
• FortiOS 6.2.6 for the 6.2.x branch
• FortiOS 6.0.12 for the 6.0.x branch
• FortiOS 5.6.14 for the 5.6.x branch

Workarounds

• Restrict network access to FortiGate management interfaces using access control lists (ACLs)
• Implement additional network segmentation between administrative segments and FortiGate appliances
• Enable and configure TLS inspection policies with enhanced logging to detect exploitation attempts
• Consider deploying a web application firewall or additional security layer in front of vulnerable FortiGate interfaces

# Example FortiOS configuration to restrict administrative access
config system interface
    edit "mgmt"
        set allowaccess ping https ssh
        set trust-ip-1 10.0.0.0/24
    next
end

config firewall local-in-policy
    edit 1
        set intf "mgmt"
        set srcaddr "trusted-admin-networks"
        set dstaddr "all"
        set action accept
        set service "HTTPS" "SSH"
    next
end