CVE-2023-27997: FortiOS SSL-VPN X-Proxy-Host Heap Buffer Overflow Triage
A critical pre-authentication heap-based buffer overflow [CWE-122] in the FortiOS sslvpnd proxy engine allows unauthenticated remote attackers to achieve arbitrary code execution via crafted HTTP handshakes referencing X-Proxy-Host headers.
Technical Root Cause Analysis
When a remote client establishes a connection to the FortiGate SSL-VPN portal (port 443 or custom admin port), the user-space daemon /bin/sslvpnd initializes an HTTP session context before initiating credential verification. The process extracts URL paths and HTTP request headers to support reverse-proxy redirection and portal bookmarking.
Specifically, inside the internal function ssl_vpn_get_proxy_host, user-controlled data supplied via URI parameters or X-Proxy-Host / redirection headers is decoded from hex or base64 encoding. The destination heap buffer length is calculated using an incorrect byte count that omits necessary padding and string-termination bounds.
When decoded bytes are copied into the newly allocated heap memory chunk, an attacker can overflow the destination boundary by several hundred bytes. This overwrites adjacent heap chunk headers and function pointers, allowing arbitrary memory manipulation and full pre-authentication root compromise.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.2 | 7.2.0 – 7.2.4 | 7.2.5+ | Upgrade firmware; disable SSL-VPN on WAN |
| FortiOS 7.0 | 7.0.0 – 7.0.11 | 7.0.12+ | Upgrade firmware; restrict local-in access |
| FortiOS 6.4 | 6.4.0 – 6.4.12 | 6.4.13+ | Upgrade or migrate to FortiOS 7.2 LTS |
| FortiOS 7.4 | Not Affected | 7.4.0+ Clean | Clean heap allocator architecture |
Platform Impact: Affects all enterprise hardware models (FG-40F through FG-3000F) and virtual appliances configured with either SSL-VPN Web Mode or Tunnel Mode. Disabling Web Mode alone does not mitigate this vulnerability.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands via SSH or console to audit active SSL-VPN tunnels, inspect daemon process states, and search crashlogs for buffer corruption indicators:
Diagnostic Commands
# 1. Audit active connected SSL-VPN users and tunnel allocations
diagnose vpn ssl list
# 2. Inspect kernel crashlog for sslvpnd segfaults and heap corruption dumps
diagnose sys log crashlog read | grep -B 2 -A 5 "sslvpnd"
# 3. Verify sslvpnd daemon process ID and memory allocation state
fnsysctl ps | grep sslvpndRemediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the Ferrite interactive platform:
Frequently Asked Questions
No. CVE-2023-27997 triggers in the pre-authentication proxy handshake routine before the portal mode is evaluated. Both Web Mode and Tunnel Mode are vulnerable when SSL-VPN is active on an interface.
Decoding encoded parameters (such as proxy redirection host headers) into undersized heap buffers causes a heap buffer overflow in sslvpnd, enabling code execution under root privileges.
The fix is present in FortiOS 7.2.5, 7.0.12, 6.4.13, 6.2.14, and all releases of FortiOS 7.4.x and 7.6.x.