Kernel TCP SYN Proxy Exhaustion under SYN Flood on Low-End Models
On 2GB RAM desktop appliances (FG-40F, 60F, 70F), external TCP SYN floods with randomized source IPs exhaust the kernel sock_alloc slab cache under software SYN proxy, causing unrecoverable OOM kernel panics within 90–120 seconds.
Technical Root Cause Analysis
When administrators configure set syn-proxy enable under Virtual IPs (VIPs) or DoS interface policies to protect internal servers, the FortiOS kernel acts as a stateful intermediary for initial TCP three-way handshakes. For every incoming SYN packet, the kernel allocates a request socket slab structure (sock_alloc and tcp_syn_proxy_request_sock) to track embryonic connections until the final ACK arrives from the client.
In FortiOS 7.2.0 through 7.2.6 on 2GB RAM desktop architectures, the garbage collection routine fails to release expired request sockets rapidly enough during high-rate attacks exceeding 40,000 pps. Because each half-open state consumes kernel unreclaimable slab memory, the kmalloc-512 and sock_alloc slabs expand uncontrollably, consuming upwards of 1.7 GB of system memory. Once user-space processes and the kernel run out of memory pages, the Linux OOM-killer fails to reclaim kernel slab, culminating in a catastrophic kernel panic: "Out of memory and no killable processes".
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.2 | 7.2.0 – 7.2.6 | 7.2.7+ | Disable syn-proxy, switch to SYN cookies |
| FortiOS 7.0 | Low Risk (Different Threshold) | 7.0.x Branch Clean | Aggressive timer eviction prevents OOM |
| FortiOS 7.4 | Not Affected | 7.4.0+ | Hardware CP9 SYN cookie offload default |
Platform Impact: High vulnerability on desktop hardware models with 2GB RAM (FortiGate 40F, 60F, 61F, 70F, 71F) publishing public VIPs with software TCP SYN proxy enabled.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these commands to verify unreclaimable slab memory consumption and check for embryonic socket accumulation in the kernel:
Diagnostic Commands
# 1. Query kernel slab allocation specifically for socket structures
diagnose hardware sysinfo slab | grep sock
# 2. View bridging and netlink buffer statistics
diagnose netlink brctl list
# 3. Check total vs. unreclaimable kernel RAM usage
diagnose hardware sysinfo memory
# 4. View active TCP session counts and embryonic rate
diagnose sys session statRemediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the Ferrite interactive platform:
Frequently Asked Questions
SYN Proxy maintains an embryonic state in memory while completing the handshake on behalf of the backend server. SYN Cookies encode the connection state directly into the TCP sequence number cryptographically, requiring zero kernel memory allocation.
Conserve mode protects user-space memory allocations. In this defect, memory is consumed in kernel slab space, which cannot be throttled by user-space conserve mode, triggering an immediate kernel panic.
FortiOS 7.2.7 GA and 7.4.0+ contain updated slab quota managers that cap embryonic socket allocations at 15% of total system RAM on low-end models.