Antivirus File Buffer Exhaustion Dropping Multi-Gigabit ISO & Archive Downloads
Multi-gigabyte ISO images, software updates, and large compressed archives stall and fail around 10MB to 50MB with gateway timeouts when inspected by Antivirus profiles due to scanunitd buffer threshold lockups.
Technical Root Cause Analysis
The FortiOS Antivirus subsystem operates in tandem with the scanunitd daemon and the av_unpack decompression library. When an HTTP/HTTPS stream is routed through a security policy with an Antivirus profile, incoming file segments are queued in an allocated memory buffer up to the configured oversize-file-threshold (by default 10MB on desktop units and 20MB on mid-range units).
In affected FortiOS 7.2 releases, when a large binary download (such as an Ubuntu ISO or Windows VHDX image) exceeded the scanning threshold, a synchronization race condition occurred between the user-space buffer deallocator and the TCP socket ring. Rather than releasing the inspection buffer and transitioning the TCP socket to uninspected stream pass-through, the worker held zero-window TCP advertisements towards the client. After 60 seconds of stalled socket ACK exchanges, the connection timed out, generating an HTTP 504 Gateway Timeout or an abrupt TCP RST packet.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.2 | 7.2.2 – 7.2.6 | 7.2.7+ | Tune oversize threshold / Flow mode |
| FortiOS 7.4 | 7.4.0 – 7.4.1 | 7.4.2+ | Resolved in stream scheduler refactor |
| FortiOS 7.0 | Low Occurrence | 7.0.12+ Clean | Buffer tuning if present |
Platform Impact: Desktop and mid-range FortiGate models (FG-40F, FG-60F, FG-80F, FG-100F, FG-200F) running proxy or flow antivirus inspection on high-speed internet links (>100 Mbps).
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands to inspect scanunit buffer utilization and check active protocol options limits:
Diagnostic Commands
# 1. Inspect global firewall session statistics and socket drop counts
diagnose sys session stat
# 2. View active firewall protocol options thresholds for HTTP and FTP
show firewall profile-protocol-options
# 3. Check live status and active memory of the scanunit daemon
diagnose test application scanunit 1
# 4. View real-time antivirus inspection worker statistics
diagnose sys session filter dport 443
diagnose sys session stat | grep -i avRemediation & Workaround Steps (Teaser Preview)
Allow reliable multi-gigabyte ISO and archive downloads while maintaining real-time antivirus defense:
Frequently Asked Questions
The default 'oversize-file-threshold' in firewall profile-protocol-options for HTTP is set to 10MB on desktop appliances and 20MB on mid-range models. In vulnerable firmware, when a streaming download crosses this threshold, the scanunitd worker fails to transition the socket back to direct pass-through, resulting in zero-window TCP freezes and socket timeout drops.
Increasing the threshold merely defers the crash and increases RAM consumption on desktop models with 2GB/4GB RAM. The recommended production fix is configuring 'oversize-file-threshold' to an appropriate size while ensuring 'uncompressed-oversize-limit' allows seamless bypass, or upgrading to 7.2.7+ where stream transition locking was overhauled.
Flow-based antivirus reduces socket buffering because files are inspected on the fly using CP9/SP5 content processors without holding the complete payload in user-space RAM, substantially improving multi-gigabyte ISO transfer reliability.