Syslog RFC 5424 Header Truncation Breaking Splunk & Microsoft Sentinel Parsers
When forwarding high-fidelity UTM security events to enterprise SIEM collectors via UDP syslog, FortiOS miglogd abruptly truncates payloads at 1024 bytes, creating unclosed quote literals and corrupting automated parsing rules.
Technical Root Cause Analysis
When deep inspection profiles (Antivirus, IPS, Web Filtering, Application Control) generate security events, the resulting log line contains extensive contextual metadata including long URI query parameters, user-agent headers, certificate distinguished names, and custom policy remarks.
Under FortiOS Bug 0883401, the internal syslog formatting thread in miglogd allocates a static 1024-byte buffer for UDP datagram assembly. Even when administrators configure RFC 5424 formatted output (set format rfc5424), miglogd performs hard truncation at byte 1024 rather than dynamically growing the datagram or fragmenting the message. When SIEM indexers (Splunk, Microsoft Sentinel, IBM QRadar) receive the incomplete datagram, the terminating quotation mark or delimiter is missing (e.g., msg="Trojan.Generic detected in https://cdn.malicious.net/dl/update?token=...). The SIEM regex parser encounters a syntax error and flags the event as an unparseable malformed record, dropping critical indicator-of-compromise (IoC) fields from automated security alerts.
Affected Firmware & Blast Radius Matrix
| FortiOS Branch | Vulnerable Builds | Confirmed Clean Build | Status & Workaround |
|---|---|---|---|
| FortiOS 7.2 | 7.2.1 – 7.2.5 | 7.2.6+ | Switch to TCP/TLS syslog with max-message-size 4096 |
| FortiOS 7.4 | Not Affected | 7.4.0+ Clean | Native 8192 byte RFC 5424 buffer supported |
| FortiOS 7.0 | 7.0.0 – 7.0.12 | 7.0.13+ | Disable url-logging in webfilter profile |
Platform Impact: Any FortiGate perimeter forwarding high-volume security logs to third-party SIEMs over standard UDP syslog port 514.
Step 01: Free Verification CLI (Safe Read-Only)
Execute these diagnostic commands to check your current syslog configuration and inspect whether messages are truncating:
Diagnostic Commands
# 1. Inspect active syslog collector configuration
show log syslogd setting
# 2. Check miglogd queue status and dropped/truncated counters
diagnose test application miglogd 6
# 3. Trigger a test syslog event to observe formatting and size
diagnose log syslogd test
# 4. Check whether RFC 5424 format is currently enforced
show log syslogd setting | grep -i "format"Remediation & Workaround Steps (Teaser Preview)
Follow the complete step-by-step remediation guide in the Ferrite interactive platform:
Frequently Asked Questions
Legacy BSD syslog (RFC 3164) restricted datagram payloads to 1024 bytes. In FortiOS builds prior to 7.2.6, miglogd enforced a static 1024-byte buffer on standard UDP syslog sockets regardless of whether the modern RFC 5424 header format was selected.
When a syslog message is truncated mid-string (for example, inside a long HTTP URL, user-agent, or virus name), key-value parsers like Splunk's KV_MODE or Sentinel's CEF regex parser cannot find the closing quotation mark. The entire second half of the log event is either dropped or merged into unsearchable raw text.
The defect is permanently resolved in FortiOS 7.2.6 and FortiOS 7.4.0+, which support configurable max-message-size (up to 8192 bytes) and dynamic packet fragment handling for RFC 5424 syslog.