Sysmon Event ID 255: Error
- Event ID
- 255
- Channel
- Microsoft-Windows-Sysmon/Operational
- Provider
- Microsoft-Windows-Sysmon
- Log file
- Microsoft-Windows-Sysmon%4Operational.evtx
- Category
- Log integrity
- Default logging
- Needs configuration
What event 255 means
Sysmon event 255 is written when Sysmon hits an internal error: heavy system load that forces it to drop events, operations that fail, bugs, or security and integrity conditions that are not met.
For an analyst the meaning is simple: the Sysmon record around this time may be incomplete. Occasional errors happen on busy hosts, but repeated errors, or errors on a host under investigation, should make you rely more on other sources (Security log, EDR) for that period. The event cannot be filtered out by the configuration.
When it is logged
Sysmon installed. This event cannot be filtered by the configuration.
Key fields
| Field | What it tells you |
|---|---|
| UtcTime | Time of the error. |
| ID | Internal identifier of the error type. |
| Description | Text describing the error, e.g. which event types were dropped from the driver queue. |
Common benign sources
- Dropped events on heavily loaded servers or during boot storms.
- Errors after upgrades or with configuration schema mismatches.
What attackers do that produces it
- Errors appearing while an attacker interferes with the Sysmon driver or service.
Investigation tips
- Read Description to learn which event types may be missing and for how long.
- Check for events 4 and 16 around the same time.
- Fill the gap with Security, System and EDR telemetry for the affected period.
Sigma rules for this event
1 SigmaHQ detection rules (release r2026-07-01) target this event.
- High · 1
- HighSysmon Configuration ErrorRule by frack113, SigmaHQ, DRL 1.1
Rules by their named authors, published by SigmaHQ under the Detection Rule License (DRL) 1.1.
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