PYSEC-2026-3644 
## Summary
In every affected release, automation recurrence parsing anchors minutely and hourly rules at a fixed date of 2000-01-01 and then walks forward one interval at a time to find the next run. A single `FREQ=MINUTELY` rule therefore enumerates roughly a quarter-century of occurrences, synchronously, on the event loop that also serves the scheduler, HTTP and WebSocket traffic. Nothing bounds the walk, and nothing moves it off the loop.

## Preconditions
Any user who can create an automation. `USER_PERMISSIONS_FEATURES_AUTOMATIONS` defaults to `false`, so on a default deployment only an admin can reach the create path; it becomes reachable by ordinary users on any deployment that has granted the automations feature, which is the normal way to make the feature usable. `UVICORN_WORKERS` defaults to 1, so there is no second worker to absorb the stall. The rule needs no unusual syntax: `FREQ=MINUTELY` with no `DTSTART`, or with a `DTSTART` set well in the past, is enough.

## Impact
Availability, against every other user of the instance. One evaluation of `RRULE:FREQ=MINUTELY` takes 18.9 s of blocking CPU; adding a ten-value `BYSECOND` list multiplies the walk and takes 64.2 s. `FREQ=HOURLY` costs 0.34 s and is not materially exploitable on its own. The cost does not stop at creation: once the automation is stored, the scheduler recomputes the next run for every claimed row on each poll, so the same walk repeats on a default 10 s interval and the instance stays wedged rather than recovering. Instances that have not enabled the automations feature for non-admin users are exposed only to an admin doing this.

## Fix
Fixed in 0.11.0. Sub-daily rules are now anchored to the current clock instead of the year-2000 date, so the walk starts at the next occurrence rather than a quarter-century behind it. A caller-supplied `DTSTART` is honoured only when the number of occurrences it implies stays under a fixed bound, and is otherwise replaced by the clock-aligned anchor. The same rules that cost 18.9 s and 64.2 s now cost under a millisecond. Upgrading fully resolves the issue, no configuration change is required.

## Root cause
Affected component: `backend/open_webui/utils/automations.py`, `_parse_rule`, reached from the automation create, update and toggle handlers in `backend/open_webui/routers/automations.py` and from the scheduler's claim path in `backend/open_webui/models/automations.py`. Affected setup: every build from 0.9.0 onward, since that is when the automations feature shipped.

The fixed anchor existed to make sub-daily intervals snap to clock boundaries, so that "every 5 minutes" lands on :00, :05, :10 rather than drifting from whenever the automation happened to be created. Snapping only needs a reference point of the right phase, but the implementation used a literal far-past date as that reference and left the recurrence library to walk forward from it. The distance between the anchor and the present is therefore attacker-influenced work that grows with real time, and it was never treated as a cost that needed a bound or a thread.

## Proof of concept
Measured cost of a single next-run computation against the shipped 0.10.2 parser:

| rule | cost |
| --- | --- |
| `RRULE:FREQ=MINUTELY` | 18,880 ms |
| `RRULE:FREQ=MINUTELY;BYSECOND=0,1,2,3,4,5,6,7,8,9` | 64,236 ms |
| `DTSTART:20000101T000000` + `RRULE:FREQ=MINUTELY` | 26,237 ms |
| `RRULE:FREQ=HOURLY` | 339 ms |

Measured at function level deliberately. Persisting such an automation has the scheduler repeat the walk on every poll and wedge the instance indefinitely, which is the actual impact but makes a live end-to-end run destructive to the test instance.

## Credits
Reported by @Classic298.
CVSS v3.1 score: 6.5