[DOCS] EQL: Document until keyword support

docs/reference/eql/limitations.asciidoc

@@ -41,5 +41,3 @@ queries that contain:
 ** {eql-ref}/pipes.html#sort[`sort`]
 ** {eql-ref}/pipes.html#unique[`unique`]
 ** {eql-ref}/pipes.html#unique-count[`unique_count`]
-
-* The `until` {eql-ref}/sequences.html[sequence keyword]

docs/reference/eql/search.asciidoc

@@ -31,6 +31,8 @@ PUT /sec_logs/_bulk?refresh
 { "@timestamp": "2020-12-07T11:07:08.000Z", "agent": { "id": "8a4f500d" }, "event": { "category": "file", "id": "bYA7gPay", "sequence": 4 }, "file": { "accessed": "2020-12-07T11:07:08.000Z", "name": "cmd.exe", "path": "C:\\Windows\\System32\\cmd.exe", "type": "file", "size": 16384 }, "process": { "name": "cmd.exe", "path": "C:\\Windows\\System32\\cmd.exe" } }
 {"index":{"_index" : "sec_logs", "_id" : "5"}}
 { "@timestamp": "2020-12-07T11:07:09.000Z", "agent": { "id": "8a4f500d" }, "event": { "category": "process", "id": "aR3NWVOs", "sequence": 5 }, "process": { "name": "regsvr32.exe", "path": "C:\\Windows\\System32\\regsvr32.exe" } }
+{"index":{"_index" : "sec_logs", "_id" : "6"}}
+{ "@timestamp": "2020-12-07T11:07:10.000Z", "agent": { "id": "8a4f500d" }, "event": { "category": "process", "id": "GTSmSqgz0U", "sequence": 6, "type": "termination" }, "process": { "name": "regsvr32.exe", "path": "C:\\Windows\\System32\\regsvr32.exe" } }
 ----
 // TESTSETUP
 
@@ -99,7 +101,7 @@ https://en.wikipedia.org/wiki/Unix_time[Unix epoch], in ascending order.
             "name": "cmd.exe",
             "path": "C:\\Windows\\System32\\cmd.exe"
           }
-        },      
+        },
         "sort": [
           1607252645000
         ]
@@ -389,6 +391,27 @@ contains the shared `agent.id` value for each matching event.
 }
 ----
 // TESTRESPONSE[s/"took": 60/"took": $body.took/]
+
+You can use the <<eql-until-keyword,`until` keyword>> to specify an expiration
+event for sequences. Matching sequences must end before this event.
+
+The following request adds
+`until [ process where event.type == "termination" ]` to the previous EQL query.
+This ensures matching sequences end before a process termination event.
+
+[source,console]
+----
+GET /sec_logs/_eql/search
+{
+  "query": """
+    sequence by agent.id with maxspan=1h
+      [ file where file.name == "cmd.exe" ]
+      [ process where stringContains(process.name, "regsvr32") ]
+    until [ process where event.type == "termination" ]
+  """
+}
+----
+// TEST[s/search/search\?filter_path\=\-\*\.sequences\.\*events\.\*fields/]
 ====
 
 [discrete]
@@ -547,7 +570,7 @@ tiebreaker for events with the same timestamp.
 }
 ----
 // TESTRESPONSE[s/"took": 34/"took": $body.took/]
-<1> The event's <<eql-search-api-timestamp-field,timestamp>>, converted to 
+<1> The event's <<eql-search-api-timestamp-field,timestamp>>, converted to
 milliseconds since the https://en.wikipedia.org/wiki/Unix_time[Unix
 epoch]
 <2> The event's `event.id` value.

docs/reference/eql/syntax.asciidoc

@@ -485,7 +485,7 @@ sequence by user.name
 ----
 ====
 
-You can combine the `sequence by` and `with maxspan` keywords to constrain a 
+You can combine the `sequence by` and `with maxspan` keywords to constrain a
 sequence by both field values and a timespan.
 
 [source,eql]
@@ -513,6 +513,89 @@ sequence by user.name with maxspan=15m
 ----
 ====
 
+[discrete]
+[[eql-until-keyword]]
+==== `until` keyword
+
+You can use the `until` keyword to specify an expiration event for sequences.
+Matching sequences must end before this event, which is not included the
+results. If this event occurs within a sequence, the sequence is not considered
+a match.
+
+[source,eql]
+----
+sequence
+  [ event_category_1 where condition_1 ]
+  [ event_category_2 where condition_2 ]
+  ...
+until [ event_category_2 where condition_2 ]
+----
+
+.*Example*
+[%collapsible]
+====
+The following EQL sequence query uses the `until` keyword to end sequences
+before a process termination event. Process termination events have an event
+category of `process` and `event.type` value of `termination`.
+
+[source,eql]
+----
+sequence
+  [ file where file.extension == "exe" ]
+  [ process where true ]
+until [ process where event.type == "termination" ]
+----
+====
+
+[TIP]
+====
+The `until` keyword can be helpful when searching for process sequences in
+Windows event logs, such as those ingested using
+{winlogbeat-ref}/index.html[Winlogbeat].
+
+In Windows, a process ID (PID) is unique only while a process is running. After
+a process terminates, its PID can be reused.
+
+You can search for a sequence of events with the same PID value using the `by`
+and `sequence by` keywords.
+
+.*Example*
+[%collapsible]
+=====
+The following EQL query uses the `sequence by` keyword to match a sequence of
+events that share the same `process.pid` value.
+
+[source,eql]
+----
+sequence by process.pid
+  [ process where process.name == "cmd.exe" ]
+  [ process where process.name == "whoami.exe" ]
+----
+=====
+
+However, due to PID reuse, this can result in a matching sequence that
+contains events across unrelated processes. To prevent false positives, you can
+use the `until` keyword to end matching sequences before a process termination
+event.
+
+.*Example*
+[%collapsible]
+=====
+The following EQL query uses the `until` keyword to end sequences before
+`process` events with an `event.type` of `termination`. These events indicate a
+process has been terminated.
+
+[source,eql]
+----
+sequence by process.pid
+  [ process where process.name == "cmd.exe" ]
+  [ process where process.name == "whoami.exe" ]
+until [ process where event.type == "termination" ]
+----
+=====
+
+====
+
 [discrete]
 [[eql-functions]]
 === Functions