Potential Notepad Markdown RCE Exploitation
ELASTIC detection rule by Elastic · Elastic License v2
Identifies a process started by Notepad after opening a Markdown file. This may indicate successful exploitation of a
Notepad markdown parsing vulnerability (CVE-2026-20841) that can lead to arbitrary code execution.
[metadata]
creation_date = "2026/02/16"
integration = ["endpoint", "windows", "m365_defender", "sentinel_one_cloud_funnel"]
maturity = "production"
updated_date = "2026/05/01"
[rule]
author = ["Elastic"]
description = """
Identifies a process started by Notepad after opening a Markdown file. This may indicate successful exploitation of a
Notepad markdown parsing vulnerability (CVE-2026-20841) that can lead to arbitrary code execution.
"""
from = "now-9m"
index = [
"endgame-*",
"logs-endpoint.events.process-*",
"logs-m365_defender.event-*",
"logs-sentinel_one_cloud_funnel.*",
"logs-windows.sysmon_operational-*"
]
language = "eql"
license = "Elastic License v2"
name = "Potential Notepad Markdown RCE Exploitation"
references = ["https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-20841"]
risk_score = 73
rule_id = "7f3521dd-fb80-4548-a7eb-8db37b898dc2"
severity = "high"
tags = [
"Domain: Endpoint",
"OS: Windows",
"Use Case: Threat Detection",
"Tactic: Execution",
"Data Source: Elastic Endgame",
"Data Source: Elastic Defend",
"Data Source: Microsoft Defender XDR",
"Data Source: Sysmon",
"Data Source: SentinelOne",
"Resources: Investigation Guide",
]
timestamp_override = "event.ingested"
type = "eql"
query = '''
process where host.os.type == "windows" and event.type == "start" and
process.parent.name : "notepad.exe" and process.parent.args : "*.md" and
not process.executable : "C:\\Program Files\\WindowsApps\\Microsoft.WindowsNotepad_*\\Notepad\\Notepad.exe"
'''
note = """## Triage and analysis
### Investigating Potential Notepad Markdown RCE Exploitation
#### Possible investigation steps
- What exact Notepad-to-child path did the alert preserve?
- Why: CVE-2026-20841 abuse centers on Markdown link handling that invokes unsafe protocol handlers, so the launched child and target are decisive.
- Focus: `process.parent.executable`, `process.parent.command_line`, `process.parent.args`, child `process.executable`, and child `process.command_line`.
- Implication: escalate when Notepad opened Markdown and launched a shell, script host, downloader, browser, archive handler, installer, or custom protocol handler with a URL, UNC path, file URI, app installer URI, or staged payload; lower suspicion only when child and target match a confirmed validation workflow.
- What does child binary identity add to command intent?
- Focus: child `process.executable`, `process.hash.sha256`, `process.code_signature.subject_name`, `process.code_signature.trusted`, and `process.pe.original_file_name`.
- Implication: escalate when the child is unsigned, user-writable, signer/name-mismatched, or unusual for Markdown link handling; reduce suspicion only when signer, path, hash history, and command target all fit the same confirmed validation workflow.
- Does parent launch context point to lure delivery or controlled use?
- Focus: Markdown path from `process.parent.args`, `process.parent.command_line`, `user.id`, `host.id`, and `host.name`.
- Implication: escalate when the path is in downloads, mail or chat caches, temp, removable media, network shares, or lure-like filenames; lower suspicion only when source path and child target match a confirmed validation case for the same user or host.
- Did the child start more exploit-chain processes?
- Focus: process events on `host.id` where `process.parent.entity_id` links to `process.entity_id`, recording descendant `process.executable` and `process.command_line`. $investigate_4
- Hint: prefer entity ID-linked descendants; treat PID-only matches as candidates tightly anchored to `@timestamp`. Inspect `process.Ext.ancestry` only when direct lineage is incomplete.
- Implication: escalate for descendant shells, script interpreters, LOLBins, installers, payload runners, or browser-to-download chains; no descendants limits process scope but does not clear the Notepad launch.
- Did the child stage artifacts for later execution?
- Focus: if file telemetry exists, review process-scoped `file.path`, `file.Ext.original.path`, and later `process.executable` reuse of a written path. $investigate_0
- Hint: recover with `host.id` + `process.entity_id`; if unavailable, use `host.id` + `process.pid` + a tight alert window. Missing file telemetry is unresolved, not benign.
- Implication: escalate when the child writes executables, scripts, shortcuts, archives, or renamed payloads to user-writable paths, or a written artifact later runs; clean available file telemetry means staging is not corroborated, not that the alert is benign.
- Did the child contact payload-delivery destinations?
- Focus: if DNS or connection telemetry exists, review `dns.question.name`, `destination.ip`, and `destination.port`. $investigate_1
- Hint: recover with `host.id` + `process.entity_id`; if unavailable, use `host.id` + `process.pid` + a tight alert window. Missing DNS or connection telemetry is unresolved, not benign.
- Implication: escalate when the child resolves or connects to rare, external, newly introduced, or delivery-oriented destinations outside its role; clean available network telemetry removes one corroborator but cannot override suspicious alert-local execution.
- If local evidence is suspicious or unresolved, does the pattern change scope?
- Focus: related alerts for the same `user.id` over 48 hours, comparing child `process.executable`, child `process.command_line`, and source-path patterns from `process.parent.args` when present. $investigate_2
- Hint: if the user view is empty or ambiguous, compare related alerts for the same `host.id` over 48 hours before broadening. $investigate_3
- Implication: broaden when related alerts show the same Notepad-child pattern, source-path family, or delivery indicator across unrelated users or hosts; use recurrence for scope only, not benign closure.
- Escalate when child intent, descendant lineage, artifact staging, or destinations point to payload delivery, even if one conditional telemetry family is absent; close only when source path, child identity, command target, and `user.id`/`host.id` context prove one benign workflow; preserve evidence and escalate when telemetry is missing or contradictory.
### False positive analysis
- Authorized CVE validation, QA, or security lab activity can trigger this rule when controlled Markdown samples intentionally launch a stable helper. Confirm `process.parent.args`, child `process.executable`, child hash or signer, child `process.command_line`, and `user.id`/`host.id` cohort all match the same validation workflow; prior alerts support stability only after those anchors align.
- Before creating an exception, validate stability for the same child identity, child command-target pattern, Markdown path family from `process.parent.args`, and `user.id` or `host.id` scope. Avoid exceptions on `process.parent.name`, `process.name`, or Notepad alone.
### Response and remediation
- If confirmed benign:
- Reverse temporary containment and record the Markdown path, child process identity, command target, and cohort evidence that confirmed the benign workflow. Build an exception only for the same stable `user.id` or `host.id` scope and child identity.
- If suspicious but unconfirmed:
- Preserve the triggering Markdown file, parent `process.parent.command_line`, child `process.entity_id`, child `process.command_line`, descendant process identifiers, written artifact paths, and DNS or IP indicators before cleanup.
- Apply reversible containment tied to observed indicators, such as temporary restriction of the observed destination or heightened monitoring on the affected `host.id` and `user.id`.
- Escalate to host isolation if the child continues spawning payloads, downloading content, or contacting suspicious destinations after preservation.
- If confirmed malicious:
- Isolate the host first, then stop the malicious child and descendant `process.entity_id` values after recording `process.entity_id`, `process.command_line`, the Markdown path from `process.parent.args`, and related artifact or destination indicators. If direct response is unavailable, hand off the preserved evidence set to the containment team.
- Block confirmed malicious domains, IPs, URLs, hashes, and child process paths identified during the investigation.
- Eradicate only the dropped files, startup artifacts, scheduled tasks, registry changes, and payload runners tied to the exploit chain, then quarantine the triggering Markdown file and remediate its delivery path.
- Post-incident hardening:
- Update the Windows Notepad App to a version that remediates CVE-2026-20841, and restrict unsafe protocol-handler launch exposure for Markdown viewers where enterprise controls support it.
- Preserve or expand file and network telemetry if missing visibility prevented confirmation of Markdown source, payload staging, or destination activity.
- Record the Markdown delivery path, child-process pattern, and adjacent protocol-handler or payload variants in the case notes.
"""
setup = """## Setup
This rule is designed for data generated by [Elastic Defend](https://www.elastic.co/security/endpoint-security), which provides native endpoint detection and response, along with event enrichments designed to work with our detection rules.
Setup instructions: https://ela.st/install-elastic-defend
### Additional data sources
This rule also supports the following third-party data sources. For setup instructions, refer to the links below:
- [Microsoft Defender XDR](https://ela.st/m365-defender)
- [SentinelOne Cloud Funnel](https://ela.st/sentinel-one-cloud-funnel)
- [Sysmon Event ID 1 - Process Creation](https://ela.st/sysmon-event-1-setup)
"""
[rule.investigation_fields]
field_names = [
"@timestamp",
"host.id",
"user.id",
"process.entity_id",
"process.pid",
"process.executable",
"process.command_line",
"process.hash.sha256",
"process.code_signature.subject_name",
"process.code_signature.trusted",
"process.code_signature.status",
"process.parent.executable",
"process.parent.command_line",
"process.parent.args",
"process.working_directory",
]
[transform]
[[transform.investigate]]
label = "File events for the same child process"
description = ""
providers = [
[
{ excluded = false, field = "event.category", queryType = "phrase", value = "file", valueType = "string" },
{ excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
{ excluded = false, field = "process.entity_id", queryType = "phrase", value = "{{process.entity_id}}", valueType = "string" }
]
]
relativeFrom = "now-1h"
relativeTo = "now"
[[transform.investigate]]
label = "Network events for the same child process"
description = ""
providers = [
[
{ excluded = false, field = "event.category", queryType = "phrase", value = "network", valueType = "string" },
{ excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
{ excluded = false, field = "process.entity_id", queryType = "phrase", value = "{{process.entity_id}}", valueType = "string" }
]
]
relativeFrom = "now-1h"
relativeTo = "now"
[[transform.investigate]]
label = "Alerts associated with the user"
description = ""
providers = [
[
{ excluded = false, field = "event.kind", queryType = "phrase", value = "signal", valueType = "string" },
{ excluded = false, field = "user.id", queryType = "phrase", value = "{{user.id}}", valueType = "string" }
]
]
relativeFrom = "now-48h/h"
relativeTo = "now"
[[transform.investigate]]
label = "Alerts associated with the host"
description = ""
providers = [
[
{ excluded = false, field = "event.kind", queryType = "phrase", value = "signal", valueType = "string" },
{ excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" }
]
]
relativeFrom = "now-48h/h"
relativeTo = "now"
[[transform.investigate]]
label = "Child process events for the Notepad-spawned child"
description = ""
providers = [
[
{ excluded = false, field = "event.category", queryType = "phrase", value = "process", valueType = "string" },
{ excluded = false, field = "event.type", queryType = "phrase", value = "start", valueType = "string" },
{ excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
{ excluded = false, field = "process.parent.entity_id", queryType = "phrase", value = "{{process.entity_id}}", valueType = "string" }
],
[
{ excluded = false, field = "event.category", queryType = "phrase", value = "process", valueType = "string" },
{ excluded = false, field = "event.type", queryType = "phrase", value = "start", valueType = "string" },
{ excluded = false, field = "host.id", queryType = "phrase", value = "{{host.id}}", valueType = "string" },
{ excluded = false, field = "process.parent.pid", queryType = "phrase", value = "{{process.pid}}", valueType = "string" }
]
]
relativeFrom = "now-1h"
relativeTo = "now"
[[rule.threat]]
framework = "MITRE ATT&CK"
[[rule.threat.technique]]
id = "T1203"
name = "Exploitation for Client Execution"
reference = "https://attack.mitre.org/techniques/T1203/"
[[rule.threat.technique]]
id = "T1204"
name = "User Execution"
reference = "https://attack.mitre.org/techniques/T1204/"
[[rule.threat.technique.subtechnique]]
id = "T1204.002"
name = "Malicious File"
reference = "https://attack.mitre.org/techniques/T1204/002/"
[rule.threat.tactic]
id = "TA0002"
name = "Execution"
reference = "https://attack.mitre.org/tactics/TA0002/"