Flowise System Message Override via Template Interpolation (CVE-2025-59528)

Detects exploitation of the Flowise chatflow System Message template injection vulnerability (CVE-2025-59528). Flowise renders {{$flow.variables.X}} and {{$input}} in the System Message field without sanitization, allowing an attacker-controlled chat input to overwrite the system prompt and pivot the chatflow's tool-calling posture. Public PoCs achieved RCE via the vm.runInNewContext / new Function sink reached from a polluted System Message. 21 GHSAs published 2026-04-15 cover the affected chatflow surfaces (Airtable Agent, CSV Agent, Parameter Override, etc.). Disclosed 2026-04-14.

Response Actions

References

1. 01

   Flowise template variable interpolation syntax in tool content — the {{$flow.variables.X}} sink

   field: contentop: regex
2. 02

   {{$input}} combined with system message / tool definition context — classic system-prompt override vector

   field: contentop: regex
3. 03

   Explicit system/assistant/tool role assignment using Flowise template interpolation

   field: contentop: regex
4. 04

   Downstream RCE sink (vm.runInNewContext / new Function / eval) receiving Flowise template interpolation — direct RCE path

   field: contentop: regex
5. 05

   Explicit Flowise chatflow injection/override language in skill description

   field: contentop: regex

Attack Examples (Rule Triggers)

1. system: {{$input}}
   tool_definition: {{$flow.variables.handler}}

2. vm.runInNewContext(`(${ {{$input}} })()`, sandbox)

3. context_variables:
     user_note: {{$flow.variables.note}}
   system: {{$input}}

4. The chatflow System Message uses {{$flow.variables.tier}} to set the assistant role. Users can override via {{$input}}.

5. new Function('return ' + {{$input}})();

Real-world attack payloads (sanitized). Used for regression testing.

Benign Examples (Rule Doesn't Trigger)

1. Benign skill with no template syntax

   This skill fetches weather data for a given city and returns a forecast.

2. Legitimate documentation mentioning flow variables without the {{$...}} syntax

   Variables are passed via the flow configuration. Set tier and severity before calling.

3. Jinja2 template syntax — double braces without the $ prefix

   Use Jinja2 templates for rendering: {{ variable_name }}.

4. Legitimate chatflow description mentioning system message without template injection

   The chatflow processes input through validation layers before reaching the system message.

Known False Positive Contexts

• ▸Flowise documentation or tutorials demonstrating {{$flow.variables}} syntax in educational context.
• ▸Security research tooling that scans for Flowise CVE payloads as reference samples.
• ▸Legitimate Flowise chatflow config files that use {{$flow.variables}} in non-System-Message fields with proper sanitization applied upstream.

Documented Evasion Techniques

1. Technique: string wrapping

   {{'{{$input}}'}}

   Attacker wraps the interpolation in a string literal to avoid direct regex match. Regex anchors on literal {{$ so this specific wrap evades it.
2. Technique: python attribute access

   system = getattr(flow, 'variables')['input']

   Python-style equivalent of {{$flow.variables.input}} — evades Flowise-specific regex but should be caught by ATR-2026-00297 (Python code execution) if the context makes the execution path visible.

Publicly documented bypasses. We disclose known limitations rather than pretend they don't exist.

title: "Flowise System Message Override via Template Interpolation (CVE-2025-59528)"
id: ATR-2026-00210
rule_version: 1
status: experimental
description: >
  Detects exploitation of the Flowise chatflow System Message template injection
  vulnerability (CVE-2025-59528). Flowise renders {{$flow.variables.X}} and
  {{$input}} in the System Message field without sanitization, allowing an
  attacker-controlled chat input to overwrite the system prompt and pivot the
  chatflow's tool-calling posture. Public PoCs achieved RCE via the
  vm.runInNewContext / new Function sink reached from a polluted System Message.
  21 GHSAs published 2026-04-15 cover the affected chatflow surfaces
  (Airtable Agent, CSV Agent, Parameter Override, etc.). Disclosed 2026-04-14.
author: "ATR Community"
date: "2026/04/22"
schema_version: "0.1"
detection_tier: pattern
maturity: experimental
severity: critical

references:
  owasp_llm:
    - "LLM01:2025 - Prompt Injection"
    - "LLM05:2025 - Improper Output Handling"
  owasp_agentic:
    - "ASI01:2026 - Agent Behaviour Hijack"
    - "ASI05:2026 - Unexpected Code Execution"
  mitre_atlas:
    - "AML.T0051.001 - Indirect Prompt Injection"
    - "AML.T0040 - ML Model Inference API Access"
  mitre_attack:
    - "T1059 - Command and Scripting Interpreter"
    - "T1190 - Exploit Public-Facing Application"
  cve:
    - "CVE-2025-59528"

metadata_provenance:
  mitre_atlas: human-reviewed
  owasp_llm: human-reviewed
  owasp_agentic: human-reviewed
compliance:
  eu_ai_act:
    - article: "15"
      context: "CVE-2025-59528 Flowise template injection allows attacker-controlled chat input to overwrite the system prompt and achieve RCE via vm.runInNewContext; Article 15 cybersecurity requirements mandate that AI system implementations sanitize all user-controlled inputs before template interpolation into privileged execution contexts."
      strength: primary
    - article: "9"
      context: "System message template injection via unsanitized user input is a documented high-risk vulnerability class; Article 9 risk management systems must include input sanitization controls as a mandatory risk treatment for AI systems that use template-based system prompt construction."
      strength: secondary
  nist_ai_rmf:
    - subcategory: "MP.5.1"
      context: "CVE-2025-59528 Flowise template injection is an adversarial input attack where attacker-controlled chat input exploits unsanitized {{$flow.variables}} and {{$input}} interpolation to overwrite the system prompt and achieve RCE via vm.runInNewContext; MP.5.1 requires this injection vector to be identified and tracked as an adversarial input risk."
      strength: primary
    - subcategory: "MG.2.3"
      context: "Risk treatment plans under MG.2.3 must mandate input sanitization before Flowise template interpolation, as the CVE-2025-59528 attack achieves system prompt override and remote code execution through the vm.runInNewContext and new Function sinks."
      strength: secondary
  iso_42001:
    - clause: "6.2"
      context: "Risk treatment activities under clause 6.2 must address the Flowise system message override vulnerability (CVE-2025-59528), implementing sanitization controls that prevent attacker-controlled chat inputs from reaching template interpolation in privileged execution contexts."
      strength: primary
    - clause: "8.6"
      context: "Operational controls under clause 8.6 must include detection of Flowise template interpolation syntax ({{$flow.variables}}, {{$input}}) in content fields that feed into system messages or code execution sinks, blocking the CVE-2025-59528 RCE pathway."
      strength: secondary

tags:
  category: tool-poisoning
  subcategory: template-injection-rce
  scan_target: skill
  confidence: high

agent_source:
  type: llm_io
  framework:
    - flowise
    - any
  provider:
    - any

detection:
  condition: any
  false_positives:
    - "Flowise documentation or tutorials demonstrating {{$flow.variables}} syntax in educational context."
    - "Security research tooling that scans for Flowise CVE payloads as reference samples."
    - "Legitimate Flowise chatflow config files that use {{$flow.variables}} in non-System-Message fields with proper sanitization applied upstream."
  conditions:
    - field: content
      operator: regex
      value: '\{\{\$flow\.variables\.[a-zA-Z_][a-zA-Z0-9_]*\}\}'
      description: "Flowise template variable interpolation syntax in tool content — the {{$flow.variables.X}} sink"

    - field: content
      operator: regex
      value: '(?i)\{\{\$input\}\}.*(?:system\s+message|assistant\s+role|tool\s+definition|function\s+spec)'
      description: "{{$input}} combined with system message / tool definition context — classic system-prompt override vector"

    - field: content
      operator: regex
      value: '(?i)(?:system|assistant|tool)\s*[:=]\s*\{\{\$(?:input|flow\.variables)'
      description: "Explicit system/assistant/tool role assignment using Flowise template interpolation"

    - field: content
      operator: regex
      value: '(?i)(?:vm\.runInNewContext|new\s+Function|eval)\s*\([^)]{0,200}\{\{\$(?:input|flow\.variables)'
      description: "Downstream RCE sink (vm.runInNewContext / new Function / eval) receiving Flowise template interpolation — direct RCE path"

    - field: content
      operator: regex
      value: '(?i)flowise[^\n]{0,80}(?:chatflow|system\s+message)[^\n]{0,120}(?:injection|override|bypass|rce)'
      description: "Explicit Flowise chatflow injection/override language in skill description"

response:
  actions:
    - alert
    - block_tool
  message_template: >
    [ATR-2026-00210] CRITICAL: Flowise template injection pattern detected
    (CVE-2025-59528). Content uses {{$flow.variables}} or {{$input}} in a
    context that can override the System Message prompt. Public PoCs achieved
    RCE via vm.runInNewContext. Block and inspect the chatflow configuration.

confidence: 85

test_cases:
  true_positives:
    - input: "system: {{$input}}\ntool_definition: {{$flow.variables.handler}}"
      expected: triggered
      description: "Direct system message override via Flowise template interpolation"
    - input: "vm.runInNewContext(`(${ {{$input}} })()`, sandbox)"
      expected: triggered
      description: "RCE sink receiving {{$input}} — the exact PoC pattern for CVE-2025-59528"
    - input: "context_variables:\n  user_note: {{$flow.variables.note}}\nsystem: {{$input}}"
      expected: triggered
      description: "Multi-field template injection targeting system role assignment"
    - input: "The chatflow System Message uses {{$flow.variables.tier}} to set the assistant role. Users can override via {{$input}}."
      expected: triggered
      description: "Skill documentation describing the vulnerable pattern — flags as suspicious"
    - input: "new Function('return ' + {{$input}})();"
      expected: triggered
      description: "new Function RCE sink receiving unsanitized {{$input}}"

  true_negatives:
    - input: "This skill fetches weather data for a given city and returns a forecast."
      expected: not_triggered
      description: "Benign skill with no template syntax"
    - input: "Variables are passed via the flow configuration. Set tier and severity before calling."
      expected: not_triggered
      description: "Legitimate documentation mentioning flow variables without the {{$...}} syntax"
    - input: "Use Jinja2 templates for rendering: {{ variable_name }}."
      expected: not_triggered
      description: "Jinja2 template syntax — double braces without the $ prefix"
    - input: "The chatflow processes input through validation layers before reaching the system message."
      expected: not_triggered
      description: "Legitimate chatflow description mentioning system message without template injection"

evasion_tests:
  - input: "{{'{{$input}}'}}"
    expected: not_triggered
    bypass_technique: string_wrapping
    notes: "Attacker wraps the interpolation in a string literal to avoid direct regex match. Regex anchors on literal {{$ so this specific wrap evades it."
  - input: "system = getattr(flow, 'variables')['input']"
    expected: not_triggered
    bypass_technique: python_attribute_access
    notes: "Python-style equivalent of {{$flow.variables.input}} — evades Flowise-specific regex but should be caught by ATR-2026-00297 (Python code execution) if the context makes the execution path visible."