FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name

GitHub Security Advisory GHSA-rj5c-58rq-j5g5 (CVE-2025-62801). FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name

Response Actions

References

1. 01

   Detects FastMCP initialization with server_name containing Windows command metacharacters (&, |, >, <, ^) that would be injected into cursor:// deeplinks and executed via cmd.exe /c start

   field: contentop: regex
2. 02

   Detects direct calls to generate_cursor_deeplink with arguments containing shell metacharacters that bypass proper URL encoding

   field: contentop: regex
3. 03

   Detects open_deeplink calls with shell=True on Windows combined with unescaped cursor:// URLs containing command separators

   field: contentop: regex

Attack Examples (Rule Triggers)

1. mcp = FastMCP(name="test&calc")

2. mcp = FastMCP(name='service|whoami')

3. mcp = FastMCP(name="backup>C:\\output.txt")

4. FastMCP(name="foo^bar&cmd /c del /s C:\\")

5. generate_cursor_deeplink('app&taskkill /f', version)

6. open_deeplink(f'cursor://install?name={user_input}&cmd')

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

Benign Examples (Rule Doesn't Trigger)

1. LLM-authored benign sample 1

   mcp = FastMCP(name="my-service-v1")

2. LLM-authored benign sample 2

   mcp = FastMCP(name='calculator-tool')

3. LLM-authored benign sample 3

   mcp = FastMCP(name="weather_api_v2.0")

4. LLM-authored benign sample 4

   server_name = 'example_mcp'; FastMCP(name=server_name)

5. LLM-authored benign sample 5

   generate_cursor_deeplink(safe_name.replace(' ', '_'), '1.0')

title: FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name
id: ATR-2026-00561
rule_version: 1
status: experimental
description: 'GitHub Security Advisory GHSA-rj5c-58rq-j5g5 (CVE-2025-62801). FastMCP vulnerable to windows command injection in FastMCP Cursor installer via server_name

  '
author: ATR Community (GHSA sync)
date: 2026/06/01
schema_version: '0.1'
detection_tier: pattern
maturity: experimental
severity: critical
references:
  cve:
  - CVE-2025-62801
  cwe:
  - CWE-78
  ghsa:
  - GHSA-rj5c-58rq-j5g5
  external:
  - https://github.com/jlowin/fastmcp/security/advisories/GHSA-rj5c-58rq-j5g5
  - https://nvd.nist.gov/vuln/detail/CVE-2025-62801
  - https://github.com/advisories/GHSA-rj5c-58rq-j5g5
metadata_provenance:
  ghsa: ghsa-sync
  cve: ghsa-sync
  cwe: ghsa-sync
tags:
  category: tool-poisoning
  scan_target: runtime
  confidence: high
agent_source:
  type: llm_io
  framework:
  - any
  provider:
  - any
detection:
  condition: any
  false_positives: []
  conditions:
  - field: content
    operator: regex
    value: (?i)FastMCP\s*\(\s*name\s*=\s*["'][^"']*[&|><^]
    description: Detects FastMCP initialization with server_name containing Windows command metacharacters (&, |, >, <, ^) that would be injected into cursor:// deeplinks and executed via cmd.exe /c start
  - field: content
    operator: regex
    value: (?i)generate_cursor_deeplink\s*\([^)]*[&|><^]
    description: Detects direct calls to generate_cursor_deeplink with arguments containing shell metacharacters that bypass proper URL encoding
  - field: content
    operator: regex
    value: (?i)open_deeplink\s*\([^)]*(?:cursor://[^)]*[&|><^]|shell\s*=\s*True)
    description: Detects open_deeplink calls with shell=True on Windows combined with unescaped cursor:// URLs containing command separators
response:
  actions:
  - alert
  notify:
  - security_team
test_cases:
  true_positives:
  - input: mcp = FastMCP(name="test&calc")
    expected: triggered
    description: LLM-authored attack sample 1
  - input: mcp = FastMCP(name='service|whoami')
    expected: triggered
    description: LLM-authored attack sample 2
  - input: mcp = FastMCP(name="backup>C:\\output.txt")
    expected: triggered
    description: LLM-authored attack sample 3
  - input: FastMCP(name="foo^bar&cmd /c del /s C:\\")
    expected: triggered
    description: LLM-authored attack sample 4
  - input: generate_cursor_deeplink('app&taskkill /f', version)
    expected: triggered
    description: LLM-authored attack sample 5
  - input: open_deeplink(f'cursor://install?name={user_input}&cmd')
    expected: triggered
    description: LLM-authored attack sample 6
  true_negatives:
  - input: mcp = FastMCP(name="my-service-v1")
    expected: not_triggered
    description: LLM-authored benign sample 1
  - input: mcp = FastMCP(name='calculator-tool')
    expected: not_triggered
    description: LLM-authored benign sample 2
  - input: mcp = FastMCP(name="weather_api_v2.0")
    expected: not_triggered
    description: LLM-authored benign sample 3
  - input: server_name = 'example_mcp'; FastMCP(name=server_name)
    expected: not_triggered
    description: LLM-authored benign sample 4
  - input: generate_cursor_deeplink(safe_name.replace(' ', '_'), '1.0')
    expected: not_triggered
    description: LLM-authored benign sample 5
_llm_authored:
  model: claude-haiku-4-5-20251001
  generalization_note: 'This rule detects Windows command injection via FastMCP server_name by matching FastMCP initialization and deeplink generation functions where shell metacharacters (&, |, >, <, ^) appear in name parameters. It generalizes beyond the literal ''test&calc'' PoC to catch any attempt to inject cmd.exe separators or redirection operators into server names that will be embedded in cursor:// URIs and executed with shell=True on Windows. The three conditions target different injection points: constructor, function call, and shell execution context.'
  note: Generation-time LLM authoring; verified by deterministic gate. Runtime detection is pure regex. Human review required before merge.