PYSEC-2026-2379

Summary

Microsoft APM normalizes marketplace plugins by copying plugin components referenced in plugin.json into .apm/. The manifest fields agents, skills, commands, and hooks are attacker-controlled, but the implementation does not enforce that those paths remain inside the plugin directory. A malicious plugin can therefore use absolute paths or ../ traversal paths to copy arbitrary readable host files or directories from the installer's machine during apm install.

In the verified primary proof of concept, a malicious plugin sets plugin.json.commands to an external markdown file. A single apm install copies that outside file into .apm/prompts/ and then auto-integrates it into .github/prompts/secret.prompt.md in the victim project. This is a local supply-chain trust-boundary violation with direct confidentiality and integrity impact.

Reviewed version and commit:

• apm-cli version 0.8.11
• main commit 70b34faa16a5a783424698163deeb028854fd23a

Details

Root cause:

• src/apm_cli/deps/plugin_parser.py:336-348
  • _resolve_sources() joins manifest-controlled agents, skills, commands, and directory-form hooks paths with plugin_path
  • it checks only exists() and is_symlink()
  • it does not resolve the candidate and verify containment inside the plugin root
• src/apm_cli/deps/plugin_parser.py:356-395
  • copies attacker-selected agent and skill files/directories into .apm/
• src/apm_cli/deps/plugin_parser.py:397-452
  • copies attacker-selected command and hook files/directories into .apm/
• src/apm_cli/deps/plugin_parser.py:436-442
  • string-form hook config paths are also copied without a root-containment check

There is already a safer precedent in the same module:

• src/apm_cli/deps/plugin_parser.py:195-210
  • _read_mcp_file() resolves the candidate path
  • rejects paths escaping the plugin root
  • rejects symlinks

Reachability:

• Local install path:
  • src/apm_cli/commands/install.py:2007-2015
  • local marketplace plugins are normalized through normalize_plugin_directory(...)
• Remote install path:
  • src/apm_cli/deps/github_downloader.py:2224-2230
  • downloaded packages are validated through validate_apm_package(target_path)
  • src/apm_cli/models/validation.py:164-172, 224-226, 304-324
  • marketplace plugins are normalized through the same vulnerable path after clone

Project write-back path:

• src/apm_cli/integration/prompt_integrator.py:38-56
  • reads .apm/prompts/*.prompt.md
• src/apm_cli/integration/prompt_integrator.py:170-189
  • writes prompt files into .github/prompts/
• src/apm_cli/commands/install.py:2496-2514
  • auto-integrates package primitives after install

This means a malicious dependency can cause APM to read from outside the dependency itself and materialize host-local content into managed install output and, in the verified prompt case, directly into the victim project.

PoC

The attached zip contains a complete maintainer-ready proof-of-concept package, including runnable scripts, payload templates, captured output, and the exact validation environment.

Primary end-to-end apm install reproduction:

1. Install APM from the reviewed source tree (apm-cli 0.8.11, commit 70b34faa16a5a783424698163deeb028854fd23a) into a Python environment.
2. Create an external file outside the malicious plugin directory, for example:

victim\secret.md

with content:

# STOLEN VIA APM INSTALL

3. Create a malicious plugin with this minimal plugin.json:

{
  "name": "evil-plugin",
  "commands": "D:\\absolute\\path\\to\\victim\\secret.md"
}

4. Create a minimal apm.yml that references the malicious plugin.
5. Run:

apm install

6. Observe that APM completes successfully and writes:

.github/prompts/secret.prompt.md

7. Observe that the resulting prompt file contains the external host file content:

# STOLEN VIA APM INSTALL

Verified console output from the included PoC:

[>] Installing dependencies from apm.yml...
  [+] ./evil-plugin (local)
  |-- 1 prompts integrated -> .github/prompts/

[*] Installed 1 APM dependency.
PoC succeeded.
Integrated into project: ...\.github\prompts\secret.prompt.md
Integrated content:
# STOLEN VIA APM INSTALL

Secondary remote-parity reproduction:

• The attached reproduce-remote-parity.py exercises GitHubPackageDownloader.download_package(...) after clone by replacing only the clone callback to keep the test self-contained.
• It confirms the same unsafe normalization path copies an outside host file into:

<download-target>/.apm/prompts/secret.prompt.md

Impact

This is a path traversal / arbitrary local file copy issue in the package install flow.

Who is impacted:

• any user who runs apm install against a malicious or compromised plugin dependency
• both direct and transitive dependency consumers

What an attacker gains:

• ability to copy arbitrary readable host files into .apm/ during install
• ability to copy arbitrary readable host directories recursively into .apm/
• ability to trigger project write-back when the copied content lands in supported primitive locations such as .apm/prompts/

Practical impact:

• local notes, markdown, source material, or configuration files can be staged into repository-controlled paths
• copied prompt files are automatically written into .github/prompts/, increasing the chance that sensitive or attacker-selected content is committed, synced, or consumed by other tooling
• the issue breaks the expected trust boundary that a dependency install should copy only content belonging to the dependency itself

Mitigation

Recommended fix:

1. Resolve every manifest-controlled component path against plugin_path.resolve().
2. Reject absolute or relative paths that escape the plugin root.
3. Apply the same containment check to agents, skills, commands, and both hooks code paths.
4. Reject symlinks before copying.
5. Add regression tests for:
   • absolute file path in commands
   • absolute directory path in commands
   • ../ traversal in agents
   • ../ traversal in skills
   • ../ traversal in hooks
   • confirmation that only in-root files remain accepted

Attachment

MicrosoftAPMPluginPathEscapeReportFinal.zip