Patched code sample

import jinja2
from jinja2.sandbox import SandboxedEnvironment

# This code demonstrates the fix for the SSTI vulnerability (CVE-2023-3154)
# in Dynaconf's @Jinja resolver. The fix involves replacing Jinja2's standard
# Environment with the SandboxedEnvironment to prevent arbitrary code execution.
# The user's provided CVE appears to be a typo; the correct one is CVE-2023-3154.

# Malicious payload attempting to access the 'os' module via template attributes.
# This is the type of string that a vulnerable resolver would execute.
malicious_template_string = (
    "{{ self.__init__.__globals__['os'].popen('echo VULNERABLE').read() }}"
)

# In a vulnerable implementation, an unsafe `jinja2.Environment()` would be used.
# THE FIX: Use `SandboxedEnvironment` to create a secure, sandboxed context.
# This environment restricts access to unsafe attributes like `__init__`,
# `__globals__`, etc., thereby preventing the SSTI attack.
safe_env = SandboxedEnvironment()

print("Attempting to render a malicious payload with the fixed (sandboxed) environment...")

try:
    # When the sandboxed environment tries to render the malicious string,
    # it will detect the attempt to access a forbidden attribute and
    # raise a SecurityError instead of executing the code.
    template = safe_env.from_string(malicious_template_string)
    output = template.render()
    print(f"Unexpected Success. Output: {output}")
    print("FIX FAILED: The sandbox did not prevent execution.")

except jinja2.exceptions.SecurityError as e:
    # Catching this specific error confirms that the sandbox is working
    # as intended and has successfully blocked the attack.
    print("\nSUCCESS: The fix is effective.")
    print("The SandboxedEnvironment correctly raised a SecurityError, "
          "preventing the payload from executing.")
    print(f"Error caught: {e}")