Use jinja2 sandboxing by default (#12733)

* This is an opt-in feature, so users should be aware of risks if using
jinja2.
* Regardless we'll add sandboxing by default to jinja2 templates -- this
  sandboxing is a best effort basis.
* Best strategy is still to make sure that jinja2 templates are only
loaded from trusted sources.

libs/langchain/langchain/prompts/base.py

@@ -15,19 +15,33 @@
 def jinja2_formatter(template: str, **kwargs: Any) -> str:
     """Format a template using jinja2.
 
-    *Security warning*: jinja2 templates are not sandboxed and may lead
-    to arbitrary Python code execution. Do not expand jinja2 templates
-    using unverified or user-controlled inputs!
+    *Security warning*: As of LangChain 0.0.329, this method uses Jinja2's
+        SandboxedEnvironment by default. However, this sand-boxing should
+        be treated as a best-effort approach rather than a guarantee of security.
+        Do not accept jinja2 templates from untrusted sources as they may lead
+        to arbitrary Python code execution.
+
+        https://jinja.palletsprojects.com/en/3.1.x/sandbox/
     """
     try:
-        from jinja2 import Template
+        from jinja2.sandbox import SandboxedEnvironment
     except ImportError:
         raise ImportError(
             "jinja2 not installed, which is needed to use the jinja2_formatter. "
             "Please install it with `pip install jinja2`."
+            "Please be cautious when using jinja2 templates. "
+            "Do not expand jinja2 templates using unverified or user-controlled "
+            "inputs as that can result in arbitrary Python code execution."
         )
 
-    return Template(template).render(**kwargs)
+    # This uses a sandboxed environment to prevent arbitrary code execution.
+    # Jinja2 uses an opt-out rather than opt-in approach for sand-boxing.
+    # Please treat this sand-boxing as a best-effort approach rather than
+    # a guarantee of security.
+    # We recommend to never use jinja2 templates with untrusted inputs.
+    # https://jinja.palletsprojects.com/en/3.1.x/sandbox/
+    # approach not a guarantee of security.
+    return SandboxedEnvironment().from_string(template).render(**kwargs)
 
 
 def validate_jinja2(template: str, input_variables: List[str]) -> None:

libs/langchain/langchain/prompts/prompt.py

@@ -22,9 +22,16 @@ class PromptTemplate(StringPromptTemplate):
     The template can be formatted using either f-strings (default) or jinja2 syntax.
 
     *Security warning*: Prefer using `template_format="f-string"` instead of
-    `template_format="jinja2"`, since jinja2 templates are not sandboxed and may
-    lead to arbitrary Python code execution. Do not construct a jinja2 `PromptTemplate`
-    from unverified or user-controlled inputs!
+        `template_format="jinja2"`, or make sure to NEVER accept jinja2 templates
+        from untrusted sources as they may lead to arbitrary Python code execution.
+
+        As of LangChain 0.0.329, Jinja2 templates will be rendered using
+        Jinja2's SandboxedEnvironment by default. This sand-boxing should
+        be treated as a best-effort approach rather than a guarantee of security,
+        as it is an opt-out rather than opt-in approach.
+
+        Despite the sand-boxing, we recommend to never use jinja2 templates
+        from untrusted sources.
 
     Example:
 
@@ -196,6 +203,18 @@ def from_template(
     ) -> PromptTemplate:
         """Load a prompt template from a template.
 
+        *Security warning*: Prefer using `template_format="f-string"` instead of
+            `template_format="jinja2"`, or make sure to NEVER accept jinja2 templates
+            from untrusted sources as they may lead to arbitrary Python code execution.
+
+            As of LangChain 0.0.329, Jinja2 templates will be rendered using
+            Jinja2's SandboxedEnvironment by default. This sand-boxing should
+            be treated as a best-effort approach rather than a guarantee of security,
+            as it is an opt-out rather than opt-in approach.
+
+            Despite the sand-boxing, we recommend to never use jinja2 templates
+            from untrusted sources.
+
         Args:
             template: The template to load.
             template_format: The format of the template. Use `jinja2` for jinja2,

libs/langchain/tests/unit_tests/prompts/test_prompt.py

@@ -177,6 +177,17 @@ def test_prompt_from_jinja2_template() -> None:
     assert prompt == expected_prompt
 
 
+@pytest.mark.requires("jinja2")
+def test_basic_sandboxing_with_jinja2() -> None:
+    """Test basic sandboxing with jinja2."""
+    import jinja2
+
+    template = " {{''.__class__.__bases__[0] }} "  # malicious code
+    prompt = PromptTemplate.from_template(template, template_format="jinja2")
+    with pytest.raises(jinja2.exceptions.SecurityError):
+        assert prompt.format() == []
+
+
 @pytest.mark.requires("jinja2")
 def test_prompt_from_jinja2_template_multiple_inputs() -> None:
     """Test with multiple input variables."""