fix(general): Safe asteval rendering

checkov/terraform/graph_builder/variable_rendering/safe_eval_functions.py

@@ -360,13 +360,18 @@ def terraform_try(*args: Any) -> Any:
 SAFE_EVAL_DICT["timeadd"] = timeadd
 SAFE_EVAL_DICT["formatdate"] = formatdate
 
+# Computed once at import time; passed to every Interpreter so asteval cannot
+# overwrite TF builtins in the shared SAFE_EVAL_DICT via node_assign.
+_SAFE_EVAL_READONLY: frozenset = frozenset(SAFE_EVAL_DICT)
+
 
 def get_asteval() -> Interpreter:
     # asteval provides a safer environment for evaluating expressions by restricting the operations to a secure subset, significantly reducing the risk of executing malicious code.
     return Interpreter(
         symtable=SAFE_EVAL_DICT,
         use_numpy=False,
-        minimal=True
+        minimal=True,
+        readonly_symbols=_SAFE_EVAL_READONLY
     )
 
 

tests/terraform/graph/variable_rendering/test_string_evaluation.py

@@ -7,7 +7,8 @@
 from checkov.terraform.graph_builder.variable_rendering.evaluate_terraform import evaluate_terraform, \
     replace_string_value, \
     remove_interpolation, _find_new_value_for_interpolation
-from checkov.terraform.graph_builder.variable_rendering.safe_eval_functions import evaluate, get_asteval
+from checkov.terraform.graph_builder.variable_rendering.safe_eval_functions import evaluate, get_asteval, \
+    SAFE_EVAL_DICT
 
 
 class TestTerraformEvaluation(TestCase):
@@ -594,4 +595,19 @@ def test_evaluate_malicious_code(description: str, input_str: str)-> None:
     assert result == expected
     asteval = get_asteval()
     asteval(input_str)
-    assert asteval.error
+    assert asteval.error
+
+
+def test_asteval_symtable_is_isolated_per_call() -> None:
+    # Attacker-controlled assignments must not clobber TF builtins in SAFE_EVAL_DICT.
+    # Specifically: a quote-breakout that delivers "merge = 0" to asteval must not
+    # overwrite the merge() builtin for subsequent evaluations (F-005).
+    before_merge = SAFE_EVAL_DICT["merge"]
+
+    asteval = get_asteval()
+    asteval("merge = 0")
+
+    # The builtin must still be the original callable — not 0
+    assert SAFE_EVAL_DICT["merge"] is before_merge
+    assert callable(SAFE_EVAL_DICT["merge"])
+    assert evaluate('merge({"a": 1}, {"b": 2})') == {"a": 1, "b": 2}