🛡️ Sentinel: [CRITICAL] Fix parameter default expression taint leakage

.jules/sentinel.md

@@ -7,3 +7,8 @@
 **Vulnerability:** The static analyzer was missing `yaml.unsafe_load` and `yaml.full_load` in its `_SERIALISATION_SINKS` mapping, potentially leading to false negatives when tracking untrusted data flowing into these dangerous deserialization functions.
 **Learning:** Even if functions are listed in rule specifications (like `_SINK_SPECS`), they also need to be properly categorized in the core taint propagation logic (`_SERIALISATION_SINKS`) to ensure the analyzer correctly sheds validation provenance (converting output to `UNKNOWN_RAW`).
 **Prevention:** When adding new sinks to rule definitions, always verify if they need to be added to core propagation mappings like `_SERIALISATION_SINKS` or `_PROPAGATING_BUILTINS`.
+
+## 2026-06-28 - Parameter Default Expression Taint Resolution Leak
+**Vulnerability:** Untrusted data could leak into `@trusted` functions without triggering `PY-WL-101` because parameter default value expressions were defaulting to the `function_taint` (which is `ASSURED` for trusted functions), masking their true taint in L2 analysis.
+**Learning:** Default parameter expressions are evaluated at runtime in the caller's scope when an argument is omitted, not as part of the function body itself. Inheriting the `function_taint` for defaults creates a blind spot where `EXTERNAL_RAW` expressions inside parameter defaults bypass taint checks.
+**Prevention:** When evaluating parameter default expressions in static analysis (`_seed_parameters`), always resolve them with a clean base state (`TaintState.INTEGRAL`) rather than inheriting `function_taint` to ensure accurate taint propagation.

src/wardline/scanner/taint/variable_level.py

@@ -615,10 +615,10 @@ def _resolve_lambda_body_at_call_inner(
     default_taints: dict[str, TaintState] = {}
     defaulted_params = positional_params[len(positional_params) - len(args.defaults) :]
     for param, default in zip(defaulted_params, args.defaults, strict=True):
-        default_taints[param.arg] = _resolve_expr(default, function_taint, taint_map, scope)
+        default_taints[param.arg] = _resolve_expr(default, TaintState.INTEGRAL, taint_map, scope)
     for param, kw_default in zip(args.kwonlyargs, args.kw_defaults, strict=True):
         if kw_default is not None:
-            default_taints[param.arg] = _resolve_expr(kw_default, function_taint, taint_map, scope)
+            default_taints[param.arg] = _resolve_expr(kw_default, TaintState.INTEGRAL, taint_map, scope)
 
     def _omitted_seed(param_name: str) -> TaintState:
         default = default_taints.get(param_name)
@@ -767,12 +767,12 @@ def _seed_parameters(
         param_idx = total_pos_args - num_defaults + i
         if 0 <= param_idx < len(all_pos_params):
             param_name = all_pos_params[param_idx].arg
-            default_taints[param_name] = _resolve_expr(default_expr, function_taint, taint_map or {}, {})
+            default_taints[param_name] = _resolve_expr(default_expr, TaintState.INTEGRAL, taint_map or {}, {})
 
     # Map keyword-only parameter defaults
     for param, kw_default_expr in zip(args.kwonlyargs, args.kw_defaults, strict=True):
         if kw_default_expr is not None:
-            default_taints[param.arg] = _resolve_expr(kw_default_expr, function_taint, taint_map or {}, {})
+            default_taints[param.arg] = _resolve_expr(kw_default_expr, TaintState.INTEGRAL, taint_map or {}, {})
 
     def handle_arg(arg: ast.arg) -> None:
         fallback = default_taints.get(arg.arg, function_taint)

tests/golden/identity/corpus/META.json

@@ -1,5 +1,5 @@
 {
   "corpus_version": 6,
   "fingerprint_scheme": "wlfp2",
-  "reason": "call-site full-span discriminator"
+  "reason": "Use INTEGRAL for default parameter taint resolution"
 }