Reduce str.replace to LOW confidence in all cases

Since the rate of false positives may be higher for
str.replace over other string constructions like str.format,
we should reduce to LOW confidence to compensate for this.

bandit/plugins/injection_sql.py

@@ -27,6 +27,13 @@
 
  - cursor.execute("SELECT %s FROM derp;" % var)
 
+Use of str.replace in the string construction can also be dangerous.
+For example:
+
+- "SELECT * FROM foo WHERE id = '[VALUE]'".replace("[VALUE]", identifier)
+
+However, such cases are always reported with LOW confidence to compensate
+for false positives, since valid ues of str.replace can be common.
 
 :Example:
 
@@ -80,6 +87,7 @@ def _check_string(data):
 def _evaluate_ast(node):
     wrapper = None
     statement = ""
+    str_replace = False
 
     if isinstance(node._bandit_parent, ast.BinOp):
         out = utils.concat_string(node, node._bandit_parent)
@@ -91,6 +99,8 @@ def _evaluate_ast(node):
         statement = node.s
         # Hierarchy for "".format() is Wrapper -> Call -> Attribute -> Str
         wrapper = node._bandit_parent._bandit_parent._bandit_parent
+        if node._bandit_parent.attr == "replace":
+            str_replace = True
     elif hasattr(ast, "JoinedStr") and isinstance(
         node._bandit_parent, ast.JoinedStr
     ):
@@ -110,19 +120,19 @@ def _evaluate_ast(node):
     if isinstance(wrapper, ast.Call):  # wrapped in "execute" call?
         names = ["execute", "executemany"]
         name = utils.get_called_name(wrapper)
-        return (name in names, statement)
+        return (name in names, statement, str_replace)
     else:
-        return (False, statement)
+        return (False, statement, str_replace)
 
 
 @test.checks("Str")
 @test.test_id("B608")
 def hardcoded_sql_expressions(context):
-    val = _evaluate_ast(context.node)
-    if _check_string(val[1]):
+    execute_call, statement, str_replace = _evaluate_ast(context.node)
+    if _check_string(statement):
         return bandit.Issue(
             severity=bandit.MEDIUM,
-            confidence=bandit.MEDIUM if val[0] else bandit.LOW,
+            confidence=bandit.MEDIUM if execute_call and not str_replace else bandit.LOW,
             cwe=issue.Cwe.SQL_INJECTION,
             text="Possible SQL injection vector through string-based "
             "query construction.",

tests/functional/test_functional.py

@@ -444,8 +444,8 @@ def test_sql_statements(self):
             },
             "CONFIDENCE": {
                 "UNDEFINED": 0,
-                "LOW": 9,
-                "MEDIUM": 11,
+                "LOW": 10,
+                "MEDIUM": 10,
                 "HIGH": 0,
             },
         }