Python: Handle f-strings in (current) taint tracking

change-notes/1.25/analysis-python.md

@@ -20,3 +20,4 @@ The following changes in version 1.25 affect Python analysis in all applications
 ## Changes to libraries
 
 * Importing `semmle.python.web.HttpRequest` will no longer import `UntrustedStringKind` transitively. `UntrustedStringKind` is the most commonly used non-abstract subclass of `ExternalStringKind`. If not imported (by one mean or another), taint-tracking queries that concern `ExternalStringKind` will not produce any results. Please ensure such queries contain an explicit import (`import semmle.python.security.strings.Untrusted`).
+* Added support for tainted f-strings.

python/ql/src/semmle/python/security/strings/Basic.qll

@@ -27,7 +27,8 @@ abstract class StringKind extends TaintKind {
       os_path_join(fromnode, tonode) or
       str_format(fromnode, tonode) or
       encode_decode(fromnode, tonode) or
-      to_str(fromnode, tonode)
+      to_str(fromnode, tonode) or
+      f_string(fromnode, tonode)
     )
     or
     result = this and copy_call(fromnode, tonode)
@@ -61,13 +62,13 @@ private class StringEqualitySanitizer extends Sanitizer {
   }
 }
 
-/* tonode = ....format(fromnode) */
+/** tonode = ....format(fromnode) */
 private predicate str_format(ControlFlowNode fromnode, CallNode tonode) {
   tonode.getFunction().(AttrNode).getName() = "format" and
   tonode.getAnArg() = fromnode
 }
 
-/* tonode = codec.[en|de]code(fromnode)*/
+/** tonode = codec.[en|de]code(fromnode) */
 private predicate encode_decode(ControlFlowNode fromnode, CallNode tonode) {
   exists(FunctionObject func, string name |
     not func.getFunction().isMethod() and
@@ -81,7 +82,7 @@ private predicate encode_decode(ControlFlowNode fromnode, CallNode tonode) {
   )
 }
 
-/* tonode = str(fromnode)*/
+/** tonode = str(fromnode) */
 private predicate to_str(ControlFlowNode fromnode, CallNode tonode) {
   tonode.getAnArg() = fromnode and
   (
@@ -91,7 +92,7 @@ private predicate to_str(ControlFlowNode fromnode, CallNode tonode) {
   )
 }
 
-/* tonode = fromnode[:] */
+/** tonode = fromnode[:] */
 private predicate slice(ControlFlowNode fromnode, SubscriptNode tonode) {
   exists(Slice all |
     all = tonode.getIndex().getNode() and
@@ -101,12 +102,17 @@ private predicate slice(ControlFlowNode fromnode, SubscriptNode tonode) {
   )
 }
 
-/* tonode = os.path.join(..., fromnode, ...) */
+/** tonode = os.path.join(..., fromnode, ...) */
 private predicate os_path_join(ControlFlowNode fromnode, CallNode tonode) {
   tonode = Value::named("os.path.join").getACall() and
   tonode.getAnArg() = fromnode
 }
 
+/** tonode = f"... {fromnode} ..." */
+private predicate f_string(ControlFlowNode fromnode, ControlFlowNode tonode) {
+  tonode.getNode().(Fstring).getAValue() = fromnode.getNode()
+}
+
 /**
  * A kind of "taint", representing a dictionary mapping str->"taint"
  *

python/ql/test/3/library-tests/taint/strings/Taint.qll

@@ -0,0 +1,44 @@
+import python
+import semmle.python.dataflow.TaintTracking
+import semmle.python.security.strings.Untrusted
+import semmle.python.security.Exceptions
+
+class SimpleSource extends TaintSource {
+  SimpleSource() { this.(NameNode).getId() = "TAINTED_STRING" }
+
+  override predicate isSourceOf(TaintKind kind) { kind instanceof ExternalStringKind }
+
+  override string toString() { result = "taint source" }
+}
+
+class ListSource extends TaintSource {
+  ListSource() { this.(NameNode).getId() = "TAINTED_LIST" }
+
+  override predicate isSourceOf(TaintKind kind) { kind instanceof ExternalStringSequenceKind }
+
+  override string toString() { result = "list taint source" }
+}
+
+class DictSource extends TaintSource {
+  DictSource() { this.(NameNode).getId() = "TAINTED_DICT" }
+
+  override predicate isSourceOf(TaintKind kind) { kind instanceof ExternalStringDictKind }
+
+  override string toString() { result = "dict taint source" }
+}
+
+class ExceptionInfoSource extends TaintSource {
+  ExceptionInfoSource() { this.(NameNode).getId() = "TAINTED_EXCEPTION_INFO" }
+
+  override predicate isSourceOf(TaintKind kind) { kind instanceof ExceptionInfo }
+
+  override string toString() { result = "Exception info source" }
+}
+
+class ExternalFileObjectSource extends TaintSource {
+  ExternalFileObjectSource() { this.(NameNode).getId() = "TAINTED_FILE" }
+
+  override predicate isSourceOf(TaintKind kind) { kind instanceof ExternalFileObject }
+
+  override string toString() { result = "Tainted file source" }
+}

python/ql/test/3/library-tests/taint/strings/TestTaint.expected

@@ -0,0 +1 @@
+| test.py:4 | ok   | fstring | Fstring | externally controlled string |

python/ql/test/3/library-tests/taint/strings/TestTaint.ql

@@ -0,0 +1,33 @@
+import python
+import semmle.python.security.TaintTracking
+import semmle.python.web.HttpRequest
+import semmle.python.security.strings.Untrusted
+import Taint
+
+from
+  Call call, Expr arg, boolean expected_taint, boolean has_taint, string test_res,
+  string taint_string
+where
+  call.getLocation().getFile().getShortName() = "test.py" and
+  (
+    call.getFunc().(Name).getId() = "ensure_tainted" and
+    expected_taint = true
+    or
+    call.getFunc().(Name).getId() = "ensure_not_tainted" and
+    expected_taint = false
+  ) and
+  arg = call.getAnArg() and
+  (
+    not exists(TaintedNode tainted | tainted.getAstNode() = arg) and
+    taint_string = "<NO TAINT>" and
+    has_taint = false
+    or
+    exists(TaintedNode tainted | tainted.getAstNode() = arg |
+      taint_string = tainted.getTaintKind().toString()
+    ) and
+    has_taint = true
+  ) and
+  if expected_taint = has_taint then test_res = "ok  " else test_res = "fail"
+// if expected_taint = has_taint then test_res = "✓" else test_res = "✕"
+select arg.getLocation().toString(), test_res, call.getScope().(Function).getName(), arg.toString(),
+  taint_string

python/ql/test/3/library-tests/taint/strings/test.py

@@ -0,0 +1,5 @@
+def fstring():
+    tainted_string = TAINTED_STRING
+    ensure_tainted(
+        f"foo {tainted_string} bar"
+    )