Restricted environment keys for ImageMath.eval()

Tests/test_imagemath.py

@@ -64,6 +64,16 @@ def test_prevent_exec(expression):
         ImageMath.eval(expression)
 
 
+def test_prevent_double_underscores():
+    with pytest.raises(ValueError):
+        ImageMath.eval("1", {"__": None})
+
+
+def test_prevent_builtins():
+    with pytest.raises(ValueError):
+        ImageMath.eval("(lambda: exec('exit()'))()", {"exec": None})
+
+
 def test_logical():
     assert pixel(ImageMath.eval("not A", images)) == 0
     assert pixel(ImageMath.eval("A and B", images)) == "L 2"

docs/releasenotes/10.2.0.rst

@@ -62,10 +62,13 @@ output only the quantization and Huffman tables for the image.
 Security
 ========
 
-TODO
-^^^^
+Restricted environment keys for ImageMath.eval
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-TODO
+:cve:`2023-50447`: If an attacker has control over the keys passed to the
+``environment`` argument of :py:meth:`PIL.ImageMath.eval`, they may be able to execute
+arbitrary code. To prevent this, keys matching the names of builtins and keys
+containing double underscores will now raise a :py:exc:`ValueError`.
 
 Other Changes
 =============

src/PIL/ImageMath.py

@@ -234,6 +234,11 @@ def eval(expression, _dict={}, **kw):
 
     # build execution namespace
     args = ops.copy()
+    for k in list(_dict.keys()) + list(kw.keys()):
+        if "__" in k or hasattr(builtins, k):
+            msg = f"'{k}' not allowed"
+            raise ValueError(msg)
+
     args.update(_dict)
     args.update(kw)
     for k, v in args.items():