Restrict builtins for ImageMath.eval

Tests/test_imagemath.py

@@ -1,3 +1,5 @@
+import pytest
+
 from PIL import Image, ImageMath
 
 
@@ -50,6 +52,11 @@ def test_ops():
     assert pixel(ImageMath.eval("float(B)**33", images)) == "F 8589934592.0"
 
 
+def test_prevent_exec():
+    with pytest.raises(ValueError):
+        ImageMath.eval("exec('pass')")
+
+
 def test_logical():
     assert pixel(ImageMath.eval("not A", images)) == 0
     assert pixel(ImageMath.eval("A and B", images)) == "L 2"

docs/releasenotes/9.0.0.rst

@@ -116,6 +116,14 @@ To prevent attempts to slow down loading times for images, if an image has conse
 duplicate tiles that only differ by their offset, only load the last tile. Credit to
 Google's `OSS-Fuzz`_ project for finding this issue.
 
+Restrict builtins available to ImageMath.eval
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To limit :py:class:`PIL.ImageMath` to working with images, Pillow will now restrict the
+builtins available to :py:meth:`PIL.ImageMath.eval`. This will help prevent problems
+arising if users evaluate arbitrary expressions, such as
+``ImageMath.eval("exec(exit())")``.
+
 Fixed ImagePath.Path array handling
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/PIL/ImageMath.py

@@ -246,7 +246,12 @@ def eval(expression, _dict={}, **kw):
         if hasattr(v, "im"):
             args[k] = _Operand(v)
 
-    out = builtins.eval(expression, args)
+    code = compile(expression, "<string>", "eval")
+    for name in code.co_names:
+        if name not in args and name != "abs":
+            raise ValueError(f"'{name}' not allowed")
+
+    out = builtins.eval(expression, {"__builtins": {"abs": abs}}, args)
     try:
         return out.im
     except AttributeError: