Do not try and crop glyphs from outside of source ImageFont image

docs/releasenotes/10.2.0.rst

@@ -77,6 +77,16 @@ Pillow will now raise a :py:exc:`ValueError` if the number of characters passed
 This threshold can be changed by setting :py:data:`PIL.ImageFont.MAX_STRING_LENGTH`. It
 can be disabled by setting ``ImageFont.MAX_STRING_LENGTH = None``.
 
+A decompression bomb check has also been added to
+:py:meth:`PIL.ImageFont.ImageFont.getmask`.
+
+ImageFont.getmask: Trim glyph size
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To protect against potential DOS attacks when using PIL fonts,
+:py:class:`PIL.ImageFont.ImageFont` now trims the size of individual glyphs so that
+they do not extend beyond the bitmap image.
+
 ImageMath.eval: Restricted environment keys
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

src/_imaging.c

@@ -2649,6 +2649,18 @@ _font_new(PyObject *self_, PyObject *args) {
         self->glyphs[i].sy0 = S16(B16(glyphdata, 14));
         self->glyphs[i].sx1 = S16(B16(glyphdata, 16));
         self->glyphs[i].sy1 = S16(B16(glyphdata, 18));
+
+        // Do not allow glyphs to extend beyond bitmap image
+        // Helps prevent DOS by stopping cropped images being larger than the original
+        if (self->glyphs[i].sx1 > self->bitmap->xsize) {
+            self->glyphs[i].dx1 -= self->glyphs[i].sx1 - self->bitmap->xsize;
+            self->glyphs[i].sx1 = self->bitmap->xsize;
+        }
+        if (self->glyphs[i].sy1 > self->bitmap->ysize) {
+            self->glyphs[i].dy1 -= self->glyphs[i].sy1 - self->bitmap->ysize;
+            self->glyphs[i].sy1 = self->bitmap->ysize;
+        }
+
         if (self->glyphs[i].dy0 < y0) {
             y0 = self->glyphs[i].dy0;
         }