diff --git a/Tests/test_file_pdf.py b/Tests/test_file_pdf.py
index e5bba483a..c7839873a 100644
--- a/Tests/test_file_pdf.py
+++ b/Tests/test_file_pdf.py
@@ -286,3 +286,13 @@ def test_pdf_append_to_bytesio():
     f = io.BytesIO(f.getvalue())
     im.save(f, format="PDF", append=True)
     assert len(f.getvalue()) > initial_size
+
+
+@pytest.mark.timeout(1)
+def test_redos():
+    malicious = b" trailer<<>>" + b"\n" * 3456
+
+    # This particular exception isn't relevant here.
+    # The important thing is it doesn't timeout, cause a ReDoS (CVE-2021-25292).
+    with pytest.raises(PdfParser.PdfFormatError):
+        PdfParser.PdfParser(buf=malicious)