Merge pull request #1060 from wiredfool/png-dos

Fix potential PNG decompression DOS

PIL/PngImagePlugin.py

@@ -72,6 +72,19 @@ _MODES = {
 
 _simple_palette = re.compile(b'^\xff+\x00\xff*$')
 
+# Maximum decompressed size for a iTXt or zTXt chunk.
+# Eliminates decompression bombs where compressed chunks can expand 1000x
+MAX_TEXT_CHUNK = ImageFile.SAFEBLOCK
+# Set the maximum total text chunk size.
+MAX_TEXT_MEMORY = 64 * MAX_TEXT_CHUNK
+
+def _safe_zlib_decompress(s):
+    dobj = zlib.decompressobj()
+    plaintext = dobj.decompress(s, MAX_TEXT_CHUNK)
+    if dobj.unconsumed_tail:
+        raise ValueError("Decompressed Data Too Large")
+    return plaintext
+
 
 # --------------------------------------------------------------------
 # Support classes.  Suitable for PNG and related formats like MNG etc.
@@ -260,6 +273,14 @@ class PngStream(ChunkStream):
         self.im_tile = None
         self.im_palette = None
 
+        self.text_memory = 0
+
+    def check_text_memory(self, chunklen):
+        self.text_memory += chunklen
+        if self.text_memory > MAX_TEXT_MEMORY:
+            raise ValueError("Too much memory used in text chunks: %s>MAX_TEXT_MEMORY" %
+                             self.text_memory)
+
     def chunk_iCCP(self, pos, length):
 
         # ICC profile
@@ -278,7 +299,7 @@ class PngStream(ChunkStream):
             raise SyntaxError("Unknown compression method %s in iCCP chunk" %
                               comp_method)
         try:
-            icc_profile = zlib.decompress(s[i+2:])
+            icc_profile = _safe_zlib_decompress(s[i+2:])
         except zlib.error:
             icc_profile = None  # FIXME
         self.im_info["icc_profile"] = icc_profile
@@ -372,6 +393,8 @@ class PngStream(ChunkStream):
                 v = v.decode('latin-1', 'replace')
 
             self.im_info[k] = self.im_text[k] = v
+            self.check_text_memory(len(v))
+
         return s
 
     def chunk_zTXt(self, pos, length):
@@ -391,7 +414,7 @@ class PngStream(ChunkStream):
             raise SyntaxError("Unknown compression method %s in zTXt chunk" %
                               comp_method)
         try:
-            v = zlib.decompress(v[1:])
+            v = _safe_zlib_decompress(v[1:])
         except zlib.error:
             v = b""
 
@@ -401,6 +424,8 @@ class PngStream(ChunkStream):
                 v = v.decode('latin-1', 'replace')
 
             self.im_info[k] = self.im_text[k] = v
+            self.check_text_memory(len(v))
+
         return s
 
     def chunk_iTXt(self, pos, length):
@@ -421,7 +446,7 @@ class PngStream(ChunkStream):
         if cf != 0:
             if cm == 0:
                 try:
-                    v = zlib.decompress(v)
+                    v = _safe_zlib_decompress(v)
                 except zlib.error:
                     return s
             else:
@@ -436,6 +461,7 @@ class PngStream(ChunkStream):
                 return s
 
         self.im_info[k] = self.im_text[k] = iTXt(v, lang, tk)
+        self.check_text_memory(len(v))
             
         return s
 

Tests/check_png_dos.py

@@ -0,0 +1,47 @@
+from helper import unittest, PillowTestCase
+from PIL import Image, PngImagePlugin
+from io import BytesIO
+import zlib
+
+TEST_FILE = "Tests/images/png_decompression_dos.png"
+
+class TestPngDos(PillowTestCase):
+    def test_dos_text(self):
+
+        try:
+            im = Image.open(TEST_FILE)
+            im.load()
+        except ValueError as msg:
+            self.assertTrue(msg, "Decompressed Data Too Large")
+            return
+
+        for s in im.text.values():
+            self.assertLess(len(s), 1024*1024, "Text chunk larger than 1M")
+
+    def test_dos_total_memory(self):
+        im = Image.new('L',(1,1))
+        compressed_data = zlib.compress('a'*1024*1023)
+
+        info = PngImagePlugin.PngInfo()
+
+        for x in range(64):
+            info.add_text('t%s'%x, compressed_data, 1)
+            info.add_itxt('i%s'%x, compressed_data, zip=True)
+
+        b = BytesIO()
+        im.save(b, 'PNG', pnginfo=info)
+        b.seek(0)
+        
+        try:
+            im2 = Image.open(b)
+        except ValueError as msg:
+            self.assertIn("Too much memory", msg)
+            return
+
+        total_len = 0
+        for txt in im2.text.values():
+            total_len += len(txt)
+        self.assertLess(total_len, 64*1024*1024, "Total text chunks greater than 64M")
+        
+if __name__ == '__main__':
+    unittest.main()

Tests/test_file_png.py

@@ -153,7 +153,7 @@ class TestFilePng(PillowTestCase):
 
         im = load(HEAD + chunk(b'iTXt', b'spam\0\1\0en\0Spam\0' +
                                zlib.compress(b"egg")[:1]) + TAIL)
-        self.assertEqual(im.info, {})
+        self.assertEqual(im.info, {'spam':''})
 
         im = load(HEAD + chunk(b'iTXt', b'spam\0\1\1en\0Spam\0' +
                                zlib.compress(b"egg")) + TAIL)

docs/handbook/image-file-formats.rst

@@ -333,7 +333,12 @@ The :py:meth:`~PIL.Image.Image.open` method sets the following
     transparent palette image.
 
 ``Open`` also sets ``Image.text`` to a list of the values of the
-``tEXt``, ``zTXt``, and ``iTXt`` chunks of the PNG image.
+``tEXt``, ``zTXt``, and ``iTXt`` chunks of the PNG image. Individual
+compressed chunks are limited to a decompressed size of
+``PngImagePlugin.MAX_TEXT_CHUNK``, by default 1MB, to prevent
+decompression bombs. Additionally, the total size of all of the text
+chunks is limited to ``PngImagePlugin.MAX_TEXT_MEMORY``, defaulting to
+64MB.
 
 The :py:meth:`~PIL.Image.Image.save` method supports the following options: