net: Ignore the magic number sniffer if it determines an application/octet-st...

net/base/mime_sniffer.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Detecting mime types is a tricky business because we need to balance
 // compatibility concerns with security issues.  Here is a survey of how other
 // browsers behave and then a description of how we intend to behave.
 //
 // HTML payload, no Content-Type header:
 // * IE 7: Render as HTML
@@ skipping 561 matching lines @@
     return content_size >= kMaxBytesToSniff;
   }
 
   // CRX files (chrome extensions) have a special sniffing algorithm. It is
   // tighter than the others because we don't have to match legacy behavior.
   if (SniffCRX(content, content_size, url, type_hint, result))
     return true;
 
   // Now we look in our large table of magic numbers to see if we can find
   // anything that matches the content.
-  if (SniffForMagicNumbers(content, content_size, result))
+  if (SniffForMagicNumbers(content, content_size, result)) {
+    // We should never sniff an octet stream as plain text. There are Unix
+    // binaries with embedded shell scripts that get sniffed incorrectly,
+    // leading to spectacular failures like bug 29354.
+    if (type_hint == "application/octet-stream" && "text/plain" == *result)
+      result->assign("application/octet-stream");
     return true;  // We've matched a magic number.  No more content needed.
+  }
 
   // Having failed thus far, we're willing to override unknown mime types and
   // text/plain.
   if (hint_is_unknown_mime_type || hint_is_text_plain) {
     if (looks_binary)
       result->assign("application/octet-stream");
     else
       result->assign("text/plain");
     // We could change our mind if a binary-looking byte appears later in
     // the content, so we only have enough content if we have the max.
     return content_size >= kMaxBytesToSniff;
   }
 
   return have_enough_content;
 }
 
 }  // namespace net