#20237: make a revision pass over the XML vulnerabilities section

1 files changed, 35 insertions, 38 deletions

diff --git a/Doc/library/xml.rst b/Doc/library/xml.rst
index f793bae..0188219 100644
--- a/Doc/library/xml.rst
+++ b/Doc/library/xml.rst
@@ -14,9 +14,9 @@ Python's interfaces for processing XML are grouped in the ``xml`` package.
 .. warning::
 
    The XML modules are not secure against erroneous or maliciously
-   constructed data.  If you need to parse untrusted or unauthenticated data see
-   :ref:`xml-vulnerabilities`.
-
+   constructed data.  If you need to parse untrusted or
+   unauthenticated data see the :ref:`xml-vulnerabilities` and
+   :ref:`defused-packages` sections.
 
 It is important to note that modules in the :mod:`xml` package require that
 there be at least one SAX-compliant XML parser available. The Expat parser is
@@ -46,16 +46,15 @@ The XML handling submodules are:
 .. _xml-vulnerabilities:
 
 XML vulnerabilities
-===================
+-------------------
 
 The XML processing modules are not secure against maliciously constructed data.
-An attacker can abuse vulnerabilities for e.g. denial of service attacks, to
-access local files, to generate network connections to other machines, or
-to or circumvent firewalls. The attacks on XML abuse unfamiliar features
-like inline `DTD`_ (document type definition) with entities.
+An attacker can abuse XML features to carry out denial of service attacks,
+access local files, generate network connections to other machines, or
+circumvent firewalls.
 
-The following table gives an overview of the known attacks and if the various
-modules are vulnerable to them.
+The following table gives an overview of the known attacks and whether
+the various modules are vulnerable to them.
 
 =========================  ========  =========  =========  ========  =========
 kind                       sax       etree      minidom    pulldom   xmlrpc
@@ -68,7 +67,7 @@ decompression bomb         No        No         No         No        **Yes**
 =========================  ========  =========  =========  ========  =========
 
 1. :mod:`xml.etree.ElementTree` doesn't expand external entities and raises a
-   ParserError when an entity occurs.
+   :exc:`ParserError` when an entity occurs.
 2. :mod:`xml.dom.minidom` doesn't expand external entities and simply returns
    the unexpanded entity verbatim.
 3. :mod:`xmlrpclib` doesn't expand external entities and omits them.
@@ -77,23 +76,21 @@ decompression bomb         No        No         No         No        **Yes**
 billion laughs / exponential entity expansion
   The `Billion Laughs`_ attack -- also known as exponential entity expansion --
   uses multiple levels of nested entities. Each entity refers to another entity
-  several times, the final entity definition contains a small string. Eventually
-  the small string is expanded to several gigabytes. The exponential expansion
-  consumes lots of CPU time, too.
+  several times, and the final entity definition contains a small string.
+  The exponential expansion results in several gigabytes of text and
+  consumes lots of memory and CPU time.
 
 quadratic blowup entity expansion
   A quadratic blowup attack is similar to a `Billion Laughs`_ attack; it abuses
   entity expansion, too. Instead of nested entities it repeats one large entity
   with a couple of thousand chars over and over again. The attack isn't as
-  efficient as the exponential case but it avoids triggering countermeasures of
-  parsers against heavily nested entities.
+  efficient as the exponential case but it avoids triggering parser countermeasures
+  that forbid deeply-nested entities.
 
 external entity expansion
   Entity declarations can contain more than just text for replacement. They can
-  also point to external resources by public identifiers or system identifiers.
-  System identifiers are standard URIs or can refer to local files. The XML
-  parser retrieves the resource with e.g. HTTP or FTP requests and embeds the
-  content into the XML document.
+  also point to external resources or local files. The XML
+  parser accesses the resource and embeds the content into the XML document.
 
 DTD retrieval
   Some XML libraries like Python's :mod:`xml.dom.pulldom` retrieve document type
@@ -101,31 +98,32 @@ DTD retrieval
   implications as the external entity expansion issue.
 
 decompression bomb
-  The issue of decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
-  that can parse compressed XML stream like gzipped HTTP streams or LZMA-ed
+  Decompression bombs (aka `ZIP bomb`_) apply to all XML libraries
+  that can parse compressed XML streams such as gzipped HTTP streams or
+  LZMA-compressed
   files. For an attacker it can reduce the amount of transmitted data by three
   magnitudes or more.
 
-The documentation of `defusedxml`_ on PyPI has further information about
+The documentation for `defusedxml`_ on PyPI has further information about
 all known attack vectors with examples and references.
 
-defused packages
-----------------
+.. _defused-packages:
 
-`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
-XML parsers that prevent any potentially malicious operation. The courses of
-action are recommended for any server code that parses untrusted XML data. The
-package also ships with example exploits and an extended documentation on more
-XML exploits like xpath injection.
+The :mod:`defusedxml` and :mod:`defusedexpat` Packages
+------------------------------------------------------
 
-`defusedexpat`_ provides a modified libexpat and patched replacment
-:mod:`pyexpat` extension module with countermeasures against entity expansion
-DoS attacks. Defusedexpat still allows a sane and configurable amount of entity
-expansions. The modifications will be merged into future releases of Python.
+`defusedxml`_ is a pure Python package with modified subclasses of all stdlib
+XML parsers that prevent any potentially malicious operation. Use of this
+package is recommended for any server code that parses untrusted XML data. The
+package also ships with example exploits and extended documentation on more
+XML exploits such as XPath injection.
 
-The workarounds and modifications are not included in patch releases as they
-break backward compatibility. After all inline DTD and entity expansion are
-well-definied XML features.
+`defusedexpat`_ provides a modified libexpat and a patched
+:mod:`pyexpat` module that have countermeasures against entity expansion
+DoS attacks. The :mod:`defusedexpat` module still allows a sane and configurable amount of entity
+expansions. The modifications may be included in some future release of Python,
+but will not be included in any bugfix releases of
+Python because they break backward compatibility.
 
 
 .. _defusedxml: https://pypi.python.org/pypi/defusedxml/
@@ -133,4 +131,3 @@ well-definied XML features.
 .. _Billion Laughs: http://en.wikipedia.org/wiki/Billion_laughs
 .. _ZIP bomb: http://en.wikipedia.org/wiki/Zip_bomb
 .. _DTD: http://en.wikipedia.org/wiki/Document_Type_Definition
-