Improve documentation about parsing URLs in lxml_html_clean.

Author: frenzymadness

README.md

@@ -4,6 +4,10 @@
 
 This project was initially a part of [lxml](https://github.com/lxml/lxml). Because HTML cleaner is designed as blocklist-based, many reports about possible security vulnerabilities were filed for lxml and that make the project problematic for security-sensitive environments. Therefore we decided to extract the problematic part to a separate project.
 
+**Important**: the HTML Cleaner in ``lxml_html_clean`` is **not** considered appropriate **for security sensitive environments**. See e.g. [bleach](https://pypi.org/project/bleach/) for an alternative.
+
+This project uses functions from Python's `urllib.parse` for URL parsing which **do not validate inputs**. For more information on potential security risks, refer to the [URL parsing security](https://docs.python.org/3/library/urllib.parse.html#url-parsing-security) documentation. A maliciously crafted URL could potentially bypass the allowed hosts check in `Cleaner`.
+
 ## Installation
 
 You can install this project directly via `pip install lxml_html_clean` or as an extra of lxml

docs/index.rst

@@ -8,6 +8,14 @@ This project was initially a part of `lxml <https://github.com/lxml/lxml>`_. Bec
 many reports about possible security vulnerabilities were filed for lxml and that make the project problematic for
 security-sensitive environments. Therefore we decided to extract the problematic part to a separate project.
 
+**Important**: the HTML Cleaner in ``lxml_html_clean`` is **not** considered appropriate **for security sensitive environments**.
+See e.g. `bleach <https://pypi.org/project/bleach/>`_ for an alternative.
+
+This project uses functions from Python's ``urllib.parse`` for URL parsing which **do not validate inputs**.
+For more information on potential security risks, refer to the
+`URL parsing security <https://docs.python.org/3/library/urllib.parse.html#url-parsing-security>`_ documentation.
+A maliciously crafted URL could potentially bypass the allowed hosts check in ``Cleaner``.
+
 Security
 --------
 

lxml_html_clean/clean.py

@@ -185,6 +185,9 @@ class Cleaner:
 
         Note that you may also need to set ``whitelist_tags``.
 
+        Note that URLs are parsed via functions from ``urllib.parse`` and
+        no input validation is performed.
+
     ``whitelist_tags``:
         A set of tags that can be included with ``host_whitelist``.
         The default is ``iframe`` and ``embed``; you may wish to