XML Processing Modules¶
Python’s interfaces for processing XML are grouped in the xml package.
The XML modules are not secure against erroneous or maliciously constructed data. If you need to parse untrusted or unauthenticated data see the XML vulnerabilities and The defusedxml Package sections.
It is important to note that modules in the xml package require that there be at least one SAX-compliant XML parser available. The Expat parser is included with Python, so the xml.parsers.expat module will always be available.
The documentation for the xml.dom and xml.sax packages are the definition of the Python bindings for the DOM and SAX interfaces.
The XML handling submodules are:
- xml.etree.ElementTree : the ElementTree API, a simple and lightweight XML processor
- xml.dom : the DOM API definition
- xml.dom.minidom : a minimal DOM implementation
- xml.dom.pulldom : support for building partial DOM trees
- xml.sax : SAX2 base classes and convenience functions
- xml.parsers.expat : the Expat parser binding
XML vulnerabilities¶
The XML processing modules are not secure against maliciously constructed data. 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 whether the various modules are vulnerable to them.
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
Vulnerable (1)
external entity expansion
- Expat 2.4.1 and newer is not vulnerable to the “billion laughs” and “quadratic blowup” vulnerabilities. Items still listed as vulnerable due to potential reliance on system-provided libraries. Check pyexpat.EXPAT_VERSION .
- xml.etree.ElementTree doesn’t expand external entities and raises a ParserError when an entity occurs.
- xml.dom.minidom doesn’t expand external entities and simply returns the unexpanded entity verbatim.
- xmlrpclib doesn’t expand external entities and omits them.
- Since Python 3.7.1, external general entities are no longer processed by default.
The Billion Laughs attack – also known as exponential entity expansion – uses multiple levels of nested entities. Each entity refers to another entity 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 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 or local files. The XML parser accesses the resource and embeds the content into the XML document.
Some XML libraries like Python’s xml.dom.pulldom retrieve document type definitions from remote or local locations. The feature has similar implications as the external entity expansion issue.
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 for defusedxml on PyPI has further information about all known attack vectors with examples and references.
The defusedxml Package¶
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.
Get all text from an XML document?
How can I get all the text content of an XML document, as a single string — like this Ruby/hpricot example but using Python. I’d like to replace XML tags with a single whitespace.
The answer you have accepted is wrong, as noted in a comment on it. Would you unaccept it so that a wrong answer is not pinned to the top of the page?
5 Answers 5
import xml.etree.ElementTree as ET tree = ET.parse('sample.xml') print(ET.tostring(tree.getroot(), encoding='utf-8', method='text')) I really like BeautifulSoup, and would rather not use regex on HTML if we can avoid it.
from bs4 import BeautifulSoup soup = BeautifulSoup(txt) # txt is simply the a string with your XML file pageText = soup.findAll(text=True) print ' '.join(pageText) Though of course, you can (and should) use BeautifulSoup to navigate the page for what you are looking for.
A solution that doesn’t require an external library like BeautifulSoup, using the built-in sax parsing framework:
from xml import sax class MyHandler(sax.handler.ContentHandler): def parse(self, filename): self.text = [] sax.parse(filename, self) return ''.join(self.text) def characters(self, data): self.text.append(data) result = MyHandler().parse("yourfile.xml") If you need all whitespace intact in the text, also define the ignorableWhitespace method in the handler class in the same way characters is defined.
This very problem is actually an example in the lxml tutorial, which suggests using one of the following XPath expressions to get all the bits of text content from the document as a list of strings:
You’ll then want to join these bits of text together into a single big string, with str.join probably using str.strip to get rid of leading and trailing whitespace on each bit and ignoring bits that are made entirely of whitespace:
>>> from lxml import etree >>> root = etree.fromstring(""" . . some text . . . foo bar . . yet more text . . """) >>> bits_of_text = root.xpath('//text()') >>> print(bits_of_text) # Note that some bits are whitespace-only ['\n some text\n ', ' ', '\n ', '\n foo bar\n ', '\n yet more text\n ', '\n even more text\n'] >>> joined_text = ' '.join( . bit.strip() for bit in bits_of_text . if bit.strip() != '' . ) >>> print(joined_text) some text foo bar yet more text even more text Note, by the way, that if you don’t want to insert spaces between the bits of text you can just do
etree.tostring(root, method='text', encoding='unicode') And if you’re dealing with HTML instead of XML, and are using lxml.html to parse your HTML, you can just call the .text_content() method of your root node to get all the text it contains (although, again, no spaces will be inserted):
>>> import lxml.html >>> root = lxml.html.document_fromstring('stuff
more
stuffbla') >>> root.text_content() 'stuffmore stuffbla'