XML external entity expansion¶
ID: py/xxe Kind: path-problem Security severity: 9.1 Severity: error Precision: high Tags: - security - external/cwe/cwe-611 - external/cwe/cwe-827 Query suites: - python-code-scanning.qls - python-security-extended.qls - python-security-and-quality.qls
Parsing untrusted XML files with a weakly configured XML parser may lead to an XML External Entity (XXE) attack. This type of attack uses external entity references to access arbitrary files on a system, carry out denial-of-service (DoS) attacks, or server-side request forgery. Even when the result of parsing is not returned to the user, DoS attacks are still possible and out-of-band data retrieval techniques may allow attackers to steal sensitive data.
The easiest way to prevent XXE attacks is to disable external entity handling when parsing untrusted data. How this is done depends on the library being used. Note that some libraries, such as recent versions of the XML libraries in the standard library of Python 3, disable entity expansion by default, so unless you have explicitly enabled entity expansion, no further action needs to be taken.
We recommend using the defusedxml PyPI package, which has been created to prevent XML attacks (both XXE and XML bombs).
The following example uses the
lxml XML parser to parse a string
xml_src. That string is from an untrusted source, so this code is vulnerable to an XXE attack, since the default parser from
lxml.etree allows local external entities to be resolved.
from flask import Flask, request import lxml.etree app = Flask(__name__) @app.post("/upload") def upload(): xml_src = request.get_data() doc = lxml.etree.fromstring(xml_src) return lxml.etree.tostring(doc)
To guard against XXE attacks with the
lxml library, you should create a parser with
resolve_entities set to
false. This means that no entity expansion is undertaken, although standard predefined entities such as
>, for writing
> inside the text of an XML element, are still allowed.
from flask import Flask, request import lxml.etree app = Flask(__name__) @app.post("/upload") def upload(): xml_src = request.get_data() parser = lxml.etree.XMLParser(resolve_entities=False) doc = lxml.etree.fromstring(xml_src, parser=parser) return lxml.etree.tostring(doc)
Timothy Morgen: XML Schema, DTD, and Entity Attacks.
Timur Yunusov, Alexey Osipov: XML Out-Of-Band Data Retrieval.
Python 3 standard library: XML Vulnerabilities.
Python 2 standard library: XML Vulnerabilities.
PortSwigger: XML external entity (XXE) injection.
Common Weakness Enumeration: CWE-611.
Common Weakness Enumeration: CWE-827.