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74 lines
2.5 KiB
Plaintext
74 lines
2.5 KiB
Plaintext
=== How to fix it in the Python Standard Library
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The following noncompliant code is vulnerable to XPath injection because untrusted data is
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concatenated to an XPath query without prior validation. Although `xml.etree.ElementTree` only
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supports a subset of XPath syntax, exploitation can still be possible.
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==== Noncompliant code example
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[source,python,diff-id=1,diff-type=noncompliant]
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----
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from xml.etree import ElementTree
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from flask import request
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@app.route('/authenticate')
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def authenticate():
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username = request.args['username']
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password = request.args['password']
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expression = "./users/user[@name='" + username + "'][@pass='" + password + "']"
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tree = ElementTree.parse('resources/users.xml')
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if tree.find(expression) is None:
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return "Invalid credentials", 401
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else:
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return "Success", 200
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----
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////
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The above example would be exploitable by using e.g. `username=”admin][.!=”` and `password=”][.!='XXXXX”`
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This creates the following query: `./users/user[@name='admin][.!='][@pass='][.!='XXXXX']`,
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where `[.!='][@pass=']` and `[.!='XXXXX']` should match on everything, resulting in authentication as admin.
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////
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==== Compliant solution
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[source,python,diff-id=1,diff-type=compliant]
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----
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import re
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from xml.etree import ElementTree
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from flask import request
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tree = ElementTree.parse('resources/users.xml')
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@app.route('/authenticate')
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def authenticate():
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username = request.args['username']
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password = request.args['password']
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if re.match("^[a-zA-Z0-9]*$", username) is None or re.match("^[a-zA-Z0-9]*$", password) is None:
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return "Username or password contains invalid characters", 400
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expression = "./users/user[@name='" + username + "'][@pass='" + password + "']"
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tree = ElementTree.parse('resources/users.xml')
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if tree.find(expression) is None:
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return "Invalid credentials", 401
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else:
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return "Success", 200
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----
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=== How does this work?
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As a rule of thumb, the best approach to protect against injections is to
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systematically ensure that untrusted data cannot break out of the initially
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intended logic.
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include::../../common/fix/parameterized-queries.adoc[]
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It is not possible to construct parameterized queries using only the Python Standard Library. +
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Please use https://pypi.org/project/lxml/[lxml] instead, which allows for parameterized queries using the `ElementTree.xpath()` method.
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include::../../common/fix/validation.adoc[]
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In the example, we ensure that the username and password only contain alphanumeric characters by doing a regex match before executing the XPath query.
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