55 lines
1.7 KiB
Plaintext
55 lines
1.7 KiB
Plaintext
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=== How to fix it in lxml
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The following noncompliant code is vulnerable to XPath injections because untrusted data is
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concatenated to an XPath query without prior validation.
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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 flask import request
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from lxml import etree
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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 + "' and @pass='" + password + "']"
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tree = etree.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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==== Compliant solution
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[source,python,diff-id=1,diff-type=compliant]
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----
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from flask import request
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from lxml import etree
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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=$username and @pass=$password]"
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tree = etree.parse('resources/users.xml')
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if tree.xpath(expression, username=username, password=password) 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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In the example, the username and password are passed as https://lxml.de/xpathxslt.html#:~:text=The%20xpath()%20method%20has%20support%20for%20XPath%20variables%3A[XPath variables] rather than concatenated to the XPath query. By using a parameterized query, injection is successfully prevented.
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