SignXML: XML Signature in Python

SignXML is an implementation of the W3C XML Signature standard in Python. This standard (also known as XMLDSig and RFC 3275) is used to provide payload security in SAML 2.0 and WS-Security, among other uses. Two versions of the standard exist (Version 1.1 and Version 2.0). SignXML implements all of the required components of the standard, and most recommended ones. Its features are:

• Use of a libxml2-based XML parser configured to defend against common XML attacks when verifying signatures
• Extensions to allow signing with and verifying X.509 certificate chains, including hostname/CN validation
• Support for exclusive XML canonicalization with inclusive prefixes (InclusiveNamespaces PrefixList, required to verify signatures generated by some SAML implementations)
• Modern Python compatibility (2.7-3.8+ and PyPy)
• Well-supported, portable, reliable dependencies: lxml, cryptography, eight, pyOpenSSL
• Comprehensive testing (including the XMLDSig interoperability suite) and continuous integration
• Simple interface with useful defaults
• Compactness, readability, and extensibility

Installation

pip3 install signxml

Note: SignXML depends on lxml and cryptography, which in turn depend on OpenSSL, LibXML, and Python tools to interface with them. You can install those as follows:

OS	Command
Ubuntu	apt-get install --no-install-recommends python3-pip python3-wheel python3-setuptools python3-openssl python3-lxml
Red Hat, Amazon Linux, CentOS	yum install python3-pip python3-pyOpenSSL python3-lxml
Mac OS	Install Homebrew, then run brew install python.

Synopsis

SignXML uses the lxml ElementTree API to work with XML data.

from lxml import etree
from signxml import XMLSigner, XMLVerifier

data_to_sign = "<Test/>"
cert = open("example.pem").read()
key = open("example.key").read()
root = etree.fromstring(data_to_sign)
signed_root = XMLSigner().sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml

To make this example self-sufficient for test purposes:

• Generate a test certificate and key using openssl req -x509 -sha256 -nodes -subj "/CN=test" -days 1 -newkey rsa:2048 -keyout example.key -out example.pem (run yum install openssl on Red Hat).
• Pass the x509_cert=cert keyword argument to XMLVerifier.verify(). (In production, ensure this is replaced with the correct configuration for the trusted CA or certificate - this determines which signatures your application trusts.)

Verifying SAML assertions

Assuming metadata.xml contains SAML metadata for the assertion source:

from lxml import etree
from base64 import b64decode
from signxml import XMLVerifier

with open("metadata.xml", "rb") as fh:
    cert = etree.parse(fh).find("//ds:X509Certificate").text

assertion_data = XMLVerifier().verify(b64decode(assertion_body), x509_cert=cert).signed_xml

Signing SAML assertions

The SAML assertion schema specifies a location for the enveloped XML signature (between <Issuer> and <Subject>). To sign a SAML assertion in a schema-compliant way, insert a signature placeholder tag at that location before calling XMLSigner: <ds:Signature Id="placeholder"></ds:Signature>.

See what is signed

It is important to understand and follow the best practice rule of "See what is signed" when verifying XML signatures. The gist of this rule is: if your application neglects to verify that the information it trusts is what was actually signed, the attacker can supply a valid signature but point you to malicious data that wasn't signed by that signature. Failure to follow this rule can lead to vulnerability against attacks like SAML signature wrapping.

In SignXML, you can ensure that the information signed is what you expect to be signed by only trusting the data returned by the verify() method. The signed_xml attribute of the return value is the XML node or string that was signed.

Recommended reading: W3C XML Signature Best Practices for Applications, OWASP: On Breaking SAML: Be Whoever You Want to Be, Duo Finds SAML Vulnerabilities Affecting Multiple Implementations

Establish trust

If you do not supply any keyword arguments to verify(), the default behavior is to trust any valid XML signature generated using a valid X.509 certificate trusted by your system's CA store. This means anyone can get an SSL certificate and generate a signature that you will trust. To establish trust in the signer, use the x509_cert argument to specify a certificate that was pre-shared out-of-band (e.g. via SAML metadata, as shown in Verifying SAML assertions), or cert_subject_name to specify a subject name that must be in the signing X.509 certificate given by the signature (verified as if it were a domain name), or ca_pem_file/ca_path to give a custom CA.

XML signature methods: enveloped, detached, enveloping

The XML Signature specification defines three ways to compose a signature with the data being signed: enveloped, detached, and enveloping signature. Enveloped is the default method. To specify the type of signature that you want to generate, pass the method argument to sign():

signed_root = XMLSigner(method=signxml.methods.detached).sign(root, key=key, cert=cert)
verified_data = XMLVerifier().verify(signed_root).signed_xml

For detached signatures, the code above will use the Id or ID attribute of root to generate a relative URI (<Reference URI="#value"). You can also override the value of URI by passing a reference_uri argument to sign(). To verify a detached signature that refers to an external entity, pass a callable resolver in XMLVerifier().verify(data, uri_resolver=...).

See the API documentation for more.

XML parsing security and compatibility with xml.etree.ElementTree

SignXML uses the lxml ElementTree library, not the ElementTree from Python's standard library, to work with XML. lxml is used due to its superior resistance to XML attacks, as well as XML canonicalization and namespace organization features. It is recommended that you pass XML string input directly to signxml before further parsing, and use lxml to work with untrusted XML input in general. If you do pass xml.etree.ElementTree objects to SignXML, you should be aware of differences in XML namespace handling between the two libraries. See the following references for more information:

• How do I use lxml safely as a web-service endpoint?
• ElementTree compatibility of lxml.etree
• XML Signatures with Python ElementTree

Authors

• Andrey Kislyuk

Links

• Project home page (GitHub)
• Documentation (Read the Docs)
• Package distribution (PyPI)
• Change log
• List of W3C XML Signature standards and drafts
• W3C Recommendation: XML Signature Syntax and Processing Version 1.1
• W3C Working Group Note: XML Signature Syntax and Processing Version 2.0
• W3C Working Group Note: XML Security 2.0 Requirements and Design Considerations
• W3C Working Group Note: XML Signature Best Practices
• XML-Signature Interoperability
• W3C Working Group Note: Test Cases for C14N 1.1 and XMLDSig Interoperability
• XMLSec: Related links
• OWASP SAML Security Cheat Sheet
• Okta Developer Docs: SAML

Bugs

Please report bugs, issues, feature requests, etc. on GitHub.

License

Licensed under the terms of the Apache License, Version 2.0.