XML signatures are a method used to ensure the integrity and authenticity of XML documents. However, if XML signatures are not validated securely, it can lead to potential vulnerabilities.
XML can be used for a wide variety of purposes. Using a signature on an XML message generally indicates this message requires authenticity and integrity. However, if the signature validation is not properly implemented this authenticity can not be guaranteed.
By not enforcing secure validation, the XML Digital Signature API is more susceptible to attacks such as signature spoofing and injections.
By disabling secure validation, the application becomes more susceptible to signature spoofing attacks. Attackers can potentially manipulate the XML signature in a way that bypasses the validation process, allowing them to forge or tamper with the signature. This can lead to the acceptance of invalid or maliciously modified signatures, compromising the integrity and authenticity of the XML documents.
Disabling secure validation can expose the application to injection attacks. Attackers can inject malicious code or entities into the XML document, taking advantage of the weakened validation process. In some cases, it can also expose the application to denial-of-service attacks. Attackers can exploit vulnerabilities in the validation process to cause excessive resource consumption or system crashes, leading to service unavailability or disruption.
The following noncompliant code example verifies an XML signature without providing a trusted signing authority. This code will accept any signature created from a generally trusted certificate, for example, a Let’s encrypt one.
from lxml import etree
from signxml import XMLVerifier
xml_file = open("signed.xml", "rb")
xml = etree.parse(xml_file)
XMLVerifier().verify(xml) # Noncompliant
from lxml import etree
from signxml import XMLVerifier
xml_file = open("signed.xml", "rb")
xml = etree.parse(xml_file)
cert_file = open("cert.pem", "rb")
cert = cert_file.read()
XMLVerifier().verify(xml, x509_cert=cert)
Here, the compliant solution provides a trusted certificate to the signature validation function. This will ensure only signatures computed with the private key associated with the provided certificate will be accepted.