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The vulnerability stems from a missing error check in the CRuby extension of Nokogiri, specifically within the `xmlC14NExecute` function. This function is called by the `canonicalize` methods for XML Documents and Nodes to perform XML Canonicalization (C14N), a process that puts XML into a standard form for signature validation. When the C14N process fails due to malformed or invalid XML input, the underlying C library returns an error. However, the Nokogiri CRuby extension fails to check this return value. Consequently, instead of halting execution and raising an exception like a RuntimeError, the method silently returns an empty string . For downstream libraries, such as those handling SAML (Security Assertion Markup Language) authentication, this behavior is critical. They rely on the canonicalized output to verify XML digital signatures. By receiving an empty string instead of a failure, these libraries may proceed to validate a signature against this incomplete or invalid data, potentially leading to a signature validation bypass. This can allow an attacker to forge SAML assertions and impersonate other users. The issue does not affect JRuby, as the Java implementation handles these errors correctly by raising an exception.
DailyCVE Form:
Platform: Nokogiri (CRuby)
Version: < 1.19.1
Vulnerability : Missing Error Check
Severity: Medium
date: [Disclosure Date]
Prediction: Patch already available (v1.19.1)
What Undercode Say:
Analysis
This CVE highlights the dangers of not checking return values from native extensions, which can lead to silent failures. In the context of SAML, this is a critical flaw, as demonstrated by its use in bypassing signature validation in downstream libraries like ruby-saml, leading to authentication bypasses .
Vulnerable Code Example
The following code snippet shows a vulnerable scenario where canonicalization fails but returns an empty string, misleading the application.
require 'nokogiri'
doc = Nokogiri::XML("<invalid> XML")
begin
If canonicalization fails pre-patch, it returns "" instead of raising an error
canon = doc.canonicalize
puts "Canonicalized XML: {canon}"
SAML library might then validate a signature against this empty string
rescue => e
puts "Error: {e}"
end
Testing for the Vulnerability
You can test if your version is vulnerable by attempting to canonicalize intentionally broken XML.
Check your current Nokogiri version
bundle list | grep nokogiri
Run a quick Ruby script to test behavior
ruby -r nokogiri -e "begin; Nokogiri::XML('<a></b>').canonicalize; rescue => e; puts 'Fixed: Exception raised'; else; puts 'Vulnerable: No exception'; end"
Exploit:
An attacker crafts a malformed SAML response that triggers a canonicalization error. The ruby-saml library uses Nokogiri to canonicalize the XML for signature verification. Due to CVE-2025-30206, Nokogiri returns an empty string instead of an error. The ruby-saml library then unwittingly validates the signature against the empty string. By manipulating other parts of the SAML document, such as using parser differentials or namespace confusion, the attacker can forge assertions, effectively allowing them to log in as any user without a valid signature .
Protection from this CVE
The primary mitigation is to update Nokogiri to version 1.19.1 or higher.
Update Nokogiri via Bundler bundle update nokogiri Or install the latest version directly gem install nokogiri -v ">= 1.19.1"
Additionally, ensure all SAML-dependent libraries (like `ruby-saml` and omniauth-saml) are updated to versions that rely on the patched Nokogiri.
Impact
A successful exploit allows for a complete bypass of SAML-based authentication. An attacker can forge a SAML response to impersonate any user within an organization that uses a vulnerable stack. This can lead to unauthorized access to sensitive applications, data breaches, and account takeover, as was demonstrated in attacks against platforms like GitLab .
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Sources:
Reported By: github.com
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