CVE-2026-79772 in Nokogiri
Summary
by MITRE • 08/25/2026
Nokogiri versions before 1.19.1 fail to check the return value from xmlC14NExecute in the canonicalize method, returning an empty string on failure instead of raising an exception. Attackers can exploit this to bypass signature validation in downstream SAML libraries by providing invalid canonicalized XML that is incorrectly accepted as valid.
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Analysis
by VulDB Data Team • 08/25/2026
The vulnerability identified in Nokogiri versions prior to 1.19.1 stems from a critical oversight in the error handling logic of the xmlC14NExecute function, which is invoked during the execution of the canonicalize method. This Ruby library serves as a foundational component for XML processing within many enterprise applications, particularly those involving secure communication protocols such as SAML and SOAP. The core technical flaw lies in the failure to validate the return status from the underlying C-level libxml2 routine responsible for performing Canonical XML transformations. When xmlC14NExecute encounters an error condition during the canonicalization process, it returns a negative value or null pointer indicating failure; however, Nokogiri’s wrapper method does not inspect this return code. Instead of propagating the error by raising an appropriate exception to halt execution and alert the developer, the library silently defaults to returning an empty string. This behavior masks underlying processing failures and creates a false sense of successful operation within the application layer.
The operational impact of this flaw is severe in security-sensitive contexts where XML canonicalization is used as a prerequisite for cryptographic signature verification. Canonical XML ensures that different representations of semantically equivalent XML documents produce identical byte sequences, which is essential for deterministic hashing during digital signing processes. When an attacker provides malformed or invalid input to the canonicalize method, the library fails silently and returns an empty string rather than signaling the error. Downstream libraries, such as those handling SAML assertions, often assume that a non-error return value indicates successful processing of valid XML data. Consequently, these downstream systems may proceed to validate signatures against this empty or corrupted output, effectively bypassing signature validation mechanisms. This allows attackers to manipulate authentication flows, potentially leading to unauthorized access, privilege escalation, or identity spoofing within applications relying on SAML-based single sign-on solutions.
From a classification perspective, this vulnerability aligns with CWE-252, which describes unchecked return values as a common cause of software weaknesses. The failure to verify the success status of critical system calls is a classic implementation error that undermines the integrity checks performed by higher-level security protocols. Furthermore, in the context of the MITRE ATT&CK framework, this flaw facilitates techniques related to Defense Evasion, specifically those involving tampering with authentication processes or bypassing signature verification mechanisms. By exploiting the silent failure mode, an adversary can manipulate the input data stream to evade detection systems that rely on strict XML schema validation and cryptographic integrity checks. The lack of explicit error signaling prevents defensive monitoring tools from triggering alerts based on anomalous processing states, thereby extending the window of opportunity for exploitation without immediate detection.
Mitigation strategies must prioritize upgrading Nokogiri to version 1.19.1 or later, where this specific return value check has been implemented and corrected. For organizations unable to immediately patch their dependencies due to compatibility constraints, alternative defensive measures include implementing strict input validation at the application boundary before passing data to the canonicalization routine. Developers should also consider wrapping calls to Nokogiri’s XML processing methods in explicit error-handling blocks that treat empty results from canonicalization operations as potential security incidents requiring investigation rather than valid outputs. Additionally, enabling verbose logging for XML parsing events can help identify instances where silent failures occur, allowing security teams to detect attempted exploitation patterns. Regular auditing of third-party library usage and ensuring that all critical cryptographic operations are accompanied by robust error propagation mechanisms are essential practices to prevent similar vulnerabilities from compromising system integrity in the future.