External risk intelligence

Apache CXF OpenID Connect Authorization Code Substitution Vulnerability

CVE advisorySeverity: HIGH (CVSS 8.1)

CVE-2026-57817

The vulnerability affects OpenID Connect Relying Parties (RPs) using Apache CXF. OIDC RPs are typically implemented as web applications or services that handle authentication flows, which are commonly exposed to the internet to facilitate user logins and identity management.

Apache Cxf

before 3.6.124.0.0 to before 4.1.84.2.0 to before 4.2.3

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

This vulnerability impacts systems using Apache CXF that handle user authentication through OpenID Connect. Specifically, it relates to how these systems verify authentication information when users log in. If not properly configured, it could allow an attacker to potentially substitute or inject authorization codes, leading to unauthorized access. The primary concern is to confirm if our implementations are relevant and potentially exposed.

Authentication check flaw in specific web technologies.
Affects systems managing user logins.
Confirm relevance and exposure for our systems.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by leveraging an improperly configured or non-compliant identity provider that fails to include the `c_hash` parameter during an OpenID Connect Hybrid Flow. This misconfiguration allows the attacker to substitute or inject a malicious authorization code, potentially leading to the compromise of user sessions and data.

Attacker exploits misconfigured identity provider.
Vulnerability triggered by missing `c_hash` parameter.
Risk includes unauthorized session access and data theft.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability could impact systems using Apache CXF in an OpenID Connect Hybrid Flow when integrated with identity providers that do not properly validate the `c_hash` parameter. In such scenarios, an attacker might be able to substitute or inject an authorization code, potentially leading to unauthorized access or manipulation of user sessions.

Authorization codes could be compromised.
Mismatched or missing `c_hash` allows injection.
Session hijacking or unauthorized access.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability, affecting Apache CXF implementations of OpenID Connect Relying Parties, likely falls under the purview of application owners and platform teams responsible for identity and access management. The first actionable step is to identify all instances of Apache CXF used as an RP, confirm their exposure and criticality, and then engage the accountable teams to plan remediation.

Application and platform teams own the fix.
Verify OpenID Connect Hybrid Flow usage.
Plan vendor coordination and patching.

Supplementary metadata

CVSS vector

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Frequently asked questions

What is Apache CXF and how is it used?

Apache CXF is an open-source services framework used to build and develop services. It functions as a Relying Party (RP) in OpenID Connect, meaning it acts as the service that trusts an Identity Provider to authenticate users and manage their login sessions for web applications.

What does CVE-2026-57817 mean in simple terms?

This vulnerability is classified as Improper Input Validation (CWE-20). It occurs because the software fails to verify the `c_hash` parameter, which is a security check meant to ensure the integrity of authorization codes. Without this check, the system cannot confirm the code originated from a legitimate source, opening the door for substitution attacks.

How is this vulnerability triggered?

An attacker triggers this by using a non-compliant or misconfigured Identity Provider that omits the `c_hash` parameter during the OpenID Connect Hybrid Flow. If the Apache CXF implementation does not strictly enforce the presence and validation of this parameter, it will accept the malicious code. The vulnerability does not trigger if the Identity Provider correctly sends and validates the required hash.

Do I need to worry about this if my service is internal?

Halo Surface Signal identifies this as a likely issue because Apache CXF Relying Parties are typically web services that manage authentication and are often exposed to the internet. While internal services have a smaller attack surface, any system using the Hybrid Flow with an identity provider is theoretically affected regardless of its network location.

What should I do to address this vulnerability?

Your first step is to audit your environment to locate all instances where Apache CXF is configured as an OpenID Connect Relying Party. Once identified, verify if you are utilizing the Hybrid Flow. If you are, prioritize upgrading to version 4.2.3, 4.1.8, or 3.6.12 to apply the necessary security controls.

References

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.