CVE-2026-57817: Apache CXF Auth Bypass Vulnerability

CVE-2026-57817 Overview

CVE-2026-57817 affects Apache CXF Relying Party (RP) implementations of OpenID Connect Core 1.0. The OpenID Connect Core 1.0 specification requires the RP to validate the c_hash parameter when operating in the Hybrid Flow. Apache CXF integrated with a non-compliant or misconfigured Identity Provider (IdP) that omits the c_hash skips this validation. This gap exposes the RP to Authorization Code Substitution and Injection attacks. Apache addressed the issue in versions 4.2.3, 4.1.8, and 3.6.12.

Critical Impact
Attackers can substitute or inject authorization codes to hijack authenticated sessions when Apache CXF RPs pair with IdPs that omit c_hash from the ID token.

Affected Products

Apache CXF versions prior to 3.6.12
Apache CXF versions prior to 4.1.8
Apache CXF versions prior to 4.2.3

Discovery Timeline

2026-08-06 - CVE-2026-57817 published to the National Vulnerability Database (NVD)
2026-08-06 - Last updated in NVD database

Technical Details for CVE-2026-57817

Vulnerability Analysis

OpenID Connect Hybrid Flow returns both an authorization code and an ID token from the authorization endpoint. The c_hash claim in the ID token cryptographically binds the ID token to the authorization code. The RP must recompute the hash of the received code and compare it against c_hash to detect substitution.

Apache CXF's RP logic does not enforce this binding when the IdP omits the claim. An attacker who intercepts or obtains a victim's authorization code can present it to the RP alongside a legitimate ID token issued for the attacker. Without c_hash verification, the RP accepts the mismatched pair and completes authentication under the wrong identity. The weakness maps to CWE-20: Improper Input Validation.

Root Cause

The root cause is missing enforcement of the OpenID Connect Core 1.0 requirement that c_hash MUST be validated in Hybrid Flow. Apache CXF treats an absent c_hash as acceptable rather than rejecting the response. Compliance with the specification requires the RP to fail closed when the claim is missing or invalid.

Attack Vector

Exploitation requires an OpenID Connect Hybrid Flow deployment where the IdP is non-compliant or misconfigured to omit c_hash. An attacker obtains an authorization code belonging to a victim, then substitutes that code during their own callback to the vulnerable RP. The RP binds the victim's code to the attacker's ID token and grants access. See the Apache Security Mailing List Thread for the vendor advisory. No verified public proof-of-concept code is available at this time.

Detection Methods for CVE-2026-57817

Indicators of Compromise

Authentication events where the subject claim in the ID token does not match the user associated with the redeemed authorization code.
OpenID Connect callback requests arriving from unexpected source IPs shortly after a legitimate user initiated login.
ID tokens issued by the IdP that lack the c_hash claim in Hybrid Flow responses.

Detection Strategies

Inspect IdP token issuance logs for Hybrid Flow (response_type=code id_token or code token id_token) responses missing c_hash.
Correlate Apache CXF authentication success events with the originating IP of the authorization request to flag session-context mismatches.
Instrument the RP to log parsed ID token claims and alert when c_hash is absent for Hybrid Flow.

Monitoring Recommendations

Forward Apache CXF and IdP logs to a centralized analytics platform for cross-correlation of authorization code redemption and ID token issuance.
Alert on repeated callback requests that reuse a single authorization code from different clients or user agents.
Track version inventory of Apache CXF deployments to identify hosts still running vulnerable releases.

How to Mitigate CVE-2026-57817

Immediate Actions Required

Upgrade Apache CXF to version 4.2.3, 4.1.8, or 3.6.12 depending on the deployed branch.
Audit connected Identity Providers to confirm they emit c_hash for all Hybrid Flow responses.
Restrict OpenID Connect flows to Authorization Code Flow with PKCE where Hybrid Flow is not required.

Patch Information

Apache fixed the issue in Apache CXF 4.2.3, 4.1.8, and 3.6.12. Full details are available in the Apache Security Mailing List Thread. Apply the patch that matches the deployed branch and restart affected services.

Workarounds

Reconfigure the IdP to include c_hash in every ID token issued through Hybrid Flow, aligning with OpenID Connect Core 1.0.
Disable Hybrid Flow at the RP and require pure Authorization Code Flow until patching is complete.
Enforce short authorization code lifetimes and single-use code redemption at the IdP to reduce the substitution window.

bash

# Configuration example
# Verify installed Apache CXF version and upgrade via Maven
mvn dependency:tree | grep cxf
# Update pom.xml to a fixed release, for example:
#   <cxf.version>4.2.3</cxf.version>
mvn clean install