CVE-2026-73191: Apache Syncope Open Redirect Vulnerability

CVE-2026-73191 Overview

CVE-2026-73191 is an open redirect vulnerability [CWE-601] affecting Apache Syncope. The flaw resides in the Syncope Secure Remote Access (SRA) component when configured for Central Authentication Service (CAS) authentication. The SRA calculates the target Apereo CAS instance URL by unconditionally trusting client-supplied forwarded HTTP headers. An attacker can craft a request that redirects users to an attacker-controlled site during the authentication flow.

The issue affects Apache Syncope versions 3.0.0-M0 through 3.0.16, 4.0.0-M0 through 4.0.7, and 4.1.0-M0 through 4.1.2. The Apache Syncope project recommends upgrading to version 4.0.8 or 4.1.3.

Critical Impact
Attackers can hijack the CAS authentication flow to redirect users to attacker-controlled hosts, enabling credential phishing and session token theft.

Affected Products

Apache Syncope 3.0.0-M0 through 3.0.16
Apache Syncope 4.0.0-M0 through 4.0.7
Apache Syncope 4.1.0-M0 through 4.1.2

Discovery Timeline

2026-09-14 - CVE-2026-73191 published to the National Vulnerability Database
2026-09-14 - Last updated in NVD database

Technical Details for CVE-2026-73191

Vulnerability Analysis

Apache Syncope is an open source identity management system. The Syncope Secure Remote Access (SRA) module acts as a reverse proxy that can integrate with an external Apereo CAS server for single sign-on. During the CAS authentication handshake, the SRA must determine the CAS server URL to which the user is redirected.

The vulnerable code path builds this URL from HTTP forwarding headers, such as X-Forwarded-Host and X-Forwarded-Proto, without validating them against an allowlist. Because these headers are supplied by the client, an unauthenticated attacker can override the destination host used in the outbound redirect. Successful exploitation requires the victim to click a crafted link, matching the user interaction requirement in the CVSS vector.

Root Cause

The root cause is unconditional trust in forwarded HTTP headers when constructing the CAS server URL inside SRA. The application does not compare the derived host against configured trusted hosts, violating the guidance in [CWE-601]. Any host value injected by the client is accepted and used verbatim in the outbound redirect response.

Attack Vector

An attacker sends a victim a link to the Syncope SRA authentication endpoint containing manipulated X-Forwarded-Host and related headers, or hosts a page that triggers such a request. When the victim initiates authentication, the SRA responds with a redirect to the attacker-supplied host presented as the CAS server. The attacker's phishing page mimics the legitimate CAS login screen and captures credentials or session assertions.

See the Apache Mailing List Thread and the Openwall OSS-Security Discussion for the upstream advisory details.

Detection Methods for CVE-2026-73191

Indicators of Compromise

HTTP requests to Syncope SRA authentication endpoints containing X-Forwarded-Host, X-Forwarded-Server, or Forwarded headers that do not match the deployment's expected hostnames.
Outbound HTTP 302 responses from the SRA whose Location header points to hosts not on the approved CAS server allowlist.
User reports of unexpected login pages or domain changes during the CAS single sign-on flow.

Detection Strategies

Inspect reverse proxy and load balancer logs for anomalous X-Forwarded-* header values reaching the SRA.
Correlate redirect responses issued by SRA with the configured CAS server URL and alert on divergence.
Deploy web application firewall rules to log requests where forwarded headers reference external domains.

Monitoring Recommendations

Baseline the set of hostnames legitimately used in SRA redirects and alert on any new destination.
Forward SRA access logs to a centralized SIEM for retention and correlation with authentication events.
Monitor for spikes in failed CAS authentications following successful redirects, indicating possible credential harvesting.

How to Mitigate CVE-2026-73191

Immediate Actions Required

Upgrade Apache Syncope to version 4.0.8 or 4.1.3 as recommended by the project.
Audit SRA deployments configured for CAS authentication and confirm patched builds are running.
Review recent SRA access logs for suspicious X-Forwarded-Host values that may indicate attempted exploitation.

Patch Information

The Apache Syncope project addressed the flaw in versions 4.0.8 and 4.1.3. Users of the 3.0.x branch should plan migration, as versions through 3.0.16 remain affected without an equivalent fix line published in the advisory. Refer to the Apache Mailing List Thread for release details.

Workarounds

Place the SRA behind a trusted reverse proxy that strips or overwrites client-supplied X-Forwarded-* and Forwarded headers before they reach the application.
Restrict inbound traffic to the SRA so that only known upstream proxies can supply forwarding headers.
Where feasible, temporarily disable CAS authentication on SRA until patched versions are deployed.

bash

# Example NGINX snippet to strip client-supplied forwarding headers
# before proxying to the Syncope SRA backend
location /syncope-sra/ {
    proxy_set_header X-Forwarded-Host   $host;
    proxy_set_header X-Forwarded-Proto  $scheme;
    proxy_set_header X-Forwarded-Server $host;
    proxy_set_header Forwarded          "";
    proxy_pass http://syncope-sra-backend/;
}