CVE-2026-16639: Drupal I18N SSO Auth Bypass Vulnerability

CVE-2026-16639 Overview

CVE-2026-16639 is an authentication bypass vulnerability in the Drupal Internationalization Single Sign-On (SSO) contributed module. The flaw allows unauthenticated attackers to bypass authentication controls by leveraging an alternate path or channel. It affects all versions of the module from 0.0.0 through 1.8.0. The issue is tracked under CWE-288: Authentication Bypass Using an Alternate Path or Channel. The vulnerability is network-exploitable, requires no privileges, and requires no user interaction. Full details are published in the Drupal Security Advisory SA-CONTRIB-2026-081.

Critical Impact
Remote attackers can bypass SSO authentication and gain unauthorized access to Drupal sites running the Internationalization Single Sign-On module, compromising confidentiality, integrity, and availability.

Affected Products

Drupal contributed module: Internationalization Single Sign-On
Versions from 0.0.0 up to and including 1.8.0
Any Drupal site relying on this module for federated login

Discovery Timeline

2026-08-25 - CVE-2026-16639 published to NVD
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-16639

Vulnerability Analysis

The Drupal Internationalization Single Sign-On module handles federated login across multilingual Drupal deployments. CVE-2026-16639 stems from an authentication bypass condition classified under [CWE-288]. An attacker can reach a protected resource or authenticated context through a channel that does not enforce the intended identity checks.

Because the attack vector is the network with low complexity, an unauthenticated remote actor can trigger the bypass with a single crafted request. Successful exploitation grants access equivalent to a legitimate authenticated user, exposing user data, administrative functions where SSO is trusted, and any downstream applications that rely on the module for identity assertion.

At the time of publication, there is no public proof-of-concept exploit, no CISA KEV listing, and no confirmed in-the-wild exploitation. The EPSS probability is 0.182%.

Root Cause

The root cause is an alternate authentication path in the module that fails to enforce the same identity validation as the primary login flow. Endpoints or request patterns intended for internal or session-continuation use accept requests without verifying that the caller possesses valid SSO credentials or a validated assertion. Vendor-side technical details are aggregated in the Drupal Security Advisory.

Attack Vector

Exploitation occurs over the network against the Drupal site's HTTP interface. The attacker sends a request to the alternate authentication path exposed by the Internationalization Single Sign-On module without providing valid credentials. The module returns a session or context that treats the caller as an authenticated principal. From that state, the attacker can access account-scoped resources or escalate through Drupal role assignments tied to the SSO identity.

No user interaction and no prior privileges are required. The vulnerability is remotely reachable by any actor able to make HTTP requests to the site.

Detection Methods for CVE-2026-16639

Indicators of Compromise

Authentication or session-creation events for user accounts without a matching successful SSO assertion in upstream identity provider logs.
HTTP requests to Internationalization Single Sign-On module endpoints from unexpected source IPs, followed by authenticated activity.
New or elevated Drupal sessions established without a preceding login form submission or IdP redirect chain.
Access to administrative or role-scoped Drupal paths from accounts that did not complete a normal SSO flow.

Detection Strategies

Inventory Drupal deployments and confirm whether the Internationalization Single Sign-On module is installed and at version 1.8.0 or earlier.
Correlate Drupal watchdog authentication events with identity provider logs to detect sessions created without a corresponding IdP assertion.
Deploy web application firewall rules that log and alert on requests to SSO module paths lacking expected assertion parameters or referrer chains.
Baseline normal SSO login sequences and alert on deviations, including direct calls to callback or continuation endpoints without prior initiation.

Monitoring Recommendations

Forward Drupal application logs, web server access logs, and IdP logs to a centralized analytics platform for cross-source correlation.
Monitor for spikes in successful authentications from single source IPs against Drupal endpoints.
Track privileged action events (role changes, content publication, module configuration) tied to sessions with anomalous origination.
Retain logs for a period sufficient to support retrospective hunts once the vendor advisory technical details are fully reviewed.

How to Mitigate CVE-2026-16639

Immediate Actions Required

Identify all Drupal sites running the Internationalization Single Sign-On module and confirm the installed version.
Upgrade the module to a fixed release as identified in the Drupal Security Advisory.
If a fix cannot be applied immediately, disable the Internationalization Single Sign-On module and terminate active sessions.
Review authentication and administrative activity logs for signs of prior unauthorized access.

Patch Information

Drupal has published guidance in advisory SA-CONTRIB-2026-081. Administrators should upgrade the Internationalization Single Sign-On module to the vendor-designated fixed version above 1.8.0. After patching, invalidate existing sessions and force reauthentication so that any session created through the bypass is dropped. Confirm the module version through drush pm:list or the Drupal administrative interface after upgrade.

Workarounds

Disable the Internationalization Single Sign-On module until the patched version is deployed.
Restrict network access to Drupal SSO endpoints using web application firewall rules or reverse proxy allowlists.
Enforce a secondary authentication check at the reverse proxy or IdP layer while the module remains vulnerable.
Rotate session keys and invalidate active sessions after applying the fix to remove any adversary-established state.

bash

# Configuration example: disable the vulnerable module via Drush
drush pm:uninstall i18n_sso
drush cache:rebuild

# After upgrading to a fixed release, verify the installed version
drush pm:list --type=module --status=enabled | grep i18n_sso