CVE-2026-16639

Executive Summary

CVE-2026-16639 is an Authentication Bypass Using an Alternate Path or Channel vulnerability (CWE-288) affecting the Drupal "Internationalization Single Sign-On" module across all versions from 0.0.0 through 1.8.0. The flaw permits an unauthenticated remote attacker to bypass the authentication mechanism by leveraging an alternate path or channel to access protected resources, potentially gaining unauthorized access to Drupal application functionality and data. The vulnerability currently has no patch available from any tracked source, no public exploit code, and no known exploitation in the wild; however, the severity is elevated by the absence of a fix and the module's likely deployment on internet-exposed Drupal instances. Given the CVSS score is N/A (not assigned), organizations must treat this as a high-priority unpatched risk and apply compensating controls immediately.

Technical Analysis

The vulnerability is classified under CWE-288: Authentication Bypass Using an Alternate Path or Channel. This weakness class occurs when an application's authentication logic can be circumvented by reaching a restricted resource through a path, channel, or protocol that bypasses the normal authentication gate. Common manifestations include:

• Direct access to backend endpoints that skip the authentication middleware.
• Exploitation of HTTP method inconsistencies (e.g., TRACE, PUT, or custom headers) that lead the application to a different code branch where authentication is not enforced.
• Use of alternate URL encodings, path traversal sequences, or case-sensitivity differences that cause the routing layer to map a request to a protected handler without invoking the authentication filter.
• For single sign-on (SSO) implementations specifically, bypasses commonly arise from improper validation of assertion/response messages, mishandling of SAML/OAuth tokens, or trusting an alternate authentication channel (e.g., HTTP header injection of a trusted identity) without proper cryptographic verification.

In the context of the Drupal Internationalization Single Sign-On module, the vulnerability likely resides within the SSO assertion-handling logic. The module processes authentication tokens or assertions from an identity provider; if the module fails to verify the source channel or path of the authentication request, an attacker could craft a request that mimics a legitimate SSO response, thereby authenticating as an arbitrary user (including an administrator) without valid credentials. The NVD description explicitly notes the affected range as "from 0.0.0 to 1.8.0," indicating that all released versions of the module are vulnerable and that the issue has not been addressed in any subsequent release.

No CVSS vector or score has been assigned by the CNA, and no EPSS score is available. This absence of scoring does not diminish the risk; rather, it reflects the lack of coordinated disclosure data. The attack surface is the Drupal web frontend, specifically any URL handled by the Internationalization Single Sign-On module. Because Drupal is widely deployed on public-facing servers, the exploit surface is significant. However, without a public PoC or exploit code, the precise exploitation mechanics remain unconfirmed outside of the vendor's internal analysis.

Exploitation Scenarios

The following attack chain is inferred strictly from the vulnerability class (CWE-288) and the NVD description. No specific implementation details, function names, or endpoints beyond the module name are publicly available; therefore, the steps are described at the class level.

Step 1 — Reconnaissance and Target Identification:
An attacker scans the internet for Drupal instances using fingerprinting techniques (identifying the CMS, version, and enabled modules). The presence of the Internationalization Single Sign-On module may be detectable through distinctive response headers, error messages, or exposed configuration paths. Tools such as Shodan or masscan can identify Drupal sites; the attacker then narrows the list to those running the module (e.g., by probing for SSO-related endpoints or observing the module's JavaScript/CSS assets).

Step 2 — Triggering the Authentication Bypass:
The attacker crafts an HTTP request that targets an alternate path or channel used by the SSO module for authentication handling. For a generic CWE-288 flaw, this could involve:

• Sending a request directly to an SSO callback or assertion-consumer endpoint that the module uses internally, bypassing the main login form.
• Manipulating HTTP headers or cookies that the module trusts as an alternate authentication channel (e.g., injecting a X-Remote-User header or forging a session token if the module relies on such channels).
• Using path normalization or encoding tricks (e.g., %2e, double slashes, or case variations) that cause the Drupal router to bypass the authentication middleware and invoke the protected handler directly.

Because the module handles SSO assertions, the most plausible trigger is sending a crafted SAML/assertion message or a forged callback request that the module accepts without validating the source or signature correctly.

Step 3 — Payload and Execution Method:
At the vulnerability-class level, the "payload" is the crafted authentication request itself — no code execution is required. The attacker supplies an assertion or token that claims an identity, and the module, due to the missing channel/path validation, trusts it. This results in the attacker being authenticated as the claimed user. If the attacker claims an elevated role (e.g., a user with the "administrator" or "site manager" role), the session is granted those privileges.

Step 4 — Post-Exploitation Impact:
Once authenticated with elevated privileges, the attacker can:

• Access, modify, or exfiltrate sensitive data stored in the Drupal CMS (user records, content, configuration files).
• Install malicious modules or themes to achieve remote code execution on the underlying server.
• Deface the website, inject malware or phishing content, or pivot to internal network resources if the Drupal server holds additional trust.

Note: No public PoC is available at the time of writing; the attack chain above is inferred from the vulnerability class and the module's functionality. The absence of a PoC should not be interpreted as the vulnerability being unexploitable — it simply reflects the lack of coordinated disclosure.

Impact Assessment

The business impact of CVE-2026-16639 is high. An unauthenticated attacker can bypass authentication entirely, potentially gaining administrative access to the Drupal application. This leads to:

• Data Confidentiality Breach: Unauthorized access to user databases, personal information, and potentially sensitive business data, which could trigger regulatory notification obligations (GDPR, HIPAA, etc.).
• Integrity Compromise: The attacker may alter site content, modify configuration, or backdoor the application for persistent access.
• Availability Risk: A compromised Drupal instance can be defaced, taken offline, or used as a launchpad for further attacks (e.g., cryptocurrency mining, DDoS botnet participation).

The affected systems are all Drupal deployments running the Internationalization Single Sign-On module version 1.8.0 or earlier. Given the module's purpose (SSO across internationalized site versions), affected deployments are likely large, multi-site organizations with distributed user bases — amplifying the blast radius of a compromise.

CVSS assessment: The CVSS score is N/A, which prevents a numeric severity comparison. However, based on the vulnerability class (authentication bypass), the vector would plausibly be critical if scored: network-exploitable, low attack complexity, no privileges required, and high confidentiality/integrity/availability impact. The lack of a patch further elevates the operational risk, making this a priority for compensating controls.

Detection Guidance

The following detection rules are designed to identify exploitation attempts against CVE-2026-16639. Since the exact exploitation pattern is unknown, these rules focus on anomalous authentication-related behavior and access to SSO endpoints, which are generic enough to apply across the likely attack vectors.

Sigma Rule

title: Suspicious SSO Authentication Bypass Attempt - Drupal Internationalization SSO
id: 4f5c9a2e-3d8b-4f1a-9c6e-2b7d8a1c4f5e
status: experimental
description: Detects unusual access patterns to Drupal SSO endpoints that may indicate an authentication bypass attempt (CVE-2026-16639).
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-16639
tags:
    - attack.t1190
    - attack.initial_access
    - cve.2026-16639
logsource:
    product: web
    service: access_log
detection:
    selection:
        cs-method: ['GET', 'POST']
        c-uri|contains:
            - '/sso/'
            - '/i18n_sso/'
            - '/user/login/sso'
            - '/assertion-consumer'
    filter_known:
        c-uri|endswith: ['/index.php', '/robots.txt']
    condition: selection and not filter_known
    timeframe: 5m
level: high
falsepositives:
    - Legitimate SSO redirects from trusted identity providers
    - Routine administrative access to SSO configuration pages
fields:
    - src_ip
    - cs-method
    - c-uri
    - c-useragent

YARA Rule

rule CVE_2026_16639_Drupal_i18n_SSO_Bypass {
    meta:
        author = "Valters IT Security"
        description = "Detects suspicious HTTP payloads targeting Drupal Internationalization SSO auth bypass"
        date = "2026-01-01"
        cve = "CVE-2026-16639"
        severity = "high"
    strings:
        $sso_path = "/sso" nocase
        $sso_i18n = "i18n_sso" nocase
        $auth_assert = "Assertion" nocase
        $role_claim = "administrator" nocase
        $header_inject = "X-Remote-User" nocase
    condition:
        any of ($sso_path, $sso_i18n) and
        (any of ($auth_assert, $role_claim) or $header_inject)
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (
    msg:"ET EXPLOIT Drupal Internationalization SSO Authentication Bypass Attempt (CVE-2026-16639)";
    flow:to_server,established;
    http.uri;
    content:"/sso";
    http.method;
    content:"POST";
    http.request_body;
    content:"Assertion";
    threshold:type limit, track by_src, count 5, seconds 60;
    classtype:attempted-admin;
    sid:202616639;
    rev:1;
)

Elastic Detection

sequence by destination.ip
  [ http where request.method in ("GET","POST") and
    lower(request.uri) like "*sso*" ]
  [ http where request.method in ("GET","POST") and
    lower(request.uri) like "*assertion*" ]
  [ network where destination.port == 80 or destination.port == 443 ]

Note: This EQL rule is intentionally broad; adjust the URI patterns based on observed traffic to reduce false positives.

Splunk SPL Query

index=web sourcetype=access_combined
  (uri IN ("*sso*", "*i18n_sso*", "*assertion-consumer*"))
  OR (uri="*user/login*" AND form_data="*Assertion*")
  OR (headers="X-Remote-User:*")
| stats count by src_ip, uri, method, status
| where count > 10
| table _time, src_ip, uri, method, status, count

Wazuh OSSEC Rule

<rule id="100239" level="12">
  <decoded>web-access</decoded>
  <match>sso|i18n_sso|assertion-consumer</match>
  <regex>POST|GET</regex>
  <description>Possible Drupal Internationalization SSO authentication bypass attempt (CVE-2026-16639)</description>
  <group>authentication_failure,web_attack,cve_2026_16639</group>
  <mitre>
    <id>T1190</id>
  </mitre>
  <info>Unauthorized access attempt to Drupal SSO endpoint. Verify source IP and correlate with login failures.</info>
</rule>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED. No patch evidence exists from any tracked source. The vendor has not released a fixed version as of the analysis date.

Immediate Mitigations (Compensating Controls):

1. Disable the Module if Not Critical: If the Internationalization Single Sign-On module is not essential to operations, disable and uninstall it immediately to eliminate the attack surface.
2. Network-Level Restriction: Use a Web Application Firewall (WAF) or reverse proxy to block access to SSO-related endpoints (paths containing sso, i18n_sso, assertion-consumer, etc.) from untrusted IP ranges. Restrict administrative and SSO endpoints to a VPN or allow-listed IP addresses.
3. Enforce Additional Authentication: Require a secondary factor (MFA) for all user accounts, particularly administrator accounts, to mitigate the impact of an authentication bypass.
4. Monitor and Alert: Deploy the detection rules above and correlate SSO-related anomalies with failed login logs and privileged account activity.

Patch/Upgrade Steps:

1. Check for Updates: Monitor the Drupal security advisory page and the module's official project page for a patched release (expected to be version 1.8.1 or higher). Apply immediately upon release.
2. Temporary Workaround: If upgrading is not possible, consider using a custom module or hook in Drupal to validate the source of SSO assertions more strictly (e.g., verify the remote IP or require a shared secret). This is a stopgap measure and should not replace a proper patch.

Long-Term Hardening:

1. Apply Drupal Security Best Practices: Keep Drupal core and all contributed modules up to date. Regularly audit enabled modules and remove unused ones.
2. Validate SSO Implementation: Review the SSO flow to ensure assertions are cryptographically signed and timestamps are checked, minimizing the risk of forged or replayed assertions.
3. Harden Web Server Configurations: Disable HTTP methods not in use (e.g., TRACE), normalize URL paths to prevent path-based bypasses, and use secure HTTP headers (CSP, X-Content-Type-Options).
4. Incident Response Preparedness: Ensure logs are shipped to a centralized SIEM and that the detection rules are active. Conduct periodic red-team exercises to validate the effectiveness of compensating controls.
5. Stay Informed on KEV/EPSS: Re-check CISA KEV and FIRST EPSS databases regularly; if the vulnerability is added, escalate remediation urgency accordingly.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-16639
2. CWE-288 (Authentication Bypass Using an Alternate Path or Channel): https://cwe.mitre.org/data/definitions/288.html
3. Drupal Security Advisories (general): https://www.drupal.org/security
4. MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
5. Internationalization Single Sign-On module page (Drupal.org): https://www.drupal.org/project/i18n_sso

Threat Actor Campaigns / Exploit-in-the-Wild: No known exploitation in the wild, no public exploit code, and no CISA KEV listing have been reported as of the analysis date. This status may change rapidly; analysts should re-verify these references periodically.