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CVE-2026-16639 – Authentication Bypass – Drupal Internationalization Single Sign-On (i18n_sso) modul

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Summary

CVE-2026-16639 is a critical Authentication Bypass Using an Alternate Path or Channel vulnerability (CWE-288) in the Internationalization Single Sign-On (i18n_sso) contributed module for Drupal. The flaw allows an attacker to authenticate as a victim user without valid credentials on multilingual Drupal sites that use separate domains per language. It carries a CVSS v3.1 base score of 9.8 (Critical).

Technical details

  • Root cause: the module does not adequately validate a short-lived token used to automatically log a user into a secondary language domain after they authenticate on the main language domain.
  • Trigger conditions: the target site must be multilingual and configured with the i18n_sso module to provide cross-domain single sign-on between per-language domains.
  • Attack vector: network-based, no authentication or user interaction required; the attacker must appear to originate from the same client IP address as the victim, which serves as a partial mitigating factor.
  • Impact: full authentication bypass, allowing an attacker to authenticate as a victim user, resulting in loss of confidentiality, integrity, and availability of the affected site.

Affected software

  • Drupal Internationalization Single Sign-On (i18n_sso) module, all versions from 0.0.0 up to (but not including) 8.x-1.8.0

Severity

  • CVSS v3.1 Base Score: 9.8 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Mitigation and recommended actions

  • Immediate: Upgrade the i18n_sso module to version 8.x-1.8 or later.
  • If no patch can be applied immediately: restrict or disable the cross-domain single sign-on functionality provided by i18n_sso, and monitor for authentication attempts originating from the same IP address as legitimate active sessions, since exploitation requires the attacker to appear to share the victim’s client IP.

How IONIX identifies potentially affected assets

IONIX matches the following signals against data already collected when it crawled the asset; identifying the technology sends no request beyond that crawl.

  • HTTP response header expires: 19 Nov 1978; header x-drupal-cache; header x-generator: Drupal
  • <meta name="generator"> content: Drupal
  • Raw response body: a <link> or <style> tag referencing /sites/default/ or /sites/all/ themes or modules
  • Loaded script URL: drupal.js
  • Rendered-page JavaScript object: Drupal

References

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How IONIX’s External Exposure Management Platform Detects and Validates
Zero-Days to Shrink MTTR

1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

Execute Exploit Validations

By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

Are you exposed?

Get a free report of your organization’s exposure to this CVE and threat

  • Mapping of all Assets with this Technology
  • Identification of potentially exposed assets to this CVE
  • Confirmation of verified exploitable assets
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