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CVE-2026-87929 – Authentication Bypass / Admin Takeover – MaxSite CMS 0.78 through 109.6

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Summary

CVE-2026-87929 is a critical authentication bypass vulnerability in MaxSite CMS, caused by a hardcoded session encryption key that ships unchanged in every installation. An unauthenticated attacker can forge an administrator session cookie and gain full administrative control of the CMS. The flaw affects MaxSite CMS versions 0.78 through 109.6 and carries a CVSS score of 9.3 (Critical).

Technical details

  • Root cause: application/config/config.php sets $config['encryption_key'] to the literal placeholder value 'encryption key', which is never rotated during installation and is therefore identical across all deployments.
  • The application’s ci_session cookie is validated using HMAC-SHA1 signing with this public key, allowing an attacker to sign a forged, PHP-serialized session array (e.g., userlogged = "1", users_groups_id = "1") that the server accepts as authentic.
  • A secondary flaw in init.php re-validates session data by passing malformed, attacker-supplied ciphertext (prefixed "MSO-") into mso_de_code(); when decryption fails, openssl_decrypt() returns false, which is cast to 0 and used in a database query (WHERE users_login = 0 AND users_password = 0), matching the administrator account due to MySQL’s loose string-to-integer comparison.
  • Attack vector: network, no authentication or user interaction required (AV:N/AC:L/PR:N/UI:N).
  • Impact: complete administrative takeover, enabling site defacement, user/account manipulation, configuration changes, and potential arbitrary code execution via administrative template/file features.

Affected software

  • MaxSite CMS versions 0.78 through 109.6 (all installations using the default/unchanged encryption key)

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
  • CVSS v4.0 Score: 9.3 (Critical) — Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
  • CWE-321: Use of Hard-coded Cryptographic Key

Mitigation and recommended actions

  • Immediate: Upgrade to a MaxSite CMS release beyond 109.6 that generates a unique, randomly-generated encryption_key per installation; if no such patched release is available, manually replace the default 'encryption key' value in application/config/config.php with a unique, high-entropy secret and invalidate all existing sessions.
  • If no patch is available: switch session storage to server-side/database-backed sessions (sess_use_database = TRUE) instead of cookie-based sessions, and ensure any decrypt/validation routine (e.g., mso_de_code()) rejects failed decryption results rather than allowing them to flow into database queries.
  • Restrict administrative panel access at the network layer (e.g., allow-list IPs or require VPN access) as a compensating control until the encryption key is rotated.
  • Rotate all administrator credentials and audit for signs of unauthorized administrative access following remediation.

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.

  • Named <meta> tag content (generator): MaxSite CMS

References

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