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CVE-2026-103475 – Sensitive Data Exposure & Code Injection via Debug/Gii Modules – yii2-starter-kit

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Summary

CVE-2026-103475 is a critical misconfiguration vulnerability in yii2-starter-kit, a starter application template for the Yii2 PHP framework, affecting all versions through 4.2.0. The template ships with the Yii Debug and Gii (code generator) modules configured to accept connections from any IP address (allowedIPs => ['*']), combined with default environment settings of YII_DEBUG=true and YII_ENV=dev. Any application deployed with these unmodified defaults exposes highly sensitive internal functionality — including session data, database queries, and a live code-generation interface — to unauthenticated, remote attackers over the network.

Technical details

  • Root cause: common/config/web.php, backend/config/web.php, and .env.dist configure the Yii debug and gii modules with a wildcard allowedIPs setting (['*']), rather than restricting access to localhost (127.0.0.1/::1).
  • Trigger conditions: The vulnerability is exploitable whenever an application built from the starter kit is deployed to a reachable network without overriding the default allowedIPs, YII_DEBUG, and YII_ENV settings — a scenario the maintainers acknowledge can happen inadvertently in production.
  • Attack vector: Network-based (AV:N), no authentication or user interaction required (PR:N/UI:N), low attack complexity (AC:L).
  • Impact:
    • Access to the Yii debug toolbar/panel, which can leak session cookies, request/response data, executed database queries, and environment/configuration variables.
    • Access to the Gii code generator, which can be used to generate and write new PHP files (e.g., models, controllers) directly into the application’s source tree, providing a path to arbitrary code execution on the server.
    • Combined, these expose confidentiality and integrity of the application at a high level (CVSS 3.1: C:H/I:H).

Affected software

  • yii-starter-kit/yii2-starter-kit — all versions up to and including 4.2.0

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
  • CVSS v4.0 Base Score: 9.3 (Critical) — CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
  • CWE-489: Active Debug Code (also referenced alongside CWE-215: Information Exposure Through Debug Information)

Mitigation and recommended actions

  • Immediate:
    • Do not deploy applications generated from yii2-starter-kit to any network-reachable environment using the default configuration.
    • Explicitly restrict the allowedIPs setting for both the debug and gii modules to ['127.0.0.1', '::1'] (or an internal-only IP allowlist) in common/config/web.php and backend/config/web.php.
    • Set YII_DEBUG=false and YII_ENV=prod in the production environment configuration (.env) before deploying.
  • If no patch is available:
    • Audit any existing deployments built from this starter kit for publicly reachable /debug and /gii endpoints and disable or firewall them immediately at the network/reverse-proxy layer.
    • Rotate session secrets and review logs for signs of prior access to debug or Gii endpoints, as these could indicate exposure of session cookies or unauthorized file writes.
    • Monitor upstream for an official release that changes the shipped defaults, and re-generate/redeploy affected applications once available.

References

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