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CVE-2026-73270 – Authentication Bypass / Information Disclosure – Erlang/OTP inets httpd (mod_auth)

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Summary

CVE-2026-73270 is an authentication bypass vulnerability in the mod_auth module of Erlang/OTP’s built-in inets httpd web server. The module validates directory-based access controls using a case-sensitive regular expression match, so on case-insensitive filesystems (Windows and macOS) an unauthenticated remote attacker can reach a protected path simply by changing its letter casing. The issue carries a CVSS v4.0 base score of 8.2 (High) and requires no authentication or user interaction to exploit.

Technical details

  • Root cause: mod_auth‘s path-protection check (secret_path/3) compares the requested URI against the configured protected directory using re:run/3 without the caseless option, making the comparison case-sensitive even though the underlying filesystem may not be.
  • Trigger conditions: The httpd server must be deployed with mod_auth-based directory protection (e.g., a <Directory /Secret> style access control block) on a host running a case-insensitive filesystem — Windows or macOS. Case-sensitive filesystems (typical Linux/FreeBSD deployments) are not affected.
  • Attack vector: Network — a remote, unauthenticated attacker sends an HTTP request to the protected resource using an alternate case (e.g., requesting /secret/file.html when the configured protected directory is /Secret). Because the filesystem resolves the case variant to the same file while mod_auth‘s comparison logic treats it as a different, unprotected path, the authentication check is skipped.
  • Impact: Confidentiality impact only — successful exploitation allows disclosure of content that should require authentication to access. There is no reported impact to integrity or availability.

Affected software

  • Erlang/OTP 17.0 up to (but not including) 27.3.4.17
  • Erlang/OTP 28.0 up to (but not including) 28.5.0.6
  • Erlang/OTP 29.0 up to (but not including) 29.0.6
  • inets application versions 5.10 up to (but not including) 9.3.2.7
  • inets application versions 9.4 up to (but not including) 9.6.2.3
  • inets application versions 9.7 up to (but not including) 9.7.2

Only deployments running the affected inets httpd server with mod_auth directory protection on Windows or macOS hosts are exploitable.

Severity

  • CVSS v4.0 Base Score: 8.2 (High)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Mitigation and recommended actions

  • Immediate: Upgrade Erlang/OTP to 27.3.4.17, 28.5.0.6, or 29.0.6 (or the corresponding patched inets releases 9.3.2.7, 9.6.2.3, or 9.7.2), whichever aligns with your current OTP branch.
  • If immediate patching is not possible:
    • Avoid running inets httpd with mod_auth directory protection on case-insensitive filesystems (Windows, macOS); prefer case-sensitive filesystems (Linux, FreeBSD) for such deployments.
    • Place a reverse proxy in front of the httpd server that normalizes/lowercases incoming request paths before they reach the application, ensuring consistent case handling with the protected directory configuration.
    • Audit access logs for requests to sensitive directories using unexpected letter casing as an indicator of exploitation attempts.

References

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