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CVE-2026-66066 – Arbitrary File Read / Remote Code Execution – Ruby on Rails Active Storage (< 7.2.3

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Summary

CVE-2026-66066 is a critical vulnerability in Ruby on Rails Active Storage. When Active Storage generates image variants with libvips, it does not disable libvips operations that are marked unsafe for untrusted content, allowing a crafted upload to invoke such an operation. This enables unauthenticated arbitrary file read and, through leaked secrets, potential remote code execution. It carries a CVSS 4.0 base score of 9.5 (Critical).

Technical details

  • Root cause: libvips ships operations that are unsafe (unfuzzed) for untrusted content; Active Storage failed to block these operations when processing variants of user-supplied files (CWE-1188, Insecure Default Initialization of Resource).
  • Trigger conditions: A Rails application that uses Active Storage with the libvips variant processor and accepts untrusted file uploads, then generates variants of those uploads.
  • Attack vector: Network. An unauthenticated attacker uploads a specially crafted file that causes an unsafe libvips operation to run during variant processing.
  • Impact: Arbitrary reading of files accessible to the Rails process, including the process environment, which commonly exposes credentials such as secret_key_base. Exposure of such secrets can enable remote code execution or lateral movement.

Affected software

  • Active Storage / Rails < 7.2.3.2
  • Active Storage / Rails >= 8.0.0, < 8.0.5.1
  • Active Storage / Rails >= 8.1.0, < 8.1.3.1

Severity

  • CVSS 4.0 base score: 9.5 (Critical)
  • Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H

Mitigation and recommended actions

  • Immediate: Upgrade to a fixed release — Rails 7.2.3.2, 8.0.5.1, or 8.1.3.1.
  • If no patch can be applied, workarounds:
    • With libvips >= 8.13, set the VIPS_BLOCK_UNTRUSTED environment variable.
    • With ruby-vips >= 2.2.1, call Vips.block_untrusted(true).
    • Remove the ruby-vips dependency if it is not required for other purposes.

References

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