Summary
Active Storage has possible arbitrary file read and remote code execution in Active Storage variant processing
Details
libvips reads and writes file formats through "loaders" and "savers" (or more generally
"operations"), many of which are backed by third-party libraries. It marks some of these operations
as "unfuzzed", meaning they are unsafe for untrusted content, and several handle formats unrelated
to web images. Active Storage did not disable the unfuzzed operations, so an attacker who can upload
a crafted file and cause a variant to be generated from it may be able to invoke one.
We are aware of a mechanism by which an attacker, by uploading a crafted file, is able to cause
disclosure of the contents of arbitrary files accessible on the filesystem of the targeted
application. One specific attack chain has been reported to us (see "Disclosure" below), but we do
not assume it is the only one that exists.
Affected applications
An application is affected if it meets all of these requirements:
- Uses libvips for Active Storage image processing. This is
config.active_storage.variant_processor = :vips,
whichload_defaults 7.0set and no later default has changed. - Allows image uploads from untrusted users.
Generating variants is not a separate requirement.
Mitigation
- Upgrade to a fixed version of
activestorage. - The minimum version of libvips must be upgraded to
>= 8.13. - Change
secret_key_baseand change any secrets accessible in the application environment (see "Expire and change secrets" below)
Earlier versions of libvips (< 8.13) cannot disable unfuzzed operations at all, and Active Storage
will raise an exception during boot in such an unsecurable environment.
Expire and change secrets
Upgrading closes the vulnerability but does not undo an exfiltrated secret if that already
occurred. An affected application should treat every secret readable by the application process as
potentially exposed and change it, including:
secret_key_base- The master key, whether stored in
config/master.keyor supplied asRAILS_MASTER_KEY, along
with everything inconfig/credentials.yml.encthat it decrypts - Credentials for the Active Storage service, such as S3, GCS, or Azure keys
- Database credentials
- Tokens and keys for any third-party service the application calls
Changing secret_key_base expires active sessions and requires users to log in again. Encrypted
cookies, signed cookies, signed global IDs, and Active Storage URLs are also affected.
Rotation should only be used as an intermediate step if necessary. Do not retain an exposed secret
as a fallback.
Workarounds
If libvips < 8.13 is being used, there are no workarounds available other than removing the
dependency on libvips from the application. Some applications may have ruby-vips declared as a
dependency only for image analysis, and those applications may be able to simply remove ruby-vips
from the Gemfile to remove libvips from the application. Applications that do not use Active Storage
can remove ruby-vips from the Gemfile to avoid the boot-time checks.
If libvips >= 8.13 is present on the system, applications can disable the unfuzzed operations
without upgrading Rails by setting the VIPS_BLOCK_UNTRUSTED environment variable, which libvips
reads while initializing.
Applications also running ruby-vips >= 2.2.1 or later can instead callVips.block_untrusted(true) from an initializer.
Releases
The fixed releases are available at the normal locations.
Versions affected
- activestorage < 7.2.3.2
- activestorage >= 8.0, < 8.0.5.1
- activestorage >= 8.1, < 8.1.3.1
Disclosure
Technical details of the attack chain are intentionally omitted from this advisory. They would add
nothing to an administrator's decision to upgrade, while making it substantially easier to attack
applications that have not yet done so.
Details will be disclosed no later than 2026-08-28, via the Rails Security
Announcements forum.
Credit
This issue was responsibly reported by 0xacb, s3np41k1r1t0 and castilho from Ethiack, and RyotaK from GMO Flatt Security Inc..
References
- libvips 8.13 release notes, blocking of unfuzzed loaders
- W.A. Arbaugh, W.L. Fithen, and J. McHugh, "Windows of Vulnerability: A Case Study
Analysis", IEEE Computer 33(12), December 2000 - https://ethiack.com/info-hub/research/kindarails2shell-rails-rce-cve
- https://blog.flatt.tech/entry/kindarails2shell_rails
Impact
In its default configuration, a Rails application that displays image variants may allow an
unauthenticated attacker to read arbitrary files from the server, including the process environment.
That environment typically holds secret_key_base and often credentials for external systems, which
may in turn allow escalation to remote code execution or lateral movement to those systems.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
activestorage to 7.2.3.2 or later; activestorage to 8.0.5.1 or later; activestorage to 8.1.3.1 or later
Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2026-66066? CVE-2026-66066 is a critical-severity security vulnerability in activestorage (rubygems), affecting versions < 7.2.3.2. It is fixed in 7.2.3.2, 8.0.5.1, 8.1.3.1.
- Which versions of activestorage are affected by CVE-2026-66066? activestorage (rubygems) versions < 7.2.3.2 is affected.
- Is there a fix for CVE-2026-66066? Yes. CVE-2026-66066 is fixed in 7.2.3.2, 8.0.5.1, 8.1.3.1. Upgrade to this version or later.
- Is CVE-2026-66066 exploitable, and should I be worried? Whether CVE-2026-66066 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-66066 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-66066?
- Upgrade
activestorageto 7.2.3.2 or later - Upgrade
activestorageto 8.0.5.1 or later - Upgrade
activestorageto 8.1.3.1 or later
- Upgrade