There is a vulnerability in ActiveStorage's S3 adapter that allows the Content-Length of a direct file upload to be modified by an end user.

Versions Affected: rails < 5.2.4.2, rails < 6.0.3.1
Not affected: Applications that do not use the direct upload functionality of the ActiveStorage S3 adapter.
Fixed Versions: rails >= 5.2.4.3, rails >= 6.0.3.1

Impact

Utilizing this vulnerability, an attacker can control the Content-Length of an S3 direct upload URL without receiving a new signature from the server. This could be used to bypass controls in place on the server to limit upload size.

Workarounds

This is a low-severity security issue. As such, no workaround is necessarily until such time as the application can be upgraded.

References

• https://nvd.nist.gov/vuln/detail/CVE-2020-8162
• https://github.com/rubysec/ruby-advisory-db/blob/master/gems/activestorage/CVE-2020-8162.yml
• https://groups.google.com/forum/#!topic/rubyonrails-security/PjU3946mreQ
• https://hackerone.com/reports/789579
• https://groups.google.com/g/rubyonrails-security/c/PjU3946mreQ
• https://www.debian.org/security/2020/dsa-4766
• aws/aws-sdk-ruby#2098