CVE-2020-8162 in Ruby on Rails
Summary
by MITRE
A client side enforcement of server side security vulnerability exists in rails < 5.2.4.2 and rails < 6.0.3.1 ActiveStorage's S3 adapter that allows the Content-Length of a direct file upload to be modified by an end user bypassing upload limits.
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Analysis
by VulDB Data Team • 06/20/2020
The vulnerability described in CVE-2020-8162 represents a critical client-side security enforcement flaw within the Ruby on Rails framework's ActiveStorage component. This issue specifically affects versions prior to 5.2.4.2 and 6.0.3.1, where the S3 adapter implementation fails to properly validate file upload constraints. The flaw enables malicious actors to manipulate the Content-Length header during direct file uploads, effectively circumventing server-side upload limits that are intended to prevent excessive resource consumption and potential denial-of-service conditions.
The technical nature of this vulnerability stems from the improper handling of user-supplied Content-Length values in the S3 adapter's direct upload mechanism. When Rails processes file uploads through ActiveStorage's S3 integration, it should enforce strict validation of upload parameters to prevent unauthorized modifications. However, the vulnerability allows attackers to modify the Content-Length header to exceed configured limits, potentially enabling them to upload files larger than permitted by the application's security policies. This represents a classic case of insufficient input validation and improper access control enforcement, categorized under CWE-20 as "Improper Input Validation" and CWE-345 as "Insufficient Verification of Data Authenticity."
The operational impact of this vulnerability extends beyond simple size limit bypasses, creating potential pathways for resource exhaustion attacks and unauthorized data consumption. Attackers could leverage this flaw to consume excessive server resources through large file uploads, potentially leading to denial-of-service conditions that affect legitimate users. The vulnerability particularly impacts web applications that rely on ActiveStorage for file handling and implement size-based upload restrictions as part of their security posture. This weakness allows adversaries to circumvent application-level controls that should prevent such resource abuse, undermining the integrity of the application's upload validation mechanisms.
Security professionals should prioritize immediate patching of affected Rails versions to mitigate this vulnerability, as it provides attackers with a straightforward method to bypass critical upload restrictions. The recommended mitigation strategy involves upgrading to Rails versions 5.2.4.2 or 6.0.3.1, which contain the necessary fixes to properly validate Content-Length headers during S3 direct uploads. Organizations should also implement additional monitoring of file upload patterns and Content-Length values to detect potential exploitation attempts. From an ATT&CK framework perspective, this vulnerability aligns with techniques involving privilege escalation and resource exhaustion, potentially enabling adversaries to consume excessive system resources and maintain persistent access through continued exploitation of the vulnerable upload functionality.