CVE-2020-8162

A client side enforcement of server side security vulnerability exists in rails < 5.2.4.2 and rails < 6.0.3.1 ActiveStorages S3 adapter that allows the Content-Length of a direct file upload to be modified by an end user bypassing upload limits.

Weakness

The product allows the attacker to upload or transfer files of dangerous types that can be automatically processed within the product’s environment.

Affected Software

Potential Mitigations

• Assume all input is malicious. Use an “accept known good” input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
• When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, “boat” may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as “red” or “blue.”
• Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code’s environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
• For example, limiting filenames to alphanumeric characters can help to restrict the introduction of unintended file extensions.
• Run the code in a “jail” or similar sandbox environment that enforces strict boundaries between the process and the operating system. This may effectively restrict which files can be accessed in a particular directory or which commands can be executed by the software.
• OS-level examples include the Unix chroot jail, AppArmor, and SELinux. In general, managed code may provide some protection. For example, java.io.FilePermission in the Java SecurityManager allows the software to specify restrictions on file operations.
• This may not be a feasible solution, and it only limits the impact to the operating system; the rest of the application may still be subject to compromise.
• Be careful to avoid CWE-243 and other weaknesses related to jails.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/1.html

References

• https://groups.google.com/g/rubyonrails-security/c/PjU3946mreQ
• https://hackerone.com/reports/789579
• https://www.debian.org/security/2020/dsa-4766