CVE-2020-15111: 
vulnerability analysis and mitigation

In Fiber before version 1.12.6, a CRLF injection vulnerability was discovered in the c.Attachment() function (https://docs.gofiber.io/ctx#attachment). The vulnerability was disclosed on July 20, 2020 and assigned CVE-2020-15111. The issue affects the filename handling in the Fiber web framework, where filenames passed to the attachment function were not properly escaped (GitHub Advisory).

The vulnerability stems from improper filename escaping in the c.Attachment() function. When a filename is provided to this function, it is not escaped before being used in the Content-Disposition header. This allows for CRLF (Carriage Return Line Feed) injection, where an attacker can inject special characters to manipulate HTTP headers. The vulnerability was fixed in version 1.12.6 by implementing proper URL escaping of filenames (GitHub Commit).

The vulnerability allows attackers to perform various malicious actions through CRLF injection. An attacker could modify the downloaded file name, redirect users to different sites, or manipulate authorization headers. This could lead to more serious attacks such as cross-site scripting (XSS), page injection, web cache poisoning, and cache-based defacement (GitHub Advisory).

The vulnerability can be exploited by uploading a file with a specially crafted filename containing CRLF characters. When this filename is processed by the c.Attachment() function, it leads to HTTP header manipulation. A proof of concept demonstrates that an attacker can inject additional HTTP headers like Location and Authorization through the filename parameter (GitHub Advisory).

The vulnerability was patched in Fiber version 1.12.6 by implementing proper URL escaping of filenames. As a workaround for users unable to upgrade, it is recommended to serialize the input before passing it to ctx.Attachment(). This is considered a good security practice regardless of the version being used (GitHub Advisory).