CVE-2020-26286: 
NixOS vulnerability analysis and mitigation

HedgeDoc, a collaborative platform for writing and sharing markdown, was found to contain a critical security vulnerability (CVE-2020-26286) in versions before 1.7.1. The vulnerability allowed unauthenticated attackers to upload arbitrary files to the upload storage backend, including HTML, JS, and PHP files (GitHub Advisory, NVD).

The vulnerability existed in the file upload functionality of HedgeDoc where the application failed to properly validate uploaded files. This allowed attackers to bypass file type restrictions and upload potentially malicious files to the server. The vulnerability was particularly severe in environments where uploaded files were directly served by a webserver that interprets code, such as LAMP installations, potentially allowing arbitrary code execution on the backend (GitHub Advisory).

The vulnerability allowed attackers to upload arbitrary files including HTML, JS, and PHP files to the server's storage backend. In configurations where the webserver was set to interpret uploaded files (such as PHP files in LAMP installations), this could lead to remote code execution, making the vulnerability potentially critical. Even in standard configurations, the ability to upload arbitrary files posed significant security risks (GitHub Advisory).

The vulnerability could be exploited by unauthenticated attackers, making it particularly dangerous. The attack vector was through the /uploadimage endpoint, which was accessible without authentication. No special conditions or complex exploitation techniques were required to exploit this vulnerability (GitHub Advisory).

The vulnerability was patched in HedgeDoc version 1.7.1. For users unable to upgrade immediately, several workarounds were available: blocking the /uploadimage endpoint using a reverse proxy, restricting MIME-types and file names served from the upload file storage. After upgrading, administrators should verify that their uploaded file storage only contains allowed files, as previously uploaded files might still be served (GitHub Release, GitHub Advisory).