CVE-2021-26835: 
NixOS vulnerability analysis and mitigation

CVE-2021-26835 is a cross-site scripting (XSS) vulnerability discovered in the markdown-editor component of Zettlr version 1.8.7. The vulnerability, which was disclosed on June 18, 2021, allows attackers to perform remote code execution via a crafted file due to insufficient filtering of XSS payloads (NVD).

The vulnerability stems from improper filtering of cross-site scripting payloads in the markdown-editor component. When a specially crafted markdown file is opened in Zettlr 1.8.7, the XSS payload can execute arbitrary code in the context of the application. The vulnerability has been assigned a CVSS v3.1 base score of 6.1 (MEDIUM) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N (NVD).

The vulnerability allows attackers to execute arbitrary code on the target system through specially crafted markdown files. When exploited, the attacker can gain unauthorized access to the system with the privileges of the application user (NVD).

The vulnerability can be exploited by creating a malicious markdown file containing XSS payloads. When this file is opened in the affected version of Zettlr, the payload executes automatically upon page refresh or user interaction with the document (GitHub Issue).

The vulnerability was patched in Zettlr version 1.8.9. The fix includes improved security measures such as attribute filtering for iFrames, implementation of a hostname whitelist for embedded content, and the option to disable iFrame pre-rendering in display preferences. Users are strongly advised to update to version 1.8.9 or later (Release Notes).