Markdown Preview Enhanced CVE-2026-54566

Exploitation requires the victim to open a Markdown file inside an attacker-controlled repository that contains malicious .crossnote/config.js and/or .crossnote/parser.js files, with the Markdown Preview Enhanced extension (prior to 0.8.29 / crossnote prior to 0.9.30) installed in VS Code. The config and parser-hook files are loaded automatically on file open - no further user action, and no module-scoped require, is needed. Limiting factors: the attacker must get their repository onto the victim's machine and the victim must open a Markdown file from it (UI:P / local vector), so this is not remotely triggerable against a default install without that interaction. Default extension behavior auto-evaluates these workspace files, so no special non-default configuration is required on the victim side.

This is a genuine high-priority issue for developer environments, though it is not a mass-exploitable network flaw. The vendor-assigned CVSS 4.0 vector (AV:L/AC:L/AT:N/PR:N/UI:P, VC:H/VI:H/VA:H, score 8.5) is internally consistent with the description: exploitation is local in the CVSS sense (it requires a malicious repository to be present and a Markdown file within it to be opened), requires no privileges or prior authentication, but does require passive user interaction (UI:P). The technical impact is maximal - full arbitrary OS command execution as the victim account via prototype-chain traversal from the vm.runInNewContext sandbox to the host Function constructor and process object - which the C/I/A:H ratings correctly reflect. No EPSS score was provided and the vulnerability is NOT on CISA KEV, and no public exploit was supplied in the intelligence, so there is no evidence of active or widespread exploitation; the root-cause and fix are well documented in the GitHub Security Advisory (GHSA-427h-jhpr-8jch) and the patch commits. The realistic risk is targeted social-engineering (luring a developer into cloning/opening a malicious repo), not opportunistic internet-wide attack. SC/SI/SA:N indicate no modeled impact beyond the victim's own account, but in practice code execution as a developer is a serious foothold. Net: treat as a priority patch for anyone using this extension, while recognizing it depends on attacker-supplied repository content plus the victim opening a file.

An attacker publishes a malicious repository (e.g., a tempting sample project, fork, or template) containing crafted .crossnote/config.js or .crossnote/parser.js files. When a developer clones it and opens any Markdown file with the Markdown Preview Enhanced extension, those files are automatically evaluated via vm.runInNewContext, and prototype-chain traversal reaches the host Function constructor and process object to run OS commands as the victim. No public exploit was provided in the intelligence, but the GitHub advisory and commit history describe the mechanism precisely, making weaponization straightforward.