CVE-2024-53920: 
Emacs vulnerability analysis and mitigation

CVE-2024-53920 affects GNU Emacs versions before 30.1, where users invoking elisp-completion-at-point (for code completion) on untrusted Emacs Lisp source code can trigger unsafe Lisp macro expansion that allows attackers to execute arbitrary code. The vulnerability was discovered in August 2018 and publicly disclosed in November 2024 (Emacs Blog).

The vulnerability stems from unsafe Lisp macro-expansion in Emacs, which runs unrestricted Emacs Lisp code during macro-expansion time. The issue occurs in elisp-mode.el when code completion or on-the-fly diagnosis features attempt to examine code by expanding macros. Common configurations that enable auto-completion features (like Corfu and Company) or code diagnosis tools (Flymake or Flycheck) can trigger this vulnerability automatically when opening affected files (Emacs Blog).

An attacker can achieve arbitrary code execution by crafting a malicious Emacs Lisp file that includes harmful macro invocations. When a user opens such a file in Emacs with certain common features enabled, the code can execute automatically without user intervention (Emacs Blog).

Users are advised to: 1) Avoid visiting untrusted .el files in Emacs, 2) Disable automatic error checking with Flymake or Flycheck in untrusted .el files, 3) Disable auto-completion features in untrusted .el files, and 4) Set enable-local-eval to nil. The vulnerability has been addressed in Emacs 30, which includes new safety mechanisms to disable Flymake and code completion induced macro-expansion in untrusted files (Emacs Blog, Debian Security).

The vulnerability has generated significant discussion in the Emacs community, with many users expressing concern about the long-standing nature of the issue. The discussion on Hacker News highlighted the challenges of securing Emacs given its architecture and age, with some users suggesting running Emacs in restricted environments like Firejail as a mitigation strategy (HN Discussion).