From: Ihor Radchenko <yantar92@posteo.net>
Date: Fri, 21 Jun 2024 15:45:25 +0200
Subject: org-link-expand-abbrev: Do not evaluate arbitrary unsafe Elisp code
Origin: https://git.savannah.gnu.org/cgit/emacs.git/commit/?h=emacs-29.4&id=c645e1d8205f0f0663ec4a2d27575b238c646c7c
Bug-Debian: https://bugs.debian.org/1074137
Bug-Debian-Security: https://security-tracker.debian.org/tracker/CVE-2024-39331

* lisp/org/ol.el (org-link-expand-abbrev): Refuse expanding %(...)
link abbrevs that specify unsafe function.  Instead, display a
warning, and do not expand the abbrev.  Clear all the text properties
from the returned link, to avoid any potential vulnerabilities caused
by properties that may contain arbitrary Elisp.
---
 lisp/org/ol.el | 40 +++++++++++++++++++++++++++++-----------
 1 file changed, 29 insertions(+), 11 deletions(-)

--- a/lisp/org/ol.el
+++ b/lisp/org/ol.el
@@ -925,16 +925,35 @@ Abbreviations are defined in `org-link-a
       (if (not as)
 	  link
 	(setq rpl (cdr as))
-	(cond
-	 ((symbolp rpl) (funcall rpl tag))
-	 ((string-match "%(\\([^)]+\\))" rpl)
-	  (replace-match
-	   (save-match-data
-	     (funcall (intern-soft (match-string 1 rpl)) tag)) t t rpl))
-	 ((string-match "%s" rpl) (replace-match (or tag "") t t rpl))
-	 ((string-match "%h" rpl)
-	  (replace-match (url-hexify-string (or tag "")) t t rpl))
-	 (t (concat rpl tag)))))))
+        ;; Drop any potentially dangerous text properties like
+        ;; `modification-hooks' that may be used as an attack vector.
+        (substring-no-properties
+	 (cond
+	  ((symbolp rpl) (funcall rpl tag))
+	  ((string-match "%(\\([^)]+\\))" rpl)
+           (let ((rpl-fun-symbol (intern-soft (match-string 1 rpl))))
+             ;; Using `unsafep-function' is not quite enough because
+             ;; Emacs considers functions like `genenv' safe, while
+             ;; they can potentially be used to expose private system
+             ;; data to attacker if abbreviated link is clicked.
+             (if (or (eq t (get rpl-fun-symbol 'org-link-abbrev-safe))
+                     (eq t (get rpl-fun-symbol 'pure)))
+                 (replace-match
+	          (save-match-data
+	            (funcall (intern-soft (match-string 1 rpl)) tag))
+	          t t rpl)
+               (org-display-warning
+                (format "Disabling unsafe link abbrev: %s
+You may mark function safe via (put '%s 'org-link-abbrev-safe t)"
+                        rpl (match-string 1 rpl)))
+               (setq org-link-abbrev-alist-local (delete as org-link-abbrev-alist-local)
+                     org-link-abbrev-alist (delete as org-link-abbrev-alist))
+               link
+	       )))
+	  ((string-match "%s" rpl) (replace-match (or tag "") t t rpl))
+	  ((string-match "%h" rpl)
+	   (replace-match (url-hexify-string (or tag "")) t t rpl))
+	  (t (concat rpl tag))))))))
 
 (defun org-link-open (link &optional arg)
   "Open a link object LINK.