From 9b7dfa2948c9e1e5e32a5812812d580c7879f4a0 Mon Sep 17 00:00:00 2001
From: Christian Brabandt <cb@256bit.org>
Date: Mon, 23 Feb 2026 19:35:25 +0000
Subject: [PATCH] patch 9.2.0075: [security]: Buffer underflow with emacs tag
 file

Problem:  When parsing a malformed Emacs-style tags file, a 1-byte
          heap-buffer-underflow read occurs if the 0x7f delimiter
          appears at the very beginning of a line. This happens
          because the code attempts to scan backward for a tag
          name from the delimiter without checking if space exists.
          (ehdgks0627, un3xploitable)
Solution: Add a check to ensure the delimiter (p_7f) is not at the
          start of the buffer (lbuf) before attempting to isolate
          the tag name.

GitHub Advisory:
https://github.com/vim/vim/security/advisories/GHSA-xcc8-r6c5-hvwv

Signed-off-by: Christian Brabandt <cb@256bit.org>


CVE: CVE-2026-28419
Upstream-Status: Backport [https://github.com/vim/vim/commit/9b7dfa2948c9e1e5e32a5812812d580c7879f4a0]
Signed-off-by: Hitendra Prajapati <hprajapati@mvista.com>
---
 src/tag.c                    |  3 +++
 src/testdir/test_taglist.vim | 16 ++++++++++++++++
 src/version.c                |  2 ++
 3 files changed, 21 insertions(+)

diff --git a/src/tag.c b/src/tag.c
index 45af67f20d..d3a73997bb 100644
--- a/src/tag.c
+++ b/src/tag.c
@@ -2023,6 +2023,9 @@ etag_fail:
     }
     else			    // second format: isolate tagname
     {
+	if (p_7f == lbuf)
+	    goto etag_fail;
+
 	// find end of tagname
 	for (p = p_7f - 1; !vim_iswordc(*p); --p)
 	    if (p == lbuf)
diff --git a/src/testdir/test_taglist.vim b/src/testdir/test_taglist.vim
index 506e64f7ae..42ecc4b76e 100644
--- a/src/testdir/test_taglist.vim
+++ b/src/testdir/test_taglist.vim
@@ -316,4 +316,20 @@ func Test_evil_emacs_tagfile()
   set tags&
 endfunc
 
+" This used to crash Vim due to a heap-buffer-underflow
+func Test_emacs_tagfile_underflow()
+  CheckFeature emacs_tags
+  " The sequence from the crash artifact:
+  let lines = [
+    \ "\x0c\xff\xffT\x19\x8a",
+    \ "\x19\x19\x0dtags\x19\x19\x19\x00\xff\xff\xff",
+    \ "\x7f3\x0c"
+    \ ]
+  call writefile(lines, 'Xtags', 'D')
+  set tags=Xtags
+  call assert_fails(':tag a', 'E431:')
+
+  set tags&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab
diff --git a/src/version.c b/src/version.c
index 7d265ab641..4f47ec2688 100644
--- a/src/version.c
+++ b/src/version.c
@@ -724,6 +724,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1686,
 /**/
     1685,
 /**/
-- 
2.50.1

