From 4f7b6083e5fe13ae0474520c769e1d3aeb25d393 Mon Sep 17 00:00:00 2001
From: zeertzjq <zeertzjq@outlook.com>
Date: Sat, 11 Apr 2026 07:39:54 +0800
Subject: [PATCH] vim-patch:9.2.0331: spellfile: stack buffer overflows in
 spell file generation (#38948)

Problem:  spell_read_aff() uses sprintf() into a fixed-size stack buffer
          without bounds checking. store_aff_word() uses STRCAT() to
          append attacker-controlled strings into newword[MAXWLEN] without
          checking remaining space. Both are reachable via :mkspell with
          crafted .aff/.dic files (xinyi234)
Solution: Replace sprintf() with vim_snprintf() in spell_read_aff().
          Replace STRCAT() with STRNCAT() with explicit remaining-space
          calculation in store_aff_word().

closes: vim/vim#19944

https://github.com/vim/vim/commit/07faa961a05bc5ea007ab70ff483ea1b32c3371d

Co-authored-by: Christian Brabandt <cb@256bit.org>
---
 src/nvim/spellfile.c                | 10 ++++++++--
 test/old/testdir/test_spellfile.vim | 27 +++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 2 deletions(-)

diff --git a/src/nvim/spellfile.c b/src/nvim/spellfile.c
index 20f4140a65..ab4f16eb3e 100644
--- a/src/nvim/spellfile.c
+++ b/src/nvim/spellfile.c
@@ -2437,6 +2437,8 @@ static afffile_T *spell_read_aff(spellinfo_T *spin, char *fname)
             char buf[MAXLINELEN];
 
             aff_entry->ae_cond = getroom_save(spin, items[4]);
+            // Note: this silently truncates the buffer, but this should
+            // not happen in practice
             snprintf(buf, sizeof(buf), *items[0] == 'P' ? "^%s" : "%s$", items[4]);
             aff_entry->ae_prog = vim_regcomp(buf, RE_MAGIC + RE_STRING + RE_STRICT);
             if (aff_entry->ae_prog == NULL) {
@@ -3391,7 +3393,9 @@ static int store_aff_word(spellinfo_T *spin, char *word, char *afflist, afffile_
                   MB_PTR_ADV(p);
                 }
               }
-              strcat(newword, p);
+              // Note: this silently truncates the buffer, but this should
+              // not happen in practice
+              xstrlcat(newword, p, MAXWLEN);
             } else {
               // suffix: chop/add at the end of the word
               xstrlcpy(newword, word, MAXWLEN);
@@ -3405,7 +3409,9 @@ static int store_aff_word(spellinfo_T *spin, char *word, char *afflist, afffile_
                 *p = NUL;
               }
               if (ae->ae_add != NULL) {
-                strcat(newword, ae->ae_add);
+                // Note: this silently truncates the buffer, but this should
+                // not happen in practice
+                xstrlcat(newword, ae->ae_add, MAXWLEN);
               }
             }
 
diff --git a/test/old/testdir/test_spellfile.vim b/test/old/testdir/test_spellfile.vim
index 351ffc9612..02878877b1 100644
--- a/test/old/testdir/test_spellfile.vim
+++ b/test/old/testdir/test_spellfile.vim
@@ -1170,5 +1170,32 @@ func Test_mkspell_empty_dic()
   call delete('XtestEmpty.spl')
 endfunc
 
+" This used to cause a buffer overflow
+func Test_mkspell_no_buffer_overflow()
+  CheckNotMSWindows
+
+  let aff_lines = ['SET ISO8859-1', 'SFX A Y 1',
+        \ 'SFX A 0 s ' .. repeat(nr2char(0xff), 491)]
+  call writefile(aff_lines, 'Xbof.aff', 'D')
+  call writefile(['1', 'word/A'], 'Xbof.dic', 'D')
+  " Must not crash; ignore any conversion/regex errors.
+  try
+    mkspell! Xbof.spl Xbof
+  catch
+  endtry
+  defer delete('Xbof.spl')
+
+  let long = repeat(nr2char(0xff), 200)
+  let aff2_lines = ['SET ISO8859-1', 'SFX A Y 1',
+        \ 'SFX A 0 ' .. long .. ' .']
+  call writefile(aff2_lines, 'Xbof2.aff', 'D')
+  call writefile(['1', long .. '/A'], 'Xbof2.dic', 'D')
+  try
+    mkspell! Xbof2.spl Xbof2
+  catch
+  endtry
+  defer delete('Xbof2.spl')
+endfunc
+
 
 " vim: shiftwidth=2 sts=2 expandtab