[Security] MacVim affected by GHSA-hwg5-3cxw-wvvg — OS command injection via backticks in 'path' option completion (vim < 9.2.0435)

Summary

MacVim's src/findfile.c and src/optiondefs.h allow backtick expressions in the 'path' option to be executed when file completion is triggered. Since 'path' can be set via modelines, an attacker can embed a malicious backtick command in a project file that executes when the victim uses file-path completion. The fix from vim 9.2.0435 (190cb3c2) has not been applied to macvim r183.

Vulnerability Details

• GHSA: GHSA-hwg5-3cxw-wvvg
• CVE: CVE-2026-44656
• Upstream fix (vim): 9.2.0435 (commit 190cb3c2b6e53290735f2c5cab1a06f703a90e69, 2026-05-03)
• Affected code: src/findfile.c + src/optiondefs.h ('path' option)
• Vulnerability type: CWE-78 — OS Command Injection

Root Cause

The 'path' option is not marked P_SECURE in src/optiondefs.h, so it can be set via modelines:

/* src/optiondefs.h line 2067 (macvim r183) */
{"path",    "pa",   P_STRING|P_EXPAND|P_VI_DEF|P_COMMA|P_NODUP,

Missing P_SECURE allows a modeline to set path+=\cmd`. In src/findfile.c, when file completion is performed for 'path'entries, backtick expressions are expanded via the shell — executingcmd`.

Attack Scenario

1. Attacker places a project file with a modeline:

   // vim: set path+=`id>/tmp/pwned` :
   

2. Victim opens the file in MacVim with modeline support enabled (default)
3. MacVim sets path to include the backtick expression
4. When the victim presses Tab for :find completion, MacVim expands the backtick and executes id>/tmp/pwned

Verification

$ grep -n '"path".*P_STRING' src/optiondefs.h
2067:    {"path",    "pa",   P_STRING|P_EXPAND|P_VI_DEF|P_COMMA|P_NODUP,

Missing P_SECURE. Also missing the backtick check in findfile.c. Patch 9.2.0435 not present:

$ git log --all --oneline | grep -i '9.2.0435\|path.*backtick\|hwg5'
(no output)

Suggested Fix

Merge vim patches up to at least 9.2.0435. The fix:

1. Adds P_SECURE to 'path' in optiondefs.h:

   {"path",    "pa",   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE|P_COMMA|P_NODUP,
   

2. Adds a backtick guard in findfile.c:

   /* do not expand backticks, could have been set via a modeline */
   if (vim_strchr(buf, '`') != NULL)
       continue;
   

References

• GHSA: https://github.com/vim/vim/security/advisories/GHSA-hwg5-3cxw-wvvg
• Upstream fix: https://github.com/vim/vim/commit/190cb3c2b6e53290735f2c5cab1a06f703a90e69

まず src/optiondefs.h と src/findfile.c から始め、欠落している変更を Vim のコミット 190cb3c2 またはバージョン 9.2.0435 と比較します。'path' オプションとバッククォートの処理が upstream の修正と一致していることを確認し、脆弱性が対処されていることを確認するために、issue の grep チェックと攻撃シナリオを使用します。