[Security] MacVim affected by GHSA-cwgx-gcj7-6qh8 — command injection via backtick expansion in tag filenames (vim < 9.2.0357)

Summary

MacVim's src/tag.c expands backtick expressions in tag file tagname fields when processing wildcard patterns. A malicious tags file containing a backtick expression like `touch /tmp/pwned` as a filename causes arbitrary shell command execution when a user issues a :tag command. The fix from vim 9.2.0357 (c78194e4) has not been applied to macvim r183.

Vulnerability Details

• GHSA: GHSA-cwgx-gcj7-6qh8
• CVE: CVE-2026-41411
• Upstream fix (vim): 9.2.0357 (commit c78194e4ee65bab5fef3b4f8de8f4e6ee47fbaa6, 2026-04-15)
• Affected code: src/tag.c line 4141
• Vulnerability type: CWE-78 — OS Command Injection

Root Cause

In src/tag.c, when a tag filename matches as a wildcard pattern, the filename is expanded — which includes backtick expansion (shell command substitution):

/* src/tag.c line 4141 (macvim r183) */
if (expand && mch_has_wildcard(fname))

Since backtick expressions (e.g., `cmd`) satisfy mch_has_wildcard(), they are expanded via the shell. A malicious tags file containing:

main	`touch /tmp/pwned`	/^int main/;"	f

causes touch /tmp/pwned to execute when the user runs :tag main.

Attack Scenario

1. Attacker provides a malicious tags file in the project (e.g., via repository or build system)
2. Victim opens a file in MacVim with set tags=Xtags pointing to the malicious file
3. Victim issues :tag main or another tag navigation command
4. MacVim expands the backtick expression and executes arbitrary shell commands

Verification

$ grep -n 'mch_has_wildcard.*fname' src/tag.c
4141:    if (expand && mch_has_wildcard(fname))

Missing the guard && vim_strchr(fname, '\') == NULL`. Patch 9.2.0357 not present:

$ git log --all --oneline | grep -i '9.2.0357\|backtick.*tag\|cwgx'
(no output)

Suggested Fix

Merge vim patches up to at least 9.2.0357. The fix adds a backtick exclusion:

/* Fixed (vim 9.2.0357): disallow backticks, they could execute arbitrary shell commands */
if (expand && mch_has_wildcard(fname) && vim_strchr(fname, '`') == NULL)

References

• GHSA: https://github.com/vim/vim/security/advisories/GHSA-cwgx-gcj7-6qh8
• Upstream fix: https://github.com/vim/vim/commit/c78194e4ee65bab5fef3b4f8de8f4e6ee47fbaa6

Read src/tag.c around line 4141 and compare it with Vim 9.2.0357 commit c78194e4. Verify that tag handling no longer expands backtick-containing filenames while preserving ordinary wildcard tag navigation; the issue provides no named regression test, so inspect existing tag tests for validation.