[Security] MacVim affected by GHSA-q4jv-r9gj-6cwv — heap buffer overflow in spellfile.c read_compound() (vim < 9.2.0450) #1660

Summary

MacVim's src/spellfile.c contains a heap buffer overflow in read_compound() when loading specially crafted spell files. An integer overflow in the allocation size computation allows a malicious .spl file to cause out-of-bounds writes. The fix from vim 9.2.0450 (92993329) has not been applied to macvim r183.

Vulnerability Details

• GHSA: GHSA-q4jv-r9gj-6cwv
• CVE: CVE-2026-45130
• Upstream fix (vim): 9.2.0450 (commit 929933294a5c56ef8e9dab03e0b8c61bbb1dc3cd, 2026-05-07)
• Affected code: src/spellfile.c — read_compound() function
• Vulnerability type: CWE-122 — Heap-based Buffer Overflow

Root Cause

In read_compound(), the todo variable (derived from the SN_COMPOUND section length in the spell file) is used directly in buffer size calculations without an upper bound check:

/* src/spellfile.c line 1278 (macvim r183) */
c = todo * 2 + 7;

A malicious spell file can set todo to a large value (e.g., 0x40000000), causing todo * 2 to overflow a 32-bit integer to 7, resulting in an undersized allocation. Subsequent writes to the buffer cause a heap overflow.

Attack Scenario

1. Attacker provides a malicious .spl spell file (e.g., in a project's spell directory)
2. Victim loads the spell file in MacVim (:setlocal spelllang=... or via modeline)
3. read_compound() allocates an undersized buffer and writes beyond it, potentially enabling arbitrary code execution

Verification

$ grep -n 'todo.*2.*7\|read_compound\|COMPOUND_MAX_LEN' src/spellfile.c
1278:    c = todo * 2 + 7;

Missing the COMPOUND_MAX_LEN guard and safe size computation. Patch 9.2.0450 not present:

$ git log --all --oneline | grep -i '9.2.0450\|spellfile\|q4jv'
(no output)

Suggested Fix

Merge vim patches up to at least 9.2.0450. The fix adds an upper bound check and uses size_t arithmetic:

/* Fixed (vim 9.2.0450): */
#define COMPOUND_MAX_LEN 100000

if ((size_t)todo > COMPOUND_MAX_LEN)
    return SP_FORMERROR;
size_t patsize = (size_t)todo * 2 + 7;
size_t flagsize = (size_t)todo + 1;
pat = alloc(patsize);
cp = alloc(flagsize);
ap = alloc(flagsize);
crp = alloc(flagsize);

References

• GHSA: GHSA-q4jv-r9gj-6cwv
• Upstream fix: vim/vim@9299332