CVE-2026-25749: Vim Buffer Overflow Vulnerability

CVE-2026-25749 Overview

CVE-2026-25749 is a heap buffer overflow vulnerability in Vim, the popular open source command line text editor. The vulnerability exists in Vim's tag file resolution logic when processing the helpfile option. Specifically, the issue is located in the get_tagfname() function in src/tag.c, where Vim copies the user-controlled helpfile option value into a fixed-size heap buffer of MAXPATHL + 1 bytes (typically 4097 bytes) using an unsafe STRCPY() operation without any bounds checking. This vulnerability has been patched in version 9.1.2132.

Critical Impact

A malicious actor with local access could craft a specially-formatted helpfile option value exceeding the buffer boundary, potentially leading to arbitrary code execution, data corruption, or denial of service through memory manipulation.

Affected Products

• Vim versions prior to 9.1.2132
• Systems with Vim configured with custom helpfile option values
• Unix/Linux distributions using vulnerable Vim packages

Discovery Timeline

• 2026-02-06 - CVE-2026-25749 published to NVD
• 2026-02-09 - Last updated in NVD database

Technical Details for CVE-2026-25749

Vulnerability Analysis

This heap buffer overflow vulnerability (CWE-122) occurs due to improper bounds checking in the helpfile option handling code. The vulnerable function get_tagfname() in src/tag.c uses the unsafe STRCPY() macro to copy user-supplied data from the p_hf (helpfile option) variable into a fixed-size buffer. Since STRCPY() performs no length validation, an attacker who can control the helpfile option value could supply a string longer than MAXPATHL - 1 bytes, causing heap memory corruption beyond the allocated buffer boundaries.

The attack requires local access and user interaction, as the attacker needs to either modify Vim configuration files or convince a user to set a malicious helpfile option. While exploitation is limited by these prerequisites, successful exploitation could result in high integrity and availability impact, potentially allowing code execution or causing the application to crash.

Root Cause

The root cause is the use of STRCPY() without bounds checking when copying the helpfile option value into a fixed-size heap buffer. The vulnerable code assumes the input will never exceed MAXPATHL bytes, but no validation enforces this assumption before the copy operation occurs.

Attack Vector

The attack vector requires local access with low privileges and user interaction. An attacker must be able to influence the helpfile option value, either through:

1. Direct modification of Vim configuration files (.vimrc, vimrc)
2. Convincing a user to execute a Vim command that sets a malicious helpfile value
3. Crafting a malicious modeline in a file opened by the victim

The following patch from the GitHub commit demonstrates the fix:

 	    if (tnp->tn_hf_idx > tag_fnames.ga_len || *p_hf == NUL)
 		return FAIL;
 	    ++tnp->tn_hf_idx;
-	    STRCPY(buf, p_hf);
+	    vim_strncpy(buf, p_hf, MAXPATHL - 1);
 	    STRCPY(gettail(buf), "tags");
 #ifdef BACKSLASH_IN_FILENAME
 	    slash_adjust(buf);

Source: GitHub Commit

The security patch documented in version9.txt provides additional context:

Patch 9.1.2132
Problem:  [security]: buffer-overflow in 'helpfile' option handling by
          using strcpy without bound checks (Rahul Hoysala)
Solution: Limit strncpy to the length of the buffer (MAXPATHL)

Source: GitHub Commit

Detection Methods for CVE-2026-25749

Indicators of Compromise

• Unexpected Vim crashes or segmentation faults when accessing help functionality
• Abnormally long helpfile option values in Vim configuration files
• Modified .vimrc or system vimrc files containing suspicious helpfile settings
• Core dumps from Vim processes showing heap corruption patterns

Detection Strategies

• Monitor Vim configuration files for unauthorized modifications to the helpfile option
• Implement file integrity monitoring on .vimrc and system Vim configuration files
• Use memory protection tools like AddressSanitizer during development to detect buffer overflows
• Deploy endpoint detection solutions to identify anomalous Vim process behavior

Monitoring Recommendations

• Set up alerts for Vim process crashes on critical systems
• Monitor system logs for segmentation fault signals from Vim processes
• Implement change detection on Vim configuration files across the environment
• Review modeline settings in files from untrusted sources before opening

How to Mitigate CVE-2026-25749

Immediate Actions Required

• Upgrade Vim to version 9.1.2132 or later immediately
• Review and audit all Vim configuration files for suspicious helpfile option values
• Consider disabling modelines temporarily with set nomodeline in vimrc until patched
• Apply vendor-provided security patches from your Linux distribution

Patch Information

The vulnerability has been addressed in Vim version 9.1.2132. The fix replaces the unsafe STRCPY() call with vim_strncpy() that enforces proper bounds checking by limiting the copy operation to MAXPATHL - 1 bytes. The patch is available through:

• GitHub Commit 0714b15940b245108e6e9d7aa2260dd849a26fa9
• Vim Release v9.1.2132
• GitHub Security Advisory GHSA-5w93-4g67-mm43

Workarounds

• Avoid setting custom helpfile option values from untrusted sources
• Disable modeline processing by adding set nomodeline to your vimrc configuration
• Use restricted mode (vim -Z) when editing files from untrusted sources
• Review any scripts or automation that programmatically set Vim options

# Configuration example - Disable modelines as a temporary mitigation
echo "set nomodeline" >> ~/.vimrc

# Verify current Vim version
vim --version | head -1

# Check current helpfile option value
vim -c ":echo &helpfile" -c ":q"