CVE-2025-55157: Vim Use-After-Free Vulnerability

CVE-2025-55157 Overview

CVE-2025-55157 is a use-after-free vulnerability in Vim, the popular open source command line text editor. When processing nested tuples in Vim script, an error during evaluation can trigger a use-after-free condition in Vim's internal tuple reference management. Specifically, the tuple_unref() function may access already freed memory due to improper lifetime handling, leading to memory corruption. The vulnerability affects Vim versions from 9.1.1231 to before 9.1.1400.

Critical Impact

A use-after-free vulnerability in Vim's tuple evaluation can cause memory corruption when processing malicious scripts, potentially leading to denial of service through application crash.

Affected Products

• Vim versions 9.1.1231 to 9.1.1399
• vim:vim (all platforms)

Discovery Timeline

• 2025-08-11 - CVE CVE-2025-55157 published to NVD
• 2025-08-12 - Last updated in NVD database

Technical Details for CVE-2025-55157

Vulnerability Analysis

This vulnerability is classified as CWE-416 (Use After Free), a memory corruption class vulnerability that occurs when a program continues to use a pointer after the memory it references has been freed. In this case, the issue resides in Vim's script evaluation engine, specifically in how nested tuple expressions are handled during evaluation failures.

When Vim evaluates a tuple expression that encounters an error during recursive evaluation via eval1(), the error path does not properly check the return status before continuing execution. This allows subsequent code to operate on tuple references that may have already been deallocated, causing the tuple_unref() function to access freed memory.

The vulnerability requires user interaction to exploit, as a victim must explicitly execute a maliciously crafted Vim script. This limits the attack surface but still poses a risk to users who run untrusted scripts or open files with embedded Vim modelines from untrusted sources.

Root Cause

The root cause is a missing return value check in the eval1() function call path within src/eval.c. When recursive tuple evaluation fails, the function did not properly return early, allowing execution to continue with potentially invalid memory references. The fix adds an explicit check for the return value of eval1() to ensure immediate return on failure.

Attack Vector

The attack requires a user to execute a specially crafted Vim script containing nested tuple constructs that trigger an evaluation error. An attacker could distribute malicious Vim scripts through various channels such as code repositories, configuration files, or documents with embedded Vim modelines. When a user opens or sources the malicious script in Vim, the use-after-free condition is triggered.

The following patch shows the security fix applied in version 9.1.1400:

 	else
 	{
 	    ret = eval1(arg, rettv, evalarg);	// recursive!
+	    if (ret != OK)
+		return ret;

 	    *arg = skipwhite_and_linebreak(*arg, evalarg);

Source: GitHub Commit Details

The patch adds a critical check after the recursive eval1() call. If the evaluation fails (returns non-OK), the function now immediately returns instead of continuing execution with potentially corrupted state.

Detection Methods for CVE-2025-55157

Indicators of Compromise

• Unexpected Vim crashes when opening files or executing scripts
• Core dumps or crash reports from Vim showing memory corruption in tuple-related functions
• Presence of suspicious Vim scripts with complex nested tuple structures designed to trigger evaluation errors

Detection Strategies

• Monitor for Vim process crashes and analyze crash dumps for signatures related to tuple_unref() or eval1() functions
• Implement file integrity monitoring for Vim configuration files and scripts in user home directories
• Deploy endpoint detection rules to flag unusual Vim script execution patterns

Monitoring Recommendations

• Configure system logging to capture Vim process termination signals (SIGSEGV, SIGABRT)
• Review audit logs for execution of Vim with suspicious script arguments
• Monitor for bulk distribution of Vim script files across network shares or repositories

How to Mitigate CVE-2025-55157

Immediate Actions Required

• Upgrade Vim to version 9.1.1400 or later immediately
• Audit any Vim scripts from untrusted sources before execution
• Disable automatic execution of modelines by adding set nomodeline to your .vimrc configuration
• Review recently downloaded or received Vim scripts for suspicious nested tuple constructs

Patch Information

The vulnerability has been patched in Vim version 9.1.1400. The fix is available through the official Vim release. The security patch commit 1307743697bbc46e1518abfea7f89caa95bcaf97 addresses the issue by adding proper error checking after recursive tuple evaluation. For complete details, refer to the GitHub Security Advisory GHSA-3r4f-mm4w-wgg6.

Workarounds

• Add set nomodeline to your Vim configuration to prevent automatic script execution from file headers
• Avoid executing Vim scripts from untrusted sources until patched
• Use Vim in restricted mode (vim -Z) when working with untrusted files to disable shell commands and script evaluation

# Configuration example - Add to ~/.vimrc to disable modelines
set nomodeline
set modelines=0

# Run Vim in restricted mode for untrusted files
vim -Z untrusted_file.txt