CVE-2025-55158: Vim Use-After-Free Vulnerability

CVE-2025-55158 Overview

CVE-2025-55158 is a double-free vulnerability affecting Vim, the popular open-source command line text editor. The flaw exists in Vim versions 9.1.1231 through 9.1.1405, where processing nested tuples during Vim9 script import operations can trigger memory corruption. Specifically, when an error occurs during evaluation, the clear_tv() function may attempt to free memory that has already been deallocated due to improper lifetime handling in the handle_import and ex_import code paths.

Critical Impact

Successful exploitation of this double-free vulnerability can lead to denial of service through application crashes when users open and execute specially crafted Vim scripts.

Affected Products

• Vim versions 9.1.1231 to 9.1.1405
• Systems running vulnerable Vim installations with Vim9 script support enabled

Discovery Timeline

• 2025-08-11 - CVE-2025-55158 published to NVD
• 2025-08-12 - Last updated in NVD database

Technical Details for CVE-2025-55158

Vulnerability Analysis

This vulnerability is classified as CWE-415 (Double Free), a memory corruption issue that occurs when the same memory location is freed more than once. In the context of CVE-2025-55158, the flaw manifests within Vim's typed value (typval_T) management system during Vim9 script import operations.

When processing nested tuples, if an error occurs during evaluation, the clear_tv() function is invoked to clean up memory. However, due to improper lifetime handling in the import code paths, the function may attempt to deallocate memory that was already freed earlier in the execution flow. This can corrupt the heap's internal data structures and lead to unpredictable behavior, including application crashes.

The attack requires user interaction—specifically, the user must explicitly open and execute a maliciously crafted Vim script. This limits the attack surface but still poses a significant risk to developers and system administrators who routinely work with Vim scripts from untrusted sources.

Root Cause

The root cause lies in the improper management of typed value lifetimes within the handle_import and ex_import code paths. When a tuple's first item is added from rettv, the original rettv structure retains its type information. If an error subsequently triggers cleanup via clear_tv(), both the tuple and the original rettv may attempt to free the same memory, resulting in a double-free condition.

Attack Vector

The vulnerability can be exploited through a network-based attack vector where an attacker distributes a specially crafted Vim script. When a victim downloads and executes this malicious script (for example, as part of a plugin installation or configuration file), the double-free condition is triggered. The attack requires user interaction to open and execute the script, which limits automated exploitation but enables targeted attacks against developers and system administrators.

The patch addresses this issue by setting the rettv type to VAR_UNKNOWN after its first item is added to the tuple, preventing the caller from attempting to free the already-transferred memory:

	    // Add the first item to the tuple from "rettv"
	    if (tuple_append_tv(tuple, rettv) == FAIL)
		return FAIL;
+	    // The first item in "rettv" is added to the tuple.  Set the rettv
+	    // type to unknown, so that the caller doesn't free it.
+	    rettv->v_type = VAR_UNKNOWN;
	}
    }

Source: GitHub Commit Details

Detection Methods for CVE-2025-55158

Indicators of Compromise

• Unexpected Vim crashes when processing Vim9 scripts containing nested tuple structures
• Core dumps or crash reports originating from the clear_tv() function
• Suspicious Vim script files containing complex nested tuple import statements
• Memory corruption errors in system logs associated with Vim processes

Detection Strategies

• Monitor for Vim process crashes and analyze crash dumps for double-free signatures
• Implement file integrity monitoring for Vim configuration directories to detect malicious script injection
• Deploy endpoint detection rules to identify execution of Vim scripts from untrusted sources
• Use memory sanitizers (ASan/MSan) in development environments to catch double-free issues

Monitoring Recommendations

• Enable system-level monitoring for abnormal Vim process terminations
• Audit Vim script execution in privileged environments where Vim is used for system administration
• Track downloads and installations of Vim plugins from unofficial sources
• Review Vim version information across your infrastructure to identify vulnerable installations

How to Mitigate CVE-2025-55158

Immediate Actions Required

• Upgrade Vim to version 9.1.1406 or later immediately
• Audit all Vim scripts from untrusted sources before execution
• Temporarily restrict Vim9 script execution in security-sensitive environments
• Review recently installed Vim plugins for potentially malicious content

Patch Information

This vulnerability has been addressed in Vim version 9.1.1406. The fix modifies the tuple handling code in src/tuple.c to properly transfer ownership of typed values during import operations. After adding the first item from rettv to the tuple, the patch sets rettv->v_type = VAR_UNKNOWN to prevent the caller from attempting to free the already-transferred memory.

For detailed patch information, refer to the GitHub Security Advisory or the official release notes.

Workarounds

• Avoid executing Vim9 scripts from untrusted sources until the patch is applied
• Use older Vim script syntax (legacy scripts) instead of Vim9 scripts when possible
• Run Vim in restricted mode (vim -Z) when working with untrusted files
• Implement sandboxing for Vim processes in multi-user environments

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

# Update Vim on Debian/Ubuntu
sudo apt update && sudo apt install vim

# Update Vim on RHEL/CentOS/Fedora
sudo dnf update vim

# Verify updated version is 9.1.1406 or higher
vim --version | grep -E "patch 1406|IMproved 9\.1\.140[6-9]|IMproved 9\.1\.14[1-9]|IMproved 9\.1\.[2-9]|IMproved 9\.[2-9]|IMproved [1-9][0-9]"