CVE-2025-55157

Executive Summary

CVE-2025-55157 is a use-after-free vulnerability in Vim, the ubiquitous open-source command-line text editor. The flaw resides in the internal handling of nested tuples during Vim script evaluation. When a script containing nested tuples triggers an error, the tuple_unref() function can access memory that has already been freed, leading to memory corruption. This vulnerability affects Vim versions from 9.1.1231 up to, but not including, 9.1.1400.

The attack vector requires direct user interaction, as a malicious Vim script must be explicitly executed by the user. While the CVSS score is currently unassigned, the nature of the vulnerability (use-after-free) can potentially lead to arbitrary code execution in the context of the Vim process. The EPSS score is 0.0%, indicating no active exploitation in the wild at the time of analysis. The vendor has released a patch in version 9.1.1400.

Supply Chain & Package Security

Vim is a critical component in many software supply chains, often included as a default editor in Linux distributions and macOS. A vulnerability in Vim can be exploited through:

• Developer Workstations: A developer executing a malicious Vim script (e.g., a .vimrc file or a plugin) could have their environment compromised.
• CI/CD Pipelines: If Vim is used in automated build or testing scripts, a crafted input file could trigger the vulnerability.
• Package Managers: Vim is distributed via package managers (apt, yum, brew, etc.). Users must ensure they update to the patched version.

CVSS Breakdown & Threat Level

The CVSS score is currently 0.0 (UNKNOWN). However, based on the vulnerability characteristics, a reasonable assessment would be:

• Attack Vector (AV): Local (L) - Requires the user to execute a script.
• Attack Complexity (AC): High (H) - Requires crafting a specific nested tuple structure that triggers the error condition.
• Privileges Required (PR): None (N) - The attacker does not need prior privileges.
• User Interaction (UI): Required (R) - The victim must execute the malicious script.
• Scope (S): Unchanged (U) - The vulnerability affects the Vim process itself.
• Confidentiality (C): High (H) - Potential for arbitrary code execution could lead to data exfiltration.
• Integrity (I): High (H) - Potential for arbitrary code execution could lead to data modification.
• Availability (A): High (H) - Potential for arbitrary code execution could lead to denial of service.

Estimated CVSS:3.1 Score: 7.3 (High)

Threat Level: Medium. The requirement for user interaction and the complexity of exploitation reduce the immediate threat, but the potential for code execution in a widely used tool warrants prompt patching.

Deep Dive & Mechanics

The vulnerability is a classic Use-After-Free (UAF) in Vim's internal memory management for tuple objects.

1. Tuple Creation: When Vim script evaluates a nested tuple, e.g., [[1, 2], [3, 4]], it creates multiple tuple objects in memory. Each tuple has a reference count to manage its lifetime.
2. Error Trigger: During the evaluation of the nested structure, if an error occurs (e.g., a type mismatch or an invalid operation within the tuple), Vim's error handling routine begins to clean up the partially constructed objects.
3. tuple_unref() Call: The cleanup process calls tuple_unref() to decrement the reference count of a tuple. If the reference count reaches zero, the tuple's memory is freed.
4. Premature Free: Due to improper lifetime handling, the tuple_unref() function may be called on a tuple that is still being referenced by another part of the evaluation logic. This causes the tuple's memory to be freed while a pointer to it still exists.
5. Use of Freed Memory: After the memory is freed, the evaluation logic continues to use the now-dangling pointer to access the tuple's data. This is the use-after-free condition.
6. Consequences: Accessing freed memory can lead to:
   • Memory Corruption: Overwriting freed memory with new data can corrupt the heap.
   • Arbitrary Code Execution: An attacker can potentially control the freed memory to overwrite function pointers or other critical data structures, leading to code execution.
   • Denial of Service: The most common outcome is a crash (segmentation fault).

The specific vulnerable code is in the tuple_unref() function within Vim's source code (likely in eval.c or a similar file). The patch in version 9.1.1400 corrects the reference counting logic to ensure that a tuple is not freed while it is still in use.

Vulnerable vs. Patched Code

The exact code diff is not publicly available, but the conceptual change is:

Vulnerable Code (Conceptual):

// In tuple_unref() or related function
void tuple_unref(tuple_T *tp) {
    if (--tp->refcount == 0) {
        // Free the tuple immediately
        vim_free(tp);
    }
    // tp is now a dangling pointer if freed
    // Subsequent code might still use tp
}

Patched Code (Conceptual):

// In tuple_unref() or related function
void tuple_unref(tuple_T *tp) {
    if (tp != NULL && --tp->refcount == 0) {
        // Ensure no other references exist before freeing
        // This might involve checking a parent reference or using a different cleanup order
        vim_free(tp);
    }
    // tp is no longer used after this point
}

The patch likely involves:

• Reordering operations: Ensuring that all references to a tuple are released before its refcount is decremented.
• Adding checks: Verifying that a tuple is not part of a larger structure before freeing it.
• Using a different cleanup strategy: Perhaps using a mark-and-sweep approach for nested structures instead of immediate reference counting.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• A vulnerable Vim version (9.1.1231 to 9.1.1399).
• A target user who will execute a malicious Vim script.

Exploitation Steps:

1. Craft a Malicious Vim Script: Create a .vim file that contains a nested tuple structure designed to trigger the error and subsequent UAF. The exact structure would need to be reverse-engineered from the patch, but a general approach is:

   " exploit.vim
   " This script attempts to trigger the UAF by creating a deeply nested tuple
   " and then causing an error during its evaluation.
   function! Exploit()
       " Create a nested tuple that will cause an error
       let nested = [[1, 2], [3, 4], [5, 6]]
       " Attempt an invalid operation on the tuple to trigger cleanup
       " For example, trying to use a string operation on a list
       echo nested . "invalid"
   endfunction
   call Exploit()
   

2. Deliver the Script: Use social engineering to convince the target to run the script. This could be:

   • Sending the script as an email attachment.
   • Hosting it on a website and tricking the user into downloading and running it.
   • Embedding it in a seemingly benign .vimrc file.

3. Execute the Script: The target runs vim -S exploit.vim or sources the script within Vim.

4. Trigger the Vulnerability: The error during tuple evaluation causes the UAF. If the memory layout is controlled, this can lead to arbitrary code execution.

Note: No public exploit code is available. This is a theoretical exploitation path.

Blue Team (Detection & Defense)

Detection:

• Monitor Vim Process Behavior: Look for unusual crashes or segmentation faults in Vim.
• File Integrity Monitoring: Monitor for unauthorized modifications to .vimrc or other Vim script files.
• User Awareness: Train users not to execute Vim scripts from untrusted sources.

Defense:

• Patch Immediately: Update Vim to version 9.1.1400 or later.
• Restrict Vim Script Execution: Use security policies to prevent Vim from sourcing scripts from untrusted locations (e.g., /tmp, email attachments).
• Use AppArmor/SELinux: Confine the Vim process to limit the impact of a potential exploit.
• Disable Vim Scripting (if not needed): In high-security environments, consider using an alternative editor or disabling Vim's scripting capabilities.

Indicators of Compromise (IOCs)

• File Hashes (SHA256): (None available, as no specific exploit file is known)
• Process Anomalies:
  • Vim process crashing with a segmentation fault (SIGSEGV).
  • Vim process exhibiting unusual memory access patterns.
• Network Indicators:
  • Outbound connections from a Vim process (if the exploit attempts to call back to a C2 server).
• File System Indicators:
  • Unexpected .vim or .vimrc files in user directories.
  • Modified Vim plugin files.

Detection Engineering

YARA Rule

rule CVE_2025_55157_Vim_UAF {
    meta:
        description = "Detects Vim scripts attempting to exploit CVE-2025-55157 use-after-free in nested tuple handling"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-55157"
        hash = "unknown"
    strings:
        $s1 = "tuple_unref" ascii wide nocase
        $s2 = "nested" ascii wide nocase
        $s3 = "refcount" ascii wide nocase
        $s4 = "vim_free" ascii wide nocase
    condition:
        all of ($s*) and filesize < 100KB
}

Sigma Rule

title: Potential Exploitation of CVE-2025-55157 in Vim
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects Vim process crashes or unusual behavior that may indicate exploitation of CVE-2025-55157.
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/vim'
        CommandLine|contains: '-S'
    condition: selection
falsepositives:
    - Legitimate use of Vim to source scripts.
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-55157 Vim Potential C2 Callback"; flow:to_server,established; content:"|00 00 00 00|"; depth:4; offset:0; classtype:attempted-admin; sid:1000001; rev:1;)

Note: This Suricata rule is a placeholder. A real rule would need to detect specific network patterns associated with a known exploit payload.

Remediation & Mitigation

Immediate Actions:

1. Patch: Update Vim to version 9.1.1400 or later. This is the only complete fix.
   • Linux (apt): sudo apt update && sudo apt upgrade vim
   • Linux (yum/dnf): sudo yum update vim or sudo dnf update vim
   • macOS (Homebrew): brew upgrade vim
   • Windows: Download the latest installer from vim.org.
2. Verify Version: After updating, run vim --version to confirm the version is 9.1.1400 or higher.

Long-Term Mitigations:

• Principle of Least Privilege: Run Vim with the minimum necessary privileges.
• Application Control: Use tools like AppArmor or SELinux to restrict Vim's capabilities.
• User Education: Train users to be cautious when opening Vim scripts from untrusted sources.
• Regular Updates: Keep all software, especially critical tools like Vim, up to date.

Valters IT

This report is generated for educational and defensive purposes. The author is not responsible for any misuse of this information.