CVE-2025-9389

Executive Summary

CVE-2025-9389 is a low-severity (CVSS 3.3) local memory corruption vulnerability in Vim 9.1.0000. The flaw resides in the __memmove_avx_unaligned_erms function within memmove-vec-unaligned-erms.S, leading to memory corruption when certain conditions are met. The attack vector is local, requiring the attacker to have access to a user account on the target system. The exploit is publicly available, though some users report it does not reproduce reliably, especially when syntax coloring is enabled. The EPSS score is 0.0%, indicating a very low probability of exploitation in the wild. A patch is available, and upgrading to a newer version of Vim is the recommended remediation.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
• Base Score: 3.3 (Low)
• Attack Vector: Local (AV:L) – The attacker must have local access to the system.
• Attack Complexity: Low (AC:L) – No special conditions are required for exploitation.
• Privileges Required: Low (PR:L) – The attacker needs a valid user account.
• User Interaction: None (UI:N) – No user interaction is required beyond the attacker’s own actions.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: None (C:N) – No data exposure.
• Integrity Impact: None (I:N) – No data modification.
• Availability Impact: Low (A:L) – Memory corruption may cause a denial of service (crash) or unpredictable behavior.

Threat Level: Low. The local attack vector and low impact reduce the overall risk. However, the public availability of exploit code warrants attention for environments where Vim is used by multiple users.

Deep Dive & Mechanics

The vulnerability is a memory corruption issue in the __memmove_avx_unaligned_erms function, which is part of the memmove-vec-unaligned-erms.S assembly file. This function is used by Vim for optimized memory operations when AVX (Advanced Vector Extensions) is available on the CPU. The corruption occurs due to improper handling of unaligned memory moves, potentially leading to buffer overflows or writes to unintended memory locations.

The exploit is triggered locally by crafting a specific input file or sequence of commands that cause Vim to invoke the vulnerable memory move routine. The exact trigger mechanism is not fully detailed, but it is known that enabling syntax coloring (:syntax on) may prevent the exploit from working, suggesting that the corruption is sensitive to the state of Vim’s internal buffers.

The low EPSS score (0.0%) and user reports of non-reproducibility indicate that the exploit may be unreliable or require specific system configurations (e.g., CPU microarchitecture, Vim build flags). Nonetheless, the existence of public exploit code means that targeted attacks are possible.

Vulnerable vs. Patched Code

The vulnerable code is in memmove-vec-unaligned-erms.S in Vim 9.1.0000. The patch modifies the assembly logic to ensure proper bounds checking and alignment handling during memory moves. Since the exact patch is not provided in the NVD data, the general fix involves:

• Adding alignment checks before performing AVX-optimized moves.
• Ensuring that the destination buffer size is validated.
• Falling back to a safe, non-optimized memory move when conditions are not met.

Vulnerable Code (Conceptual):

; Simplified vulnerable logic
__memmove_avx_unaligned_erms:
    ; Assume src and dst are valid
    vmovdqu ymm0, [src]
    vmovdqu [dst], ymm0
    ; No bounds check on dst size

Patched Code (Conceptual):

; Patched logic with bounds check
__memmove_avx_unaligned_erms:
    cmp dst_size, 32
    jb .Lfallback
    vmovdqu ymm0, [src]
    vmovdqu [dst], ymm0
    ret
.Lfallback:
    ; Use safe memmove
    jmp memmove_safe

Exploitation & Lab Setup

Red Team

Prerequisites:

• Local access to a system running Vim 9.1.0000.
• Ability to create and open a crafted file with Vim.

Exploit Steps:

1. Obtain the public exploit code (e.g., from Exploit-DB or GitHub).
2. Compile or prepare the exploit payload (likely a specially crafted text file).
3. Execute Vim on the target file: vim crafted_file.txt
4. The memory corruption may cause Vim to crash or behave unexpectedly, potentially allowing further exploitation (e.g., privilege escalation if Vim is setuid).

Note: The exploit may not work if syntax coloring is enabled. Disable it with :syntax off before opening the file.

Blue Team

Detection:

• Monitor for unusual Vim crashes or segmentation faults.
• Look for processes that open files with suspicious patterns (e.g., very long lines, specific byte sequences).
• Use process auditing to detect when Vim is invoked with unusual arguments.

Prevention:

• Apply the patch or upgrade Vim to a version > 9.1.0000.
• Restrict local user access to minimize attack surface.
• Use AppArmor or SELinux to confine Vim’s capabilities.

Indicators of Compromise (IOCs)

• File Hashes: (None provided; generate upon exploit retrieval)
• Process Names: vim, vim.basic, vim.tiny
• Log Entries: Vim crash logs in /var/log/syslog or ~/.viminfo showing segmentation faults.
• Network: No network indicators (local vulnerability).
• Registry/Config: Disabled syntax coloring (:syntax off) in Vim configuration.

Detection Engineering

YARA Rule

rule CVE_2025_9389_Vim_Memory_Corruption {
    meta:
        description = "Detects Vim binary with vulnerable memmove function"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-9389"
    strings:
        $vuln_func = "__memmove_avx_unaligned_erms"
        $vuln_file = "memmove-vec-unaligned-erms.S"
    condition:
        $vuln_func or $vuln_file
}

Sigma Rule

title: Vim Memory Corruption Exploit Attempt
id: cve-2025-9389
status: experimental
description: Detects potential exploitation of CVE-2025-9389 via Vim crashes
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith: '/vim'
        CommandLine|contains: 'crafted_file.txt'
    condition: selection
falsepositives:
    - Legitimate use of Vim with unusual filenames
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET any (msg:"CVE-2025-9389 Vim Memory Corruption Exploit"; flow:to_server; content:"|00|"; depth:1; sid:1000001; rev:1;)

Note: Suricata rule is generic as the exploit is local; network detection is not applicable.

Remediation & Mitigation

1. Patch: Upgrade Vim to version > 9.1.0000. The official patch is available from the Vim repository.
2. Workaround: Disable syntax coloring (:syntax off) if upgrading is not immediately possible.
3. Access Control: Restrict local user accounts and use principle of least privilege.
4. Monitoring: Enable process auditing for Vim and monitor for crashes.
5. Backup: Ensure regular backups of critical data to mitigate potential denial-of-service.

Valters IT
DevSecOps & Cyber Threat Intelligence