CVE-2025-27731

Executive Summary

CVE-2025-27731 is a local privilege escalation vulnerability in OpenSSH for Windows. An authorized attacker with low-privileged access to a Windows host running the affected OpenSSH server can exploit improper input validation to elevate privileges to SYSTEM or Administrator level. The vulnerability has a CVSS score of 7.8 (High) and currently no public proof-of-concept or patch is available. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild at this time, but the risk remains significant for targeted attacks.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Local (L) – The attacker must have local access to the system.
• Attack Complexity (AC): Low (L) – No special conditions are required.
• Privileges Required (PR): Low (L) – The attacker needs initial low-privileged access.
• User Interaction (UI): None (N) – No user action is needed.
• Scope (S): Unchanged (U) – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High (H) – Full system compromise leads to data exposure.
• Integrity (I): High (H) – Attacker can modify system files and configurations.
• Availability (A): High (H) – Attacker can disrupt services or cause denial of service.

Threat Level: High – Local privilege escalation with low complexity and no user interaction.

Deep Dive & Mechanics

OpenSSH for Windows is a port of the OpenSSH suite, commonly used for secure remote administration. The vulnerability resides in the input validation logic of the SSH server component (sshd.exe). When processing certain crafted inputs (e.g., environment variables, authentication requests, or channel requests), the server fails to properly sanitize the data before passing it to Windows API calls or internal functions.

An attacker with a valid shell session or authenticated SSH connection can send specially crafted packets that trigger a buffer overflow, type confusion, or improper permission check. This allows the attacker to execute arbitrary code in the context of the SSH service, which typically runs with SYSTEM privileges.

The root cause is a lack of bounds checking or input sanitization in a function that handles user-supplied data. The exact vulnerable code path is not publicly disclosed, but based on similar CVEs, it likely involves the handling of SSH_MSG_CHANNEL_REQUEST or environment variable processing.

Vulnerable vs. Patched Code

Since no patch is available, a direct code comparison cannot be made. However, the vulnerable code likely resembles:

// Vulnerable pseudo-code
void process_channel_request(char *input) {
    char buffer[256];
    strcpy(buffer, input); // No bounds check
    // ... further processing
}

A patched version would include input validation and bounds checking:

// Patched pseudo-code
void process_channel_request(char *input) {
    char buffer[256];
    if (strlen(input) >= sizeof(buffer)) {
        log_error("Input too long");
        return;
    }
    strcpy_s(buffer, sizeof(buffer), input);
    // ... further processing
}

Exploitation & Lab Setup

Red Team

Lab Setup:

1. Install a vulnerable version of OpenSSH for Windows on a Windows 10/11 or Windows Server VM.
2. Create a low-privileged user account.
3. Connect via SSH as the low-privileged user.

Exploitation Steps:

1. Enumerate the SSH server version: ssh -V or nmap -sV -p 22 <target>.
2. Use a custom exploit script (when available) to send a crafted SSH packet.
3. The exploit triggers the input validation flaw, spawning a SYSTEM shell.

Example exploit skeleton (conceptual):

import paramiko

def exploit(target, port, username, password):
    client = paramiko.SSHClient()
    client.set_missing_host_key_policy(paramiko.AutoAddPolicy())
    client.connect(target, port, username, password)
    transport = client.get_transport()
    # Send crafted channel request
    transport.open_session()
    # ... exploit payload
    client.close()

Blue Team

Detection:

• Monitor for unusual SSH channel requests or environment variable sizes.
• Look for processes spawned by sshd.exe with unexpected command lines.
• Enable detailed SSH logging (LogLevel DEBUG3 in sshd_config).

Hunting:

• Search for Event ID 4624 (logon) followed by Event ID 4688 (process creation) with parent process sshd.exe.
• Look for anomalous network connections from sshd.exe to internal hosts.

Indicators of Compromise (IOCs)

• File Hashes: (None available yet)
• Network: Unusual SSH packets with large payloads or malformed headers.
• Process: sshd.exe spawning cmd.exe or powershell.exe with SYSTEM integrity level.
• Registry: Modifications to HKLM\SYSTEM\CurrentControlSet\Services\sshd.
• Logs: SSH debug logs showing repeated failed validation attempts.

Detection Engineering

YARA Rule

rule CVE_2025_27731_OpenSSH_Windows_Elevation {
    meta:
        description = "Detects exploitation attempts of CVE-2025-27731 in OpenSSH for Windows"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-27731"
    strings:
        $s1 = "SSH_MSG_CHANNEL_REQUEST" ascii wide nocase
        $s2 = "env" ascii wide nocase
        $s3 = "exec" ascii wide nocase
        $s4 = "shell" ascii wide nocase
        $hex1 = { 00 00 00 00 00 00 00 00 } // placeholder for large payload
    condition:
        uint16(0) == 0x4D5A and // MZ header
        any of ($s*) and
        #hex1 > 10
}

Sigma Rule

title: CVE-2025-27731 OpenSSH for Windows Privilege Escalation
id: 12345678-1234-1234-1234-123456789012
status: experimental
description: Detects potential exploitation of CVE-2025-27731 via anomalous SSH channel requests
author: Valters IT
date: 2025-04-10
logsource:
    product: windows
    service: security
detection:
    selection:
        EventID: 4688
        ParentProcessName|endswith: '\sshd.exe'
        NewProcessName|endswith:
            - '\cmd.exe'
            - '\powershell.exe'
            - '\wscript.exe'
            - '\cscript.exe'
    condition: selection
falsepositives:
    - Legitimate administrative use
level: high

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 22 (msg:"CVE-2025-27731 OpenSSH for Windows Privilege Escalation Attempt"; flow:to_server,established; content:"|00 00 00 00|"; depth:4; offset:0; content:"SSH-"; within:10; content:"channel"; within:50; content:"request"; within:100; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

• Patch: No official patch available. Monitor OpenSSH for Windows releases and apply immediately when available.
• Workaround: Restrict SSH access to trusted users only. Use firewall rules to limit source IPs.
• Mitigation: Enable Windows Defender Application Control (WDAC) to block unauthorized executables. Use AppLocker to restrict script execution.
• Monitoring: Enable advanced audit logging for process creation and SSH service events.
• Compensating Controls: Implement Privileged Access Workstations (PAWs) for administrative SSH sessions.

Valters IT
DevSecOps & Cyber Threat Intelligence