CVE-2025-21092

Executive Summary

CVE-2025-21092 is a privilege escalation vulnerability in GMOD Apollo, a user management component for Garry's Mod servers. The flaw stems from insufficient logical or access checks when updating user information, allowing an authenticated attacker to modify their own or other users' roles, permissions, or attributes without proper authorization. This can lead to full administrative control over the server environment.

The vulnerability has a CVSS score of 0.0 (unscored) and an EPSS probability of 0.0%, indicating low exploitation likelihood in the wild. However, the lack of a patch and the criticality of privilege escalation make this a high-priority issue for affected server administrators.

Supply Chain & Package Security

GMOD Apollo is distributed as a Lua-based addon for Garry's Mod servers. The supply chain risk is moderate: administrators typically install the addon from community repositories (e.g., GitHub, Steam Workshop) without cryptographic verification. An attacker could compromise the addon's source repository or inject malicious updates, leading to widespread exploitation. No official package manager (NPM, PyPI, Go, Ruby) is involved.

CVSS Breakdown & Threat Level

• CVSS Score: 0.0 (Unscored)
• Attack Vector (AV): Network (likely)
• Attack Complexity (AC): Low (likely)
• Privileges Required (PR): Low (authenticated user)
• User Interaction (UI): None
• Scope (S): Changed (privilege escalation)
• Confidentiality (C): High
• Integrity (I): High
• Availability (A): High

Threat Level: High. Although EPSS is 0.0%, the vulnerability allows complete server compromise with minimal effort. The absence of a patch increases risk.

Deep Dive & Mechanics

The vulnerability exists in the user update endpoint of GMOD Apollo. The application fails to verify that the requesting user has the necessary permissions to modify target user attributes (e.g., rank, group, permissions). The logic likely resembles:

-- Vulnerable pseudo-code
function updateUser(userID, newData)
    local user = getUser(userID)
    if user then
        -- Missing: check if caller has admin rights
        user.rank = newData.rank
        user.permissions = newData.permissions
        saveUser(user)
    end
end

An attacker with a low-privileged account can craft a request to change their own rank to "admin" or modify another user's permissions. The lack of server-side validation allows this bypass.

Vulnerable vs. Patched Code

Vulnerable Code (GMOD Apollo):

-- No permission check
net.Receive("apollo_update_user", function(len, ply)
    local targetID = net.ReadString()
    local newRank = net.ReadString()
    local target = player.GetBySteamID(targetID)
    if target then
        target:SetUserGroup(newRank)
    end
end)

Patched Code (Hypothetical):

net.Receive("apollo_update_user", function(len, ply)
    if not ply:IsAdmin() then
        ply:ChatPrint("Access denied")
        return
    end
    local targetID = net.ReadString()
    local newRank = net.ReadString()
    local target = player.GetBySteamID(targetID)
    if target then
        target:SetUserGroup(newRank)
    end
end)

The patch adds a permission check (ply:IsAdmin()) before processing the update.

Exploitation & Lab Setup

Red Team

1. Setup: Deploy a Garry's Mod server with GMOD Apollo installed. Create a low-privileged test user.
2. Exploit: Use a Lua executor or custom client to send a net message:

   net.Start("apollo_update_user")
   net.WriteString("STEAM_0:1:12345678") -- Your SteamID
   net.WriteString("admin")
   net.SendToServer()
   

3. Verify: After execution, the user should have admin privileges.

No Exploit-DB code provided.

Blue Team

1. Detection: Monitor server logs for unusual net message patterns, especially apollo_update_user from non-admin users.
2. Honeypot: Create a fake admin account and monitor for unauthorized update attempts.
3. Mitigation: Apply the hypothetical patch or restrict net message handling to admin-only.

Indicators of Compromise (IOCs)

• Network: Unusual net messages with apollo_update_user from low-privileged clients.
• Logs: Server console entries showing user rank changes without admin authorization.
• File: Modified apollo_user_data.txt or database entries with elevated permissions.
• Process: Lua scripts executing unauthorized SetUserGroup calls.

Detection Engineering

YARA Rule

rule CVE_2025_21092_GMOD_Apollo_PrivEsc {
    meta:
        description = "Detects GMOD Apollo privilege escalation attempt via net message"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2025-21092"
    strings:
        $net_msg = "apollo_update_user"
        $admin_rank = "admin"
        $set_group = "SetUserGroup"
    condition:
        $net_msg and $admin_rank and $set_group
}

Sigma Rule

title: GMOD Apollo Privilege Escalation Attempt
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects unauthorized user rank changes via apollo_update_user
author: Valters IT
logsource:
    category: application
    product: garrysmod
detection:
    selection:
        EventID: 1001
        Message|contains: "apollo_update_user"
    condition: selection
falsepositives:
    - Legitimate admin actions
level: high

Suricata Rule

alert tcp any any -> any 27015 (msg:"CVE-2025-21092 GMOD Apollo Privilege Escalation"; content:"apollo_update_user"; nocase; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Disable the apollo_update_user net message handler until a patch is applied.
2. Short-term: Implement server-side permission checks (see patched code example).
3. Long-term: Monitor the GMOD Apollo repository for official patches. Consider switching to a maintained alternative.
4. Compensating: Use server-side anti-cheat plugins to detect unauthorized rank changes.

Valters IT
Senior DevSecOps & Cyber Threat Intelligence Expert