CVE-2024-35538

Executive Summary

CVE-2024-35538 is a Client IP Spoofing vulnerability in Typecho v1.3.0, a popular open-source blogging platform. The flaw allows an unauthenticated attacker to falsify their IP address by injecting arbitrary values into the X-Forwarded-For or Client-Ip HTTP headers. This can be exploited to bypass IP-based access controls, logging, and rate-limiting mechanisms, potentially enabling further attacks such as brute-force attempts or privilege escalation. The vulnerability has a CVSS score of 0.0 (pending official assessment), but based on the vector, it is considered low severity with a high attack surface due to the lack of authentication required. No official patch has been released, and no public exploit code is available, though the technique is well-documented.

Supply Chain & Package Security

Typecho is a PHP-based application distributed via GitHub and its official website. The vulnerability exists in the core application code, not in any third-party package. Users should verify the integrity of their installation and monitor for updates from the vendor.

CVSS Breakdown & Threat Level

• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): None (N) – No user action required.
• Scope (S): Unchanged (U) – Impact confined to the vulnerable component.
• Confidentiality (C): None (N) – No data disclosure.
• Integrity (I): Low (L) – IP-based controls can be bypassed.
• Availability (A): None (N) – No denial of service.

Threat Level: Low (CVSS 3.1 base score ~3.5). However, the risk is elevated in environments relying on IP-based authentication or logging for security decisions.

Deep Dive & Mechanics

The vulnerability stems from Typecho's improper handling of HTTP headers used for client IP identification. The application trusts the X-Forwarded-For and Client-Ip headers without validation, allowing an attacker to set these headers to any arbitrary IP address. This is a classic case of CWE-348 (Use of Less Trusted Source). The affected code likely resides in the IP retrieval function, which directly reads from $_SERVER['HTTP_X_FORWARDED_FOR'] or $_SERVER['HTTP_CLIENT_IP'] without verifying the source.

Attack Scenario:

1. An attacker sends an HTTP request to a Typecho instance with a spoofed X-Forwarded-For: 192.168.1.100 header.
2. The application logs the attacker's IP as 192.168.1.100 instead of the real IP.
3. If IP-based access controls (e.g., admin panel restrictions) are in place, the attacker can bypass them by spoofing a whitelisted IP.
4. The attacker can also evade rate-limiting or brute-force detection mechanisms that rely on IP tracking.

Vulnerable vs. Patched Code

Vulnerable Code (Typecho v1.3.0):

// In a function like getIp()
function getIp() {
    if (!empty($_SERVER['HTTP_CLIENT_IP'])) {
        return $_SERVER['HTTP_CLIENT_IP'];
    } elseif (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
        return $_SERVER['HTTP_X_FORWARDED_FOR'];
    } else {
        return $_SERVER['REMOTE_ADDR'];
    }
}

Patched Code (Recommended Fix):

function getIp() {
    // Only trust REMOTE_ADDR as it cannot be spoofed by the client
    return $_SERVER['REMOTE_ADDR'];
}

Alternatively, if proxy support is required, validate against a trusted proxy list:

function getIp() {
    $trustedProxies = ['10.0.0.0/8', '172.16.0.0/12', '192.168.0.0/16'];
    if (in_array($_SERVER['REMOTE_ADDR'], $trustedProxies)) {
        if (!empty($_SERVER['HTTP_X_FORWARDED_FOR'])) {
            return $_SERVER['HTTP_X_FORWARDED_FOR'];
        }
    }
    return $_SERVER['REMOTE_ADDR'];
}

Exploitation & Lab Setup

Red Team

Prerequisites: A Typecho v1.3.0 instance accessible over HTTP.

Exploitation Steps:

1. Identify the target Typecho installation (e.g., via Shodan dork http.title:"Typecho").
2. Send a crafted HTTP request with spoofed headers:

   curl -H "X-Forwarded-For: 10.0.0.1" -H "Client-Ip: 10.0.0.1" http://target.com/admin/login.php
   

3. If the admin panel restricts access to a specific IP range (e.g., 10.0.0.0/8), the attacker can bypass it by spoofing an IP in that range.
4. For brute-force attacks, spoof a different IP for each request to evade rate-limiting:

   for i in {1..100}; do
     curl -H "X-Forwarded-For: 192.168.1.$i" -d "username=admin&password=test" http://target.com/login.php
   done
   

No public exploit code is available; the above demonstrates the technique.

Blue Team

Detection:

• Monitor for multiple requests with varying X-Forwarded-For or Client-Ip headers from the same source IP.
• Alert on requests where the REMOTE_ADDR does not match the forwarded IP (unless behind a trusted proxy).
• Use WAF rules to block requests with suspicious or non-standard IP headers.

Honeypot: Deploy a fake Typecho instance and log all X-Forwarded-For and Client-Ip headers for analysis.

Indicators of Compromise (IOCs)

• Network IOCs:
  • HTTP requests with X-Forwarded-For or Client-Ip headers containing private or reserved IP addresses (e.g., 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
  • Multiple requests from the same source IP with different X-Forwarded-For values.
• Log IOCs:
  • Server access logs showing mismatches between REMOTE_ADDR and X-Forwarded-For values.
  • Admin panel login attempts from IPs that should not have access.

Detection Engineering

YARA Rule

rule CVE_2024_35538_Client_IP_Spoofing {
    meta:
        description = "Detects Typecho v1.3.0 Client IP Spoofing attempts"
        author = "Valters IT"
        date = "2025-04-17"
        reference = "CVE-2024-35538"
    strings:
        $header1 = "X-Forwarded-For"
        $header2 = "Client-Ip"
        $ip_pattern = /(?:10\.\d{1,3}\.\d{1,3}\.\d{1,3}|172\.(?:1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}|192\.168\.\d{1,3}\.\d{1,3})/
    condition:
        any of ($header*) and $ip_pattern
}

Sigma Rule

title: Typecho Client IP Spoofing Attempt
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects attempts to exploit CVE-2024-35538 by spoofing X-Forwarded-For or Client-Ip headers
author: Valters IT
logsource:
    category: webserver
    product: apache
    service: access
detection:
    selection:
        cs-uri-query|contains:
            - "X-Forwarded-For"
            - "Client-Ip"
    condition: selection
falsepositives:
    - Legitimate proxy usage
level: medium

Suricata Rule

alert http any any -> any any (msg:"CVE-2024-35538 Typecho Client IP Spoofing Attempt"; flow:to_server; content:"X-Forwarded-For"; http_header; pcre:"/X-Forwarded-For:\s*(?:10\.\d{1,3}\.\d{1,3}\.\d{1,3}|172\.(?:1[6-9]|2\d|3[01])\.\d{1,3}\.\d{1,3}|192\.168\.\d{1,3}\.\d{1,3})/i"; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Apply the patch: Modify the IP retrieval function to only trust REMOTE_ADDR (see patched code above).
2. Update Typecho: Monitor the official repository for a security release and upgrade immediately.
3. WAF Rules: Block requests with X-Forwarded-For or Client-Ip headers unless from trusted proxies.
4. Logging: Enable detailed logging of all HTTP headers for forensic analysis.

Long-Term Measures:

• Implement IP validation using a trusted proxy list if reverse proxies are used.
• Use CAPTCHA or multi-factor authentication to reduce reliance on IP-based controls.
• Conduct regular security audits of custom code.

Valters IT