CVE-2024-56200

Executive Summary

CVE-2024-56200 is a high-severity vulnerability affecting Altair, a fork of Misskey v12. The issue stems from a lack of request validation and authentication in the image proxy component responsible for compressing and resizing remote files. An unauthenticated attacker can exploit this flaw to cause a denial-of-service (DoS) condition by abnormally increasing CPU usage on the server or placing a heavy load on the network. The vulnerability has been fixed in version v12.24Q4.1, and no workarounds are available. The EPSS score is 0.0%, indicating low exploitation likelihood at this time, but the potential for impact remains significant due to the lack of authentication required.

Supply Chain & Package Security

This vulnerability is not related to a package manager (NPM, PyPI, Go, Ruby) but rather to a self-hosted application (Altair/Misskey fork). Organizations using Altair should verify their deployment version and apply the patch immediately.

CVSS Breakdown & Threat Level

• CVSS Score: 8.6 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Attack Vector: Network (AV:N)
• Attack Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Confidentiality Impact: None (C:N)
• Integrity Impact: None (I:N)
• Availability Impact: High (A:H)

The vulnerability is classified as High severity due to the ease of exploitation (no authentication, low complexity) and the direct impact on availability. An attacker can cause a denial-of-service condition without any prior access.

Deep Dive & Mechanics

The vulnerability resides in the image proxy functionality of Altair, which is used to compress and resize remote images fetched from external URLs. The proxy lacks proper request validation and authentication, meaning any unauthenticated user can send arbitrary requests to the proxy endpoint.

Mechanism:

1. The image proxy accepts a URL parameter pointing to an external image.
2. It fetches the image, compresses, and resizes it.
3. Due to missing validation, an attacker can craft requests that cause the proxy to:
   • Fetch extremely large files or infinite streams.
   • Trigger recursive or resource-intensive operations.
   • Overwhelm the server's CPU and network resources.

Attack Scenario: An attacker sends a large number of requests to the image proxy with URLs pointing to very large files or slow-to-respond servers. The proxy attempts to process each request, consuming CPU cycles and network bandwidth, leading to service degradation or complete unavailability.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-v12.24Q4.1): The image proxy endpoint likely lacks authentication middleware and input validation. Example pseudocode:

// Vulnerable: No authentication, no size limits
app.get('/proxy/image', async (req, res) => {
  const url = req.query.url;
  const image = await fetch(url); // No validation on URL or size
  const compressed = await compress(image);
  res.send(compressed);
});

Patched Code (v12.24Q4.1): The fix introduces authentication checks and request validation, such as limiting file size and validating URLs.

// Patched: Authentication required, size limits enforced
app.get('/proxy/image', authenticate, async (req, res) => {
  const url = req.query.url;
  if (!isValidUrl(url) || !isAllowedSize(url)) {
    return res.status(400).send('Invalid request');
  }
  const image = await fetch(url);
  const compressed = await compress(image);
  res.send(compressed);
});

Exploitation & Lab Setup

Red Team (Offensive)

Prerequisites:

• Target running Altair v12.x (pre-12.24Q4.1)
• Network access to the image proxy endpoint (e.g., https://target.tld/proxy/image?url=)

Exploitation Steps:

1. Identify the image proxy endpoint by reviewing application documentation or probing common paths.
2. Craft a request to a very large file or a slow server:

   curl "https://target.tld/proxy/image?url=https://slow.server/largefile.bin"
   

3. Automate multiple concurrent requests to amplify the DoS effect:

   for i in {1..100}; do
     curl "https://target.tld/proxy/image?url=https://slow.server/largefile.bin" &
   done
   

4. Monitor server CPU and network load to confirm impact.

Note: No public exploit code is available at this time.

Blue Team (Defensive)

Detection:

• Monitor for unusual spikes in CPU usage on the Altair server.
• Analyze web server logs for repeated requests to the image proxy endpoint with suspicious URLs (e.g., pointing to large files or known slow servers).
• Use network monitoring to detect high outbound traffic from the proxy to external hosts.

Mitigation:

• Immediately upgrade to v12.24Q4.1.
• If upgrade is not possible, implement a Web Application Firewall (WAF) rule to block requests to the image proxy from unauthenticated users.
• Rate-limit requests to the proxy endpoint.
• Restrict outbound connections from the proxy to only trusted domains.

Indicators of Compromise (IOCs)

• Network IOCs:

  • High volume of requests to /proxy/image endpoint.
  • Outbound connections from the server to unusual or slow external hosts.
  • Large data transfers from the server to external IPs.

• System IOCs:

  • Elevated CPU usage (near 100%) on the Altair server.
  • Increased network bandwidth consumption.
  • Application logs showing repeated errors or timeouts from the image proxy.

• Log Patterns:

  • Web server logs with multiple GET /proxy/image?url=... entries from the same source IP.
  • Error logs indicating failed image processing due to resource exhaustion.

Detection Engineering

YARA Rule

rule CVE_2024_56200_Altair_Image_Proxy_DoS {
    meta:
        description = "Detects exploitation attempts of CVE-2024-56200 via image proxy requests"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2024-56200"
    strings:
        $proxy_path = "/proxy/image"
        $url_param = "url="
        $large_file = /\.(zip|tar|gz|bin|iso|img)/i
    condition:
        $proxy_path and $url_param and #large_file > 0
}

Sigma Rule

title: CVE-2024-56200 Altair Image Proxy DoS Attempt
id: 8a3f5c2e-7d1b-4f9a-8e6c-3b2a1d0f5e7c
status: experimental
description: Detects multiple requests to Altair image proxy with large file URLs
author: Valters IT
date: 2025-04-10
logsource:
    category: webserver
    product: apache
detection:
    selection:
        cs-method: GET
        cs-uri-stem: /proxy/image
        cs-uri-query|contains: 'url='
    condition: selection
    timeframe: 5m
    condition: selection | count() > 50
falsepositives:
    - Legitimate use of image proxy for large files
level: high

Suricata Rule

alert http any any -> $HOME_NET any (
    msg:"CVE-2024-56200 Altair Image Proxy DoS Attempt";
    flow:to_server,established;
    content:"/proxy/image"; http_uri;
    content:"url="; http_uri;
    pcre:"/\.(zip|tar|gz|bin|iso|img)/iU";
    classtype:attempted-dos;
    sid:1000001;
    rev:1;
    reference:cve,2024-56200;
)

Remediation & Mitigation

Immediate Actions:

1. Upgrade: Update Altair to version v12.24Q4.1 or later. This is the only known fix.
2. WAF Rules: Block unauthenticated requests to /proxy/image endpoint.
3. Rate Limiting: Implement rate limiting on the proxy endpoint to prevent abuse.
4. Network Restrictions: Restrict outbound connections from the proxy to only trusted domains.

Long-term Measures:

• Conduct a security audit of all endpoints for missing authentication.
• Implement input validation for all user-supplied URLs.
• Monitor for similar vulnerabilities in other components.

Workarounds: None available.

Valters IT signature: This report is generated by Valters IT Cyber Threat Intelligence. For questions, contact threatintel@valters.it.