CVE-2024-21510: Sinatra SSRF Vulnerability

CVE-2024-21510 Overview

CVE-2024-21510 is an Open Redirect vulnerability affecting the Sinatra Ruby web application framework. The vulnerability stems from improper handling of the X-Forwarded-Host (XFH) header, which allows attackers to manipulate redirect locations in applications using Sinatra's redirect functionality. This weakness (CWE-807: Reliance on Untrusted Inputs in a Security Decision) enables multiple attack vectors including Open Redirect, Cache Poisoning, and Routing-based Server-Side Request Forgery (SSRF).

Critical Impact

Attackers can redirect users to malicious sites, poison web caches, or exploit routing-based SSRF when Sinatra applications are deployed behind reverse proxies like Nginx without proper X-Forwarded-Host header validation.

Affected Products

• Sinatra Ruby framework (all versions from 0.0.0)
• Web applications using Sinatra's redirect functionality
• Deployments behind reverse proxies (Nginx, etc.) without X-Forwarded-Host filtering

Discovery Timeline

• 2024-11-01 - CVE-2024-21510 published to NVD
• 2024-11-01 - Last updated in NVD database

Technical Details for CVE-2024-21510

Vulnerability Analysis

The vulnerability exists in Sinatra's redirect handling logic within lib/sinatra/base.rb. When a Sinatra application uses the redirect method, the framework constructs the redirect URL by trusting the X-Forwarded-Host header without proper validation. This header is commonly set by reverse proxies and load balancers to indicate the original host requested by the client.

The core issue is that Sinatra uses the X-Forwarded-Host value when constructing absolute redirect URLs, allowing an attacker to inject an arbitrary hostname. When combined with caching infrastructure or reverse proxy configurations, this can lead to severe security implications beyond simple open redirects.

Root Cause

The root cause is classified as CWE-807: Reliance on Untrusted Inputs in a Security Decision. Sinatra's redirect logic trusts the X-Forwarded-Host header value when building redirect responses without validating that the host is an expected or allowed value. This header is client-controllable in many deployment scenarios, particularly when applications are not properly configured to strip or validate forwarded headers at the reverse proxy layer.

Attack Vector

The attack is network-based and requires user interaction (clicking a malicious link or being directed to a crafted URL). An attacker can exploit this vulnerability by:

1. Crafting a request to a Sinatra endpoint that performs a redirect
2. Including a malicious X-Forwarded-Host header with an attacker-controlled domain
3. The redirect response will point to the attacker's domain instead of the legitimate application host

When exploited against caching infrastructure, the poisoned redirect can be cached and served to subsequent users, amplifying the attack's impact. In routing-based SSRF scenarios, the manipulated host header can cause internal services to make requests to attacker-controlled servers.

The vulnerability mechanism involves the redirect function reading the X-Forwarded-Host header to construct the Location header in HTTP redirect responses. Without proper sanitization, an attacker-supplied value like evil.com in the XFH header causes the application to generate redirects pointing to the attacker's domain. Technical implementation details can be found in the Sinatra base.rb source code and the related GitHub Pull Request discussion.

Detection Methods for CVE-2024-21510

Indicators of Compromise

• Unusual X-Forwarded-Host header values in application logs that don't match expected hostnames
• Redirect responses (HTTP 301/302/303/307/308) containing unexpected or external domains in the Location header
• Increased cache hit rates for redirect responses that point to external domains
• User reports of being redirected to phishing or malicious sites from trusted application links

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block requests with suspicious X-Forwarded-Host header values
• Monitor application and proxy access logs for requests containing X-Forwarded-Host headers with unexpected domains
• Configure intrusion detection systems (IDS) to alert on redirect responses pointing to domains outside an allowlist
• Review cache logs for poisoned entries containing redirects to external hosts

Monitoring Recommendations

• Enable detailed logging of HTTP headers at both the reverse proxy and application layers
• Set up alerts for redirect responses where the Location header domain differs from configured application domains
• Monitor for anomalous patterns in cache behavior, particularly around redirect caching
• Implement continuous security scanning to identify Sinatra applications vulnerable to header manipulation

How to Mitigate CVE-2024-21510

Immediate Actions Required

• Configure reverse proxies (Nginx, Apache, etc.) to explicitly set or override the X-Forwarded-Host header with trusted values
• Implement application-level validation of redirect destinations against an allowlist of permitted hosts
• Review and update Sinatra applications to avoid relying on the X-Forwarded-Host header for security-sensitive redirect operations
• Disable caching of redirect responses or implement cache key normalization that ignores the XFH header

Patch Information

A fix has been proposed via GitHub Pull Request #2010. Organizations should monitor the Sinatra project for official releases containing this fix. In the meantime, implement the workarounds described below to mitigate the vulnerability. Additional details are available in the Snyk Vulnerability Report.

Workarounds

• Configure Nginx or other reverse proxies to strip or override the X-Forwarded-Host header before forwarding requests to Sinatra applications
• Implement a Rack middleware that validates or removes the X-Forwarded-Host header before it reaches Sinatra
• Use relative redirects instead of absolute URLs where possible in application code
• Add application-level checks to validate that redirect targets belong to expected domains

# Nginx configuration to override X-Forwarded-Host
# Add to your server block or location block

# Option 1: Set X-Forwarded-Host to the actual server name
proxy_set_header X-Forwarded-Host $host;

# Option 2: Explicitly clear the header if not needed
proxy_set_header X-Forwarded-Host "";

# Option 3: Set to a specific trusted value
proxy_set_header X-Forwarded-Host "trusted-domain.example.com";