CVE-2020-37056: Crystal Shard Auth Bypass Vulnerability

CVE-2020-37056 Overview

Crystal Shard http-protection version 0.2.0 contains an IP spoofing vulnerability (CWE-290: Authentication Bypass by Spoofing) that allows attackers to bypass protection middleware by manipulating request headers. Attackers can hardcode consistent IP values across X-Forwarded-For, X-Client-IP, and X-Real-IP headers to circumvent security checks and gain unauthorized access to protected resources.

Critical Impact

Attackers can bypass IP-based security controls by spoofing request headers, potentially gaining unauthorized access to protected endpoints and circumventing rate limiting or IP blocking mechanisms.

Affected Products

• Crystal Shard http-protection version 0.2.0

Discovery Timeline

• 2026-01-30 - CVE CVE-2020-37056 published to NVD
• 2026-02-03 - Last updated in NVD database

Technical Details for CVE-2020-37056

Vulnerability Analysis

This vulnerability stems from improper validation of client-provided IP address headers in the http-protection middleware. The library relies on HTTP headers such as X-Forwarded-For, X-Client-IP, and X-Real-IP to determine the originating IP address of incoming requests. However, the middleware fails to properly validate or sanitize these headers, allowing attackers to supply arbitrary IP addresses that the application will trust as legitimate.

The root cause relates to CWE-290 (Authentication Bypass by Spoofing), where the application incorrectly assumes that client-provided header values are trustworthy. This is particularly dangerous in web applications that use IP-based access controls, rate limiting, or geo-blocking features.

Root Cause

The vulnerability exists because the http-protection library directly trusts and uses IP address values from client-controllable HTTP headers without proper validation. When multiple IP-related headers are present in a request, the middleware does not verify their authenticity or cross-reference them against the actual connection source IP. This design flaw allows attackers to consistently spoof their perceived IP address by setting identical values across all relevant headers.

Attack Vector

The attack is network-based and requires no authentication or user interaction to exploit. An attacker can craft malicious HTTP requests with spoofed IP headers to bypass IP-based security controls. By setting X-Forwarded-For, X-Client-IP, and X-Real-IP headers to consistent trusted or whitelisted IP addresses, attackers can:

• Bypass IP-based access control lists (ACLs)
• Circumvent rate limiting protections
• Evade geo-blocking or region-based restrictions
• Access internal or admin-only endpoints protected by IP filtering

The attack involves setting spoofed IP headers in HTTP requests. For example, an attacker would include headers like X-Forwarded-For: 127.0.0.1, X-Client-IP: 127.0.0.1, and X-Real-IP: 127.0.0.1 to appear as if the request originates from localhost. Technical details and proof-of-concept information can be found in the Exploit-DB #48533 advisory.

Detection Methods for CVE-2020-37056

Indicators of Compromise

• HTTP requests containing identical values across X-Forwarded-For, X-Client-IP, and X-Real-IP headers that do not match the actual source IP
• Requests with IP headers set to internal addresses (127.0.0.1, 10.x.x.x, 192.168.x.x) originating from external sources
• Unusual access patterns to IP-restricted endpoints from seemingly trusted IPs
• Multiple requests from different actual source IPs but with identical spoofed header values

Detection Strategies

• Implement logging that captures both the actual connection source IP and all IP-related headers for comparison analysis
• Deploy Web Application Firewall (WAF) rules to flag requests where IP headers contain internal/loopback addresses from external connections
• Create alerts for requests where X-Forwarded-For, X-Client-IP, and X-Real-IP headers all contain identical values
• Monitor for unusual access patterns to previously IP-restricted resources

Monitoring Recommendations

• Enable verbose HTTP header logging on reverse proxies and application servers
• Configure SIEM rules to correlate actual source IPs with claimed header values
• Implement real-time alerting for requests attempting to access sensitive endpoints with suspicious IP header combinations
• Review access logs for protected endpoints to identify potential bypass attempts

How to Mitigate CVE-2020-37056

Immediate Actions Required

• Audit current usage of http-protection library and identify all applications using version 0.2.0
• Implement additional IP validation at the application or infrastructure level that does not solely rely on client headers
• Configure reverse proxies to overwrite or strip incoming IP headers before setting trusted values
• Review and strengthen IP-based access controls with additional authentication layers

Patch Information

Consult the GitHub HTTP Protection Repository for the latest version information and available patches. Review the VulnCheck Advisory on IP Spoofing for additional remediation guidance.

Workarounds

• Configure upstream reverse proxies (nginx, HAProxy) to set IP headers from the actual connection source, overwriting any client-provided values
• Implement application-level validation that cross-references IP headers with the actual TCP connection source
• Add defense-in-depth security measures that do not rely solely on IP-based authentication
• Consider implementing additional authentication mechanisms for sensitive endpoints beyond IP-based access controls

# nginx configuration example - overwrite client IP headers
# Add to server or location block to ensure trusted IP headers

proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_set_header X-Client-IP $remote_addr;

# Strip any incoming X-Client-IP header from untrusted sources
proxy_set_header X-Original-Forwarded-For $http_x_forwarded_for;