CVE-2026-103592 in simple-router
Summary
by MITRE • 10/01/2026
simple-php-router through 5.4.1.7 contains an IP restriction bypass vulnerability in the IpRestrictAccess middleware that allows remote unauthenticated attackers to bypass IP whitelist and blacklist protections. Attackers can spoof X-Forwarded-For, CF-Connecting-IP, or Client-IP headers to impersonate whitelisted addresses or evade blacklists, gaining access to IP-restricted routes.
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Analysis
by VulDB Data Team • 10/01/2026
The vulnerability identified in simple-php-router versions through 5.4.1.7 represents a critical failure in the implementation of network-level access controls within the IpRestrictAccess middleware component. This flaw stems from an insecure trust relationship with client-supplied HTTP headers, specifically X-Forwarded-For, CF-Connecting-IP, and Client-IP. In modern web architectures utilizing reverse proxies or content delivery networks, these headers are commonly used to convey the original IP address of a connecting user when the direct connection is made through an intermediary. However, the middleware in question fails to validate whether these header values originate from a trusted proxy server rather than directly from the end client. Consequently, any remote unauthenticated attacker can manipulate these fields within their HTTP requests to fabricate source addresses that match entries in the configured IP whitelist or fall outside of blacklisted ranges.
This mechanism allows for a straightforward bypass of IP-based restriction policies through header spoofing. By injecting specific values into the X-Forwarded-For, CF-Connecting-IP, or Client-IP headers, an attacker can impersonate authorized internal addresses that are permitted access to sensitive routes or administrative interfaces. Conversely, attackers targeting blacklisted restrictions can mask their true origin by presenting a fabricated IP address not present in the deny list. This bypass effectively neutralizes the intended security boundary provided by the middleware, granting unauthorized entities access to resources that should be restricted based on network location. The vulnerability is particularly severe because it requires no authentication credentials and exploits standard HTTP protocol behaviors that are often misconfigured or misunderstood by developers implementing basic IP filtering logic.
From a technical classification perspective, this flaw aligns with CWE-287 Improper Authentication, as the system fails to correctly verify the identity of the requester based on their network address due to reliance on untrusted input for critical security decisions. It also relates closely to CWE-918 Server-Side Request Forgery (SSRF) principles in terms of trusting client-controlled data for server-side logic execution without sufficient validation, although it is more accurately categorized under CWE-749 Exposed Dangerous Method or specifically as a failure in access control enforcement due to untrusted header consumption. In the context of the MITRE ATT&CK framework, this vulnerability facilitates Initial Access techniques such as Exploitation for Credential Access if combined with other flaws, but primarily it supports Reconnaissance and Lateral Movement by allowing unauthorized entry into restricted network segments or administrative panels that are not protected by additional authentication layers.
The operational impact of this vulnerability is significant for applications relying solely on IP whitelisting for security. If an application exposes sensitive endpoints like API gateways, admin dashboards, or internal service interfaces behind such middleware without supplementary protections like multi-factor authentication or certificate-based mutual TLS, the entire access control strategy collapses. Attackers can gain persistent unauthorized access to backend systems, potentially leading to data exfiltration, configuration tampering, or further exploitation of other vulnerabilities within the application logic that are only accessible from trusted IP ranges. The ease of exploitation means that automated scanning tools and opportunistic attackers can quickly identify and exploit this misconfiguration across large numbers of deployments using vulnerable versions of simple-php-router.
To mitigate this vulnerability, developers must implement strict validation for proxy headers rather than trusting them blindly. A robust mitigation strategy involves configuring the middleware to only accept X-Forwarded-For or similar headers if they originate from a known and trusted reverse proxy server IP address defined in a whitelist configuration within the application itself. Alternatively, applications should rely on direct connection information such as $_SERVER['REMOTE_ADDR'] when no trusted proxies are involved, ensuring that the source IP is derived directly from the TCP socket rather than HTTP metadata which can be easily spoofed by any client with network access to the web server. Upgrading to a patched version of simple-php-router where this logic has been corrected is essential for immediate remediation. Additionally, defense-in-depth principles should be applied by ensuring that critical routes are protected by multiple layers of security controls, including strong authentication mechanisms and rate limiting, so that IP-based restrictions serve only as an additional layer rather than the sole barrier against unauthorized access.