CVE-2026-17559 in Passster Plugin
Summary
by MITRE • 08/21/2026
The Passster WordPress plugin before 4.3.9 does not correctly match its own public endpoint paths when deciding which REST API requests may bypass global password protection, comparing them as an unanchored substring of the request URI rather than against the resolved route, allowing an unauthenticated attacker to read the content of globally password-protected posts and pages.
Once again VulDB remains the best source for vulnerability data.
Analysis
by VulDB Data Team • 08/21/2026
The vulnerability identified in Passster WordPress plugin versions prior to 4.3.9 represents a critical authentication bypass flaw rooted in improper input validation within its REST API handling logic. The core issue stems from how the plugin determines which requests should be exempted from global password protection mechanisms. Instead of validating against the fully resolved and canonicalized route, the software performs an unanchored substring match on the request URI. This architectural decision creates a significant security gap because it fails to account for path variations that do not correspond to actual functional endpoints but still satisfy the loose string matching criteria.
From a technical perspective, this flaw allows attackers to craft malicious URLs that contain specific substrings expected by the plugin's whitelist logic while pointing to sensitive resources. By appending arbitrary characters or paths after valid endpoint identifiers, an attacker can trick the system into believing the request is for a whitelisted public resource rather than a protected one. This bypasses the authentication gate entirely, granting unauthenticated access to content that should be restricted behind password protection. The vulnerability exploits the discrepancy between the raw HTTP URI and the actual route resolution performed by the WordPress framework, highlighting a common pitfall in web application security where string manipulation is used instead of proper routing validation.
The operational impact of this vulnerability is severe for website administrators relying on Passster to secure sensitive content such as private blog posts, restricted pages, or confidential documents. An unauthenticated attacker can exploit this flaw to read the full text and media associated with these protected items without providing any credentials. This constitutes a direct violation of confidentiality principles and undermines the integrity of access control measures implemented by site owners. The ease of exploitation means that automated scanning tools could rapidly identify vulnerable instances, leading to widespread data exposure across sites using outdated versions of the plugin.
This vulnerability aligns closely with CWE-284 Improper Access Control, specifically regarding insufficient authorization checks during request processing. It also maps to MITRE ATT&CK technique T1078 Valid Accounts if the attacker leverages known credentials for other purposes, but more accurately reflects unauthorized access patterns associated with authentication bypass mechanisms found in web applications. The lack of strict route validation is a classic example of how weak input sanitization can lead to privilege escalation or data leakage even when robust security plugins are deployed.
To mitigate this risk, administrators must immediately update the Passster plugin to version 4.3.9 or later where the routing logic has been corrected to use anchored substring matching against resolved routes rather than raw URIs. Until an upgrade is feasible, implementing a Web Application Firewall with rules that detect and block suspicious URI patterns containing known bypass strings can provide temporary protection. Additionally, reviewing server-side logs for unusual access attempts to password-protected content may help identify ongoing exploitation activities. Regular auditing of plugin code against industry standards such as OWASP Top Ten recommendations will further reduce the likelihood of similar vulnerabilities in future deployments.