CVE-2021-20293 in RESTEasy
Summary
by MITRE • 06/10/2021
A reflected Cross-Site Scripting (XSS) flaw was found in RESTEasy in all versions of RESTEasy up to 4.6.0.Final, where it did not properly handle URL encoding when calling @javax.ws.rs.PathParam without any @Produces MediaType. This flaw allows an attacker to launch a reflected XSS attack. The highest threat from this vulnerability is to data confidentiality and integrity.
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Analysis
by VulDB Data Team • 06/13/2021
The vulnerability CVE-2021-20293 represents a critical reflected cross-site scripting flaw within the RESTEasy framework that affects all versions up to 4.6.0.Final. This vulnerability stems from improper handling of URL encoding mechanisms when processing javax.ws.rs.PathParam annotations without explicit Produces MediaType declarations. The flaw exists at the core of how RESTEasy processes incoming HTTP requests and translates URL parameters into application logic, creating an exploitable condition that can be leveraged by malicious actors to execute arbitrary JavaScript code within victim browsers.
The technical implementation of this vulnerability occurs when RESTEasy processes URL path parameters that contain encoded characters without proper sanitization or validation. When an application uses PathParam annotations in REST endpoints without specifying a Produces MediaType, the framework fails to properly decode and validate URL-encoded input before incorporating it into HTTP response content. This creates a scenario where maliciously crafted URLs containing encoded script payloads can be reflected back to users through HTTP responses, particularly in error messages or dynamic content generation. The vulnerability specifically manifests when RESTEasy's internal parameter processing logic does not adequately handle the transition from URL-encoded strings to their decoded forms, allowing script code to persist and execute in the victim's browser context.
The operational impact of this vulnerability extends beyond simple script execution, as it can compromise both data confidentiality and integrity within affected applications. Attackers can exploit this flaw to steal session cookies, perform unauthorized actions on behalf of users, redirect victims to malicious sites, or inject persistent malware payloads. The reflected nature of the attack means that the malicious payload must be crafted specifically for each victim, requiring targeted delivery through phishing emails, compromised links, or social engineering tactics. The vulnerability affects applications that rely on RESTEasy's default parameter handling behavior, making it particularly dangerous in enterprise environments where RESTEasy is widely deployed for building web services and APIs.
Organizations should immediately implement mitigations including upgrading to RESTEasy versions 4.6.1.Final or later where this vulnerability has been addressed through proper URL decoding and input validation mechanisms. Additional protective measures include implementing comprehensive input validation at application level, employing Content Security Policy (CSP) headers to restrict script execution, and conducting thorough security testing of all REST endpoints that utilize @PathParam annotations. The vulnerability aligns with CWE-79 (Cross-site Scripting) and represents a specific implementation weakness in RESTEasy's parameter handling that can be mapped to ATT&CK technique T1566.001 (Phishing: Spearphishing Attachment) and T1059.007 (Command and Scripting Interpreter: JavaScript) when exploited in real-world scenarios. Organizations should also consider implementing web application firewalls and runtime application self-protection mechanisms to detect and prevent exploitation attempts targeting this specific vulnerability pattern.