CVE-2020-1695: Redhat Resteasy XSS Vulnerability

CVE-2020-1695 Overview

A flaw was found in all RESTEasy 3.x.x versions prior to 3.12.0.Final and all RESTEasy 4.x.x versions prior to 4.6.0.Final, where an improper input validation results in returning an illegal header that integrates into the server's response. This flaw may result in an injection, which leads to unexpected behavior when the HTTP response is constructed.

Critical Impact

Attackers can exploit improper input validation in RESTEasy to inject malicious headers into HTTP responses, potentially leading to HTTP response splitting attacks, cache poisoning, or cross-site scripting vulnerabilities.

Affected Products

• Red Hat RESTEasy versions 3.x.x prior to 3.12.0.Final
• Red Hat RESTEasy versions 4.x.x prior to 4.6.0.Final
• Fedora 32 and Fedora 33

Discovery Timeline

• 2020-05-19 - CVE-2020-1695 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-1695

Vulnerability Analysis

This vulnerability stems from improper input validation (CWE-20) in RESTEasy, a JAX-RS implementation and framework for building RESTful web services on the Java platform. When processing user-controlled input that becomes part of HTTP response headers, RESTEasy fails to properly sanitize or validate the input, allowing attackers to inject newline characters (CRLF) or other control characters that can manipulate the HTTP response structure.

The vulnerability affects the header generation mechanism within RESTEasy, where untrusted data can be incorporated into response headers without adequate validation. This creates an HTTP Response Splitting condition where an attacker can terminate the legitimate header section and inject arbitrary headers or even a complete HTTP response body.

Root Cause

The root cause lies in RESTEasy's failure to validate and sanitize input data before incorporating it into HTTP response headers. Specifically, the framework does not properly filter or reject header values containing carriage return (\r) and line feed (\n) characters, which are used as delimiters in the HTTP protocol. This allows attackers to craft malicious input that breaks out of the intended header context.

Attack Vector

The attack can be conducted remotely over the network without requiring authentication or user interaction. An attacker would submit specially crafted input through any RESTEasy endpoint that reflects user-controlled data in HTTP response headers. This could include:

• Custom header values set from request parameters
• Cookie values reflected in responses
• Redirect URLs or location headers derived from user input

By injecting CRLF sequences followed by malicious headers or content, an attacker can achieve HTTP response splitting, potentially leading to cache poisoning, session fixation, or cross-site scripting attacks depending on the application context.

The vulnerability mechanism involves crafting input with embedded CRLF sequences that, when processed by RESTEasy, terminate the current header and allow injection of additional headers. For example, an attacker might provide a value like legitimate-value\r\nX-Injected-Header: malicious-value which, when not properly sanitized, would result in two separate headers being sent to the client. For detailed technical analysis, refer to the Red Hat Bug Report CVE-2020-1695.

Detection Methods for CVE-2020-1695

Indicators of Compromise

• Unusual HTTP response headers appearing in web server logs that contain unexpected newline characters or multiple header values
• Evidence of CRLF injection patterns (%0d%0a, \r\n) in application request logs
• Cache entries containing unexpected or duplicated content that may indicate cache poisoning
• Reports of XSS or session manipulation from users despite proper input sanitization elsewhere in the application

Detection Strategies

• Implement web application firewall (WAF) rules to detect and block CRLF injection patterns in incoming requests
• Deploy runtime application self-protection (RASP) solutions to monitor header generation and flag anomalous patterns
• Use dependency scanning tools to identify vulnerable RESTEasy versions (3.x.x < 3.12.0.Final and 4.x.x < 4.6.0.Final) in your Java applications
• Review application code for endpoints that reflect user input in response headers

Monitoring Recommendations

• Enable detailed logging for HTTP response headers in web servers and application frameworks
• Configure SIEM rules to alert on patterns indicative of HTTP response splitting attempts
• Monitor for unusual cache behavior that could indicate successful cache poisoning attacks
• Implement anomaly detection for HTTP response sizes and header counts that deviate from expected baselines

How to Mitigate CVE-2020-1695

Immediate Actions Required

• Upgrade RESTEasy 3.x.x installations to version 3.12.0.Final or later
• Upgrade RESTEasy 4.x.x installations to version 4.6.0.Final or later
• Audit application code for endpoints that incorporate user input into HTTP response headers
• Deploy WAF rules to block CRLF injection patterns as a temporary mitigation while patching

Patch Information

Red Hat has released patched versions of RESTEasy that address this vulnerability. The fix is included in RESTEasy 3.12.0.Final for the 3.x branch and 4.6.0.Final for the 4.x branch. Fedora users should apply the updates announced in the Fedora Package Announcement. For additional details, consult the Red Hat Bug Report CVE-2020-1695.

Workarounds

• Implement custom input validation filters at the application level to reject header values containing CRLF characters before they reach RESTEasy
• Use a reverse proxy or WAF configured to sanitize or reject requests containing HTTP response splitting patterns
• Review and restrict which application endpoints can set custom HTTP headers based on user input
• Consider implementing Content Security Policy (CSP) headers to mitigate potential XSS impact from successful exploitation

# Example: Check RESTEasy version in Maven project
mvn dependency:tree | grep resteasy

# Update RESTEasy dependency in pom.xml to patched version
# For 3.x branch: <version>3.12.0.Final</version>
# For 4.x branch: <version>4.6.0.Final</version>