CVE-2021-0268: Juniper Junos J-web XSS Vulnerability

CVE-2021-0268 Overview

CVE-2021-0268 is an HTTP Response Splitting vulnerability affecting the J-web interface of Juniper Networks Junos OS. This weakness stems from improper neutralization of CRLF (Carriage Return Line Feed) sequences in HTTP headers, which can lead to buffer overflows, segmentation faults, and other severe impacts. An unauthenticated attacker can exploit this vulnerability to modify the integrity of affected devices and exfiltrate sensitive information without requiring any authentication.

The vulnerability can be leveraged to facilitate cross-site scripting (XSS) attacks, cookie manipulation including modifying session cookies and stealing authentication cookies, and other web-based attacks. Attackers can exploit this weakness by directing legitimate users to malicious links that appear to originate from the affected Juniper device's web interface.

Critical Impact

Unauthenticated remote attackers can modify device integrity, steal sensitive data, and execute XSS attacks through the J-web management interface without requiring any special access or permissions.

Affected Products

• Juniper Networks Junos OS 18.1 versions prior to 18.1R3-S11
• Juniper Networks Junos OS 18.2 versions prior to 18.2R3-S5
• Juniper Networks Junos OS 18.3 versions prior to 18.3R2-S4 and 18.3R3-S3
• Juniper Networks Junos OS 18.4 versions prior to 18.4R2-S5 and 18.4R3-S3
• Juniper Networks Junos OS 19.1 versions prior to 19.1R2-S2 and 19.1R3-S2
• Juniper Networks Junos OS 19.2 versions prior to 19.2R1-S5 and 19.2R2
• Juniper Networks Junos OS 19.3 versions prior to 19.3R3
• Juniper Networks Junos OS 19.4 versions prior to 19.4R1-S3, 19.4R2, and 19.4R3
• Juniper Networks Junos OS 20.1 versions prior to 20.1R1-S2 and 20.1R2

Discovery Timeline

• 2021-04-22 - CVE-2021-0268 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-0268

Vulnerability Analysis

This vulnerability exists in the J-web component of Juniper Networks Junos OS and is classified under CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) and CWE-79 (Improper Neutralization of Input During Web Page Generation). The J-web interface fails to properly sanitize CRLF sequences in HTTP headers before processing them, allowing attackers to inject malicious content into HTTP responses.

The exploitation of this vulnerability does not require authentication, making it particularly dangerous for organizations that expose J-web interfaces to untrusted networks. When successfully exploited, the vulnerability can cause buffer overflows and segmentation faults in the J-web service, potentially leading to service disruption or arbitrary code execution.

Root Cause

The root cause of this vulnerability lies in the improper input validation within the J-web HTTP header processing logic. The application fails to neutralize or reject CRLF sequences (\r\n) that are embedded in user-controlled input before incorporating that input into HTTP response headers. This allows attackers to terminate the HTTP header section prematurely and inject arbitrary content, including additional HTTP headers or response body content.

Attack Vector

The attack vector is network-based and requires user interaction. An attacker crafts a malicious URL that contains CRLF sequences designed to split the HTTP response. When a victim user clicks on this seemingly legitimate link to the J-web interface, the malicious payload is processed by the vulnerable J-web component. The attacker-controlled content is then injected into the HTTP response, enabling various attack scenarios:

• Cross-Site Scripting (XSS): Injecting malicious JavaScript that executes in the victim's browser context
• Cookie Manipulation: Stealing or modifying session cookies to hijack authenticated sessions
• Cache Poisoning: Injecting malicious content that may be cached by intermediate proxies
• Content Injection: Displaying phishing content or malicious forms to unsuspecting users

The attack mechanism typically involves crafting a URL containing encoded CRLF characters (%0d%0a) followed by malicious headers or content. When the J-web interface processes this input without proper sanitization, the response is split, allowing the attacker's payload to be interpreted as part of the legitimate response.

Detection Methods for CVE-2021-0268

Indicators of Compromise

• Unusual HTTP requests to J-web interface containing URL-encoded CRLF sequences (%0d%0a or %0D%0A)
• HTTP access logs showing requests with abnormally long or malformed header content
• Unexpected JavaScript execution or cookie modifications reported by users accessing J-web
• Evidence of session cookie theft or unauthorized administrative access attempts

Detection Strategies

• Monitor J-web access logs for requests containing CRLF character encodings in URL parameters or headers
• Implement web application firewall (WAF) rules to detect and block HTTP response splitting attempts
• Deploy network intrusion detection signatures targeting CRLF injection patterns in HTTP traffic to J-web interfaces
• Review authentication logs for anomalous session activity that may indicate cookie theft

Monitoring Recommendations

• Enable detailed logging for all J-web interface access and review logs regularly for suspicious patterns
• Configure alerts for multiple failed authentication attempts or session anomalies following J-web access
• Monitor network traffic to J-web interfaces for requests containing encoded special characters
• Implement browser-based security controls and Content Security Policy headers where possible

How to Mitigate CVE-2021-0268

Immediate Actions Required

• Upgrade affected Junos OS installations to the patched versions specified in the Juniper security advisory
• Disable J-web interface on devices that do not require web-based management
• Restrict J-web access to trusted management networks only using firewall rules or access control lists
• Implement strong session management practices and educate users about suspicious links

Patch Information

Juniper Networks has released patches addressing this vulnerability across multiple Junos OS versions. Organizations should upgrade to the following minimum versions:

• Version 18.1: Upgrade to 18.1R3-S11 or later
• Version 18.2: Upgrade to 18.2R3-S5 or later
• Version 18.3: Upgrade to 18.3R2-S4, 18.3R3-S3, or later
• Version 18.4: Upgrade to 18.4R2-S5, 18.4R3-S3, or later
• Version 19.1: Upgrade to 19.1R2-S2, 19.1R3-S2, or later
• Version 19.2: Upgrade to 19.2R1-S5, 19.2R2, or later
• Version 19.3: Upgrade to 19.3R3 or later
• Version 19.4: Upgrade to 19.4R1-S3, 19.4R2, 19.4R3, or later
• Version 20.1: Upgrade to 20.1R1-S2, 20.1R2, or later

For complete patch information, refer to the Juniper Security Advisory JSA11159.

Workarounds

• Disable J-web interface entirely if web-based management is not required using CLI management instead
• Implement strict network access controls to limit J-web access to trusted internal management networks only
• Deploy a reverse proxy with HTTP response splitting protection in front of J-web interfaces
• Use SSH-based CLI management as an alternative to web-based administration until patches can be applied

# Disable J-web interface on Junos OS devices
configure
delete system services web-management http
delete system services web-management https
commit
exit