Trustwave's 2024 Retail Report Series Highlights Alarming E-Commerce Threats and Growing Fraud Against Retailers. Learn More

Trustwave's 2024 Retail Report Series Highlights Alarming E-Commerce Threats and Growing Fraud Against Retailers. Learn More

Services
Managed Detection & Response

Eliminate active threats with 24/7 threat detection, investigation, and response.

Co-Managed SOC (SIEM)

Maximize your SIEM investment, stop alert fatigue, and enhance your team with hybrid security operations support.

Advisory & Diagnostics

Advance your cybersecurity program and get expert guidance where you need it most.

Penetration Testing

Test your physical locations and IT infrastructure to shore up weaknesses before exploitation.

Database Security

Prevent unauthorized access and exceed compliance requirements.

Email Security

Stop email threats others miss and secure your organization against the #1 ransomware attack vector.

Digital Forensics & Incident Response

Prepare for the inevitable with 24/7 global breach response in-region and available on-site.

Firewall & Technology Management

Mitigate risk of a cyberattack with 24/7 incident and health monitoring and the latest threat intelligence.

Solutions
BY TOPIC
Microsoft Security
Unlock the full power of Microsoft Security
Offensive Security
Solutions to maximize your security ROI
Rapidly Secure New Environments
Security for rapid response situations
Securing the Cloud
Safely navigate and stay protected
Securing the IoT Landscape
Test, monitor and secure network objects
Why Trustwave
About Us
Awards and Accolades
Trustwave SpiderLabs Team
Trustwave Fusion Security Operations Platform
Trustwave Security Colony
Partners
Technology Alliance Partners
Key alliances who align and support our ecosystem of security offerings
Trustwave PartnerOne Program
Join forces with Trustwave to protect against the most advance cybersecurity threats

ModSecurity Rule for Full-width/Half-width Unicode Evasion Detection

You have probably heard it by now, but US-CERT released a Vulnerability Note last week entitled "HTTP content scanning systems full-width/half-width Unicode encoding bypass." The short of it is that many HTTP content scanning systems (think IDS/IPS/WAFs) may not be able to properly decode data that is encoded using Unicode full-width/half-width encoding thus allowing a possible evasion issue for malicious traffic.

This is yet another Impedance Mismatch issue where one host may interpret data a certain way while another interprets in differently. In this case, security devices that use decoding functions may not properly decode the data and cannot therefore apply certain signatures. If this is the case, then you would have a false negative if the request was malicious and the destination host is able to decode the data and process it.

The $1,000,000 question here is does this issue affect ModSecurity? Yes. The %u syntax is a Microsoft-specific extension. While ModSecurity does not decode such encodings by default (meaning you have to explicitly address the issue in your rules if you need this feature) you can choose to decode them using the urlDecodeUni transformation function. In the current version of ModSecurity this transformation function cannot deal with the above-mentioned evasion technique. (On the positive side, the transformation function behaves exactly as documented in the reference manual.) Fortunately, it is quite easy to create a ModSecurity rule that can identify and block any use of this type of encoding. Here is the example rule that you can use:

# Disallow use of full-width unicode
SecRule REQUEST_FILENAME|ARGS|ARGS_NAMES|REQUEST_HEADERS|XML:/*|!REQUEST_HEADERS:Referer \
"\%u[fF]{2}[0-9a-fA-F]{2}" \
"t:none,deny,log,auditlog,status:400,msg:'Unicode Full/Half Width Abuse Attack Attempt',id:'950116',severity:'4'"

This rule is also included in the latest development release of the Core Rules.

ABOUT TRUSTWAVE

Trustwave is a globally recognized cybersecurity leader that reduces cyber risk and fortifies organizations against disruptive and damaging cyber threats. Our comprehensive offensive and defensive cybersecurity portfolio detects what others cannot, responds with greater speed and effectiveness, optimizes client investment, and improves security resilience. Learn more about us.

Latest Intelligence

Discover how our specialists can tailor a security program to fit the needs of
your organization.

Request a Demo