The Payment Card Industry Data Security Standard (PCI DSS) applies to all organizations that store, process, or transmit cardholder data or sensitive authentication data, as well as any systems, people, or processes that could affect their security. This includes not only merchants and service providers, but also supporting components such as authentication servers, access controls, and Web Application Firewalls (WAFs).
The OWASP CRS (Core Rule Set) is a widely adopted, open-source set of WAF rules designed to detect and block a broad range of web application attacks. When properly deployed, CRS provides robust, customizable protection against many threats relevant to PCI DSS compliance.
In recent updates, the CRS team decided to remove PCI DSS specific tags from individual rules. As the PCI standard has evolved, at the time of writing it is at version 4.0.1, maintaining a rigid mapping of requirements to specific CRS rule IDs became impractical and potentially misleading. The focus has then shifted to showing how CRS, as a whole, reinforces your PCI DSS compliance posture. This post contributes to that effort by illustrating how CRS adds a critical layer of real-time inspection for web-facing applications.
A First Line of Defense – PCI DSS Requirement 6.4.2
PCI DSS Requirement 6.4.2 mandates that public-facing web applications be protected by “an automated technical solution” that continually detects and prevents web-based attacks (for example, the attacks included in PCI DSS Requirement 6.2.4). CRS is designed precisely for this role: when deployed as part of a WAF, CRS inspects inbound requests in real time, blocking malicious input before it ever reaches the application layer.
Injection attacks, including SQL, XPath and command injection, are covered by rule groups: 930–944, e.g., APPLICATION-ATTACK-SQLI, APPLICATION-ATTACK-RCE, APPLICATION-ATTACK-XSS.
Attacks on data and data structures, such as buffer overflows or payload-based exploitation, are detected through generic anomaly and payload inspection in 933 APPLICATION-ATTACK-PHP, 934 APPLICATION-ATTACK-GENERIC.
Business logic abuse, including misuse of APIs, client-side functionality, or bypass of workflows are addressed via pattern enforcement and heuristics in 911 METHOD-ENFORCEMENT, 920 PROTOCOL-ENFORCEMENT, 922 MULTIPART-ATTACK, 941 APPLICATION-ATTACK-XSS, 943 APPLICATION-ATTACK-SESSION-FIXATION.
Access control bypass attempts, such as session fixation, broken authentication, or forged identity tokens are mitigated in 913 SCANNER-DETECTION, 920 PROTOCOL-ENFORCEMENT, 943 APPLICATION-ATTACK-SESSION-FIXATION.
High-risk vulnerabilities exploitation are prevented, when applicable, with dedicated rules. For example, famous Shellshock or more recent Log4shell vulnerabilities have their own rules respectively in 932 APPLICATION-ATTACK-RCE and 944 APPLICATION-ATTACK-JAVA.
In short, PCI DSS Requirement 6.2.4 calls for secure software development practices to mitigate common attack types. PCI Requirement 6.4.2 then mandates a WAF to defend against those same threats in production. CRS bridges the two - blocking known exploit vectors and offering virtual patching where code changes aren’t yet in place.
Logging and Visibility – PCI DSS Requirement 10.2
CRS doesn’t just stop threats, it logs them. Every triggered rule produces structured audit data aligned with PCI DSS Requirement 10.2: recording request origin, timestamp, rule ID, action taken, and in some cases, matched payload data.
This logging supports incident detection, alerting, and forensic investigations. Integrations with SIEM (Security Information and Event Management) tools enable real-time correlation with other system events, making CRS an effective part of a broader PCI-aligned logging strategy.
Enforcing Policy at Scale – PCI DSS Requirement 12.1
Security policies are only as strong as their implementation. PCI DSS Requirement 12.1 calls for maintaining a formal information security policy and CRS can help to enforce those policies technically.
Using CRS, teams can define positive and negative security models at the traffic level:
• Allow only specific HTTP methods (e.g., GET/POST) - 920 PROTOCOL-ENFORCEMENT
• Reject unexpected encodings or character sets - 920 PROTOCOL-ENFORCEMENT
• Limit request size and parameter complexity
This approach turns abstract policy into concrete, auditable controls making it easier to demonstrate enforcement during assessments.
About Zero-Day attacks
CRS also includes several rules that enforce strict positive filtering across different parts of the HTTP request, efficiently reducing attack surface and reducing vulnerability to “zero-day” attacks.
Some of the most (in)famous Paranoia Level 4 rules, 920273 or 920274 (part of the 920 PROTOCOL-ENFORCEMENT group), are enforcing very strict ASCII character ranges on parameters, request bodies, and headers, limiting input to safe alphanumerics and minimal punctuation.
These controls are not signature-based. Instead, they reject any unexpected character use, making them effective against zero-day payloads that often exploit encoding tricks, parser inconsistencies, or malformed input. Doing so, CRS can block unknown attack techniques before they reach application logic. Log4shell, for example, was blocked at Paranoia Level 4 even before the team had created a dedicated rule to detect the attack at lower Paranoia Levels.
Rules like 920273 and 920274 offer strong zero-day defenses but are also common sources of false positives, blocking legitimate traffic. Overly aggressive tuning can lead to their premature removal, weakening the overall protection.
Conclusion
CRS can significantly strengthen your PCI DSS compliance posture, but its true value depends on thoughtful implementation and careful tuning. False positives are part of the game, especially at higher paranoia levels. This isn’t a flaw, it’s a feature that reflects CRS’s strict security posture. In sensitive environments like online banking, Paranoia Level 3 is the recommended baseline. At that level, tuning isn’t optional, it’s essential. CRS is designed for this, rules can be refined, narrowed, or scoped more precisely without needing to turn off entire protections.
However, tuning requires expertise. It’s easy to go too far - disabling entire rule groups or applying broad exclusions like ignoring all query parameters or disabling a large group of rules by tag. This is where well-intentioned tuning becomes risky. Auditors and reviewers should look for signs of thoughtful deployment: precise targeting, documented rationales for rule removals, and a clear understanding of what’s being excluded and why.
Security shouldn’t be sacrificed for silence in logs. Done right, CRS tuning maintains a strong defense while adapting to the specifics of your applications. It’s not just about making things work - it’s about making them work securely.
Special thanks to the OWASP CRS team and community for their continued work in making the web safer. Deep appreciation to Max and na1ex for their guidance and review.