Category: Vulnerability and Software Security

Secure Coding

Also known as: Secure Programming

Simply put

Secure coding is the practice of writing software in a way that helps defend against cyberattacks and reduces security vulnerabilities. It involves following a set of programming practices and guidelines intended to make software more robust, reliable, and resilient. The goal is to find, fix, and prevent weaknesses in code before they can be exploited.

Formal definition

Secure coding, also referred to as secure programming, is a discipline of writing source code according to defined practices and guidelines that help defend against exploitation and reduce the introduction of security vulnerabilities. It draws on structured references such as the OWASP Secure Coding Practices Quick Reference Guide, and is reinforced through hands-on developer security training focused on finding, fixing, and preventing vulnerabilities in real code. Secure coding is intended to improve software resilience against attacks but does not by itself guarantee freedom from vulnerabilities; its effectiveness depends on implementation, review, and validation. Note that PCI DSS addresses secure software development within its own requirements, and software-specific validation is governed by separate standards such as the PCI Software Security Framework; confirm applicable controls against the current published standard.

Why it matters

Software vulnerabilities are a common entry point for attackers seeking to compromise payment applications and the systems that store, process, or transmit cardholder data. When weaknesses such as injection flaws, broken authentication, or insecure data handling are introduced during development, they can later be exploited to expose sensitive information or disrupt operations. Secure coding aims to reduce the introduction of these weaknesses at the source, so that flaws are found and fixed before code reaches production rather than after an incident.

For organizations in the payments ecosystem, secure coding is directly relevant to compliance obligations. PCI DSS addresses secure software development within its own requirements, and software-specific validation is governed by separate standards such as the PCI Software Security Framework. Because requirement numbering and wording differ between versions, teams should confirm applicable controls against the current published standard rather than assuming a fixed reference. Adhering to structured practices helps demonstrate that development follows defined guidelines intended to make software more robust and resilient.

It is important to set realistic expectations: secure coding is intended to improve software resilience against attacks but does not by itself guarantee freedom from vulnerabilities. Its effectiveness depends on implementation, review, and validation. Secure coding practices work best as part of a broader program that includes developer training, code review, and testing, rather than as a single control that eliminates risk.

Who it's relevant to

Software developers and engineering teams

Developers who write and maintain payment applications apply secure coding practices and guidelines to reduce the introduction of vulnerabilities. Hands-on training helps them learn how to find, fix, and prevent weaknesses in real code as part of day-to-day development.

Application security teams

Application security specialists use structured references such as the OWASP Secure Coding Practices Quick Reference Guide to define expectations, review code against consistent guidance, and validate that practices are followed. They also help ensure secure coding is reinforced through review and testing, since the practice does not by itself guarantee freedom from vulnerabilities.

Compliance officers and assessors

Those responsible for PCI compliance need to map secure development activities to applicable requirements. PCI DSS addresses secure software development within its own requirements, while software-specific validation is governed by separate standards such as the PCI Software Security Framework; applicable controls should be confirmed against the current published standard.

Engineering and product leaders

Leaders responsible for software delivery invest in developer security training and secure coding programs to improve software resilience against attacks. They should treat secure coding as one component of a broader assurance program rather than a standalone guarantee against exploitation.

Inside Secure Coding

Input validation and output encoding

Practices that treat untrusted input as potentially hostile, validating it against expected format and length, and encoding output to the appropriate context. These help reduce injection and cross-site scripting risks but do not by themselves guarantee an application is free of vulnerabilities.

Protection of cardholder data in code

Coding practices that avoid persisting sensitive authentication data (full track data, CAV2/CVC2/CVV2/CID, PINs and PIN blocks) after authorization, even in encrypted form, and that apply defined controls such as masking, truncation, or tokenization to cardholder data (PAN, cardholder name, expiration date, service code) where retention is justified.

Authentication and session management

Code-level handling of credentials, session tokens, and access controls. Secure coding addresses how authentication is implemented in software, which is distinct from broader authentication schemes such as EMV chip authentication, 3-D Secure, or multi-factor authentication that operate at other points in a transaction.

Error handling and logging

Handling exceptions without exposing sensitive data or internal detail, and generating logs that support monitoring while avoiding the recording of sensitive authentication data or full PAN in cleartext.

Use of vetted cryptographic functions

Reliance on established, standard cryptographic libraries rather than custom implementations. Note that encryption, tokenization, truncation, masking, and hashing transform data differently, and their effect on scope depends on implementation and validation, not on the label used.

Relationship to PCI standards

Secure coding expectations for payment applications and software are addressed within PCI DSS software development requirements and, separately, within the PCI Software Security Framework, which supersedes the earlier PA-DSS. Requirement numbering and wording differ between PCI DSS versions, so readers should confirm against the current published standard.

Common questions

Answers to the questions practitioners most commonly ask about Secure Coding.

Does secure coding by itself make an application PCI DSS compliant?

No. Secure coding practices help reduce common software vulnerabilities, but they are one component among many. PCI DSS addresses application security through requirements covering the software development lifecycle, code review, and protection against known vulnerability classes, but compliance also depends on network controls, access management, monitoring, encryption of cardholder data, and other areas. Note that specific requirement numbering and wording differ between PCI DSS versions, so confirm the exact expectations against the current published standard. Secure coding is intended to lower risk, not to establish compliance on its own.

Is secure coding the same thing as satisfying PA-DSS or the PCI Software Security Framework?

No. Secure coding is a set of development practices, while PA-DSS and the PCI Software Security Framework are distinct standards with their own validation processes. Payment software validation historically fell under PA-DSS and has been transitioning to the PCI Software Security Framework, which includes the Secure Software Standard and the Secure Software Lifecycle Standard. Applying secure coding techniques may support meeting those standards, but the standards impose their own defined requirements and formal assessment. Confirm which standard applies to your software and validate against its current published version.

How should input validation be applied to reduce injection risks?

Input validation is intended to help reduce injection and related risks by constraining data to expected formats, lengths, and types before it is processed. Common practices include validating on the server side rather than relying solely on client-side checks, using allow-lists where feasible, and combining validation with context-appropriate output encoding and parameterized queries. Note that validation alone does not eliminate injection risk; it is one layer that works alongside other controls, and its effectiveness depends on consistent implementation across all input paths.

How can secure coding practices avoid exposing cardholder data or sensitive authentication data?

Developers should ensure that sensitive authentication data, such as full track data, card verification values, and PINs or PIN blocks, is not stored after authorization, even in encrypted form. For cardholder data such as PAN, secure coding should support approved handling controls, for example masking in displays, truncation where full values are not needed, and avoiding logging of full PAN or authentication data. The transformation applied, whether encryption, tokenization, truncation, masking, or hashing, affects protection and potential scope differently, and its effect depends on implementation and validation rather than the label alone.

What role does code review play in a secure coding process?

Code review is intended to identify security-relevant defects before code reaches production, and it may include manual peer review, automated static analysis, or a combination. Reviews commonly focus on injection risks, authentication and session handling, access control, error handling, and secure storage and transmission of data. Note that reviews carry trade-offs: automated tools can produce false positives and false negatives, and manual review depth varies with reviewer skill and time. Confirm code review expectations against the current published version of the applicable standard, as wording differs between versions.

How should developers handle secrets such as keys and credentials in code?

Secure coding practices generally call for keeping secrets such as cryptographic keys, passwords, and API credentials out of source code and version control, and instead managing them through protected mechanisms such as secrets management systems or secure configuration. This is intended to help reduce exposure from source code disclosure or repository access. Handling of cryptographic keys used to protect cardholder data is also subject to key management requirements in the applicable standard, so coordinate coding practices with those controls and confirm the current requirements.

Common misconceptions

Encrypting sensitive authentication data in code makes it acceptable to store after authorization.

Sensitive authentication data such as full track data, CAV2/CVC2/CVV2/CID, and PINs or PIN blocks must not be stored after authorization even when encrypted. Encryption in code does not create an exception to this prohibition.

Secure coding is fully governed by PCI DSS alone.

Software security expectations span multiple standards. PCI DSS includes software development requirements, while the PCI Software Security Framework (which supersedes PA-DSS) governs validated payment software separately. The applicable standard depends on the software and how it is deployed and validated.

Applying input validation and using a crypto library guarantees a secure application.

These practices are intended to help reduce specific classes of vulnerability, but no single control eliminates risk. Secure coding is a layered discipline, and its effectiveness depends on correct, validated implementation across the codebase.

Best practices

Validate untrusted input against expected type, length, and format, and apply context-appropriate output encoding to help reduce injection and scripting vulnerabilities.

Ensure code never persists sensitive authentication data after authorization, and apply defined controls such as tokenization, truncation, or masking to cardholder data where retention is justified.

Use vetted, standard cryptographic libraries instead of custom implementations, and choose the data-protection method (encryption, tokenization, truncation, masking, or hashing) based on validated effect rather than label.

Implement error handling and logging so that exceptions and log entries do not expose sensitive authentication data or cleartext PAN.

Confirm secure coding requirements against the current published PCI DSS and, where applicable, the PCI Software Security Framework, rather than relying on fixed requirement numbers from prior versions.

Incorporate code review and security testing throughout development to identify weaknesses early, recognizing that testing reduces but does not eliminate residual risk.