Ensuring Secure Web Application Output Encoding
Web applications must correctly encode all their outputs to prevent security risks.
Plain language
When web applications output information to users, they need to encode this information properly to prevent security risks like data breaches or malware attacks. If this isn't done, cybercriminals might take advantage of it by tricking the application into running harmful scripts, which could lead to loss of sensitive information or control over the site.
Framework
ASD Information Security Manual (ISM)
Control effect
Preventative
Classifications
NC, OS, P, S, TS
ISM last updated
Feb 2022
Control Stack last updated
19 Mar 2026
E8 maturity levels
N/A
Guideline
Guidelines for software developmentSection
Web application developmentOfficial control statement
Output encoding is performed on all output produced by web applications.
Why it matters
Without output encoding on all web application responses, attackers can inject scripts (XSS), resulting in session theft, data leakage, and site hijacking.
Operational notes
Confirm all dynamic output is context-appropriately encoded (HTML, attribute, JS, URL) and regression test templates and APIs after changes to prevent XSS.
Implementation tips
- The IT team should review the web application code to identify parts where output is sent to users, such as web pages or data files. Check that parts like text boxes and other outputs are properly encoded using established coding libraries or methods. This means turning potential harmful content into harmless text that users and computers can safely read.
- Developers should apply consistent output encoding to all parts of web applications where data is shown to users. Ensure they use reliable coding strategies such as HTML or JavaScript encoding, making it much harder for harmful scripts to run by accident.
- The IT security team should conduct regular tests on web applications using security tools to scan for any areas left without proper encoding. Use online tools or software auditing services to find loopholes where encoding is missing, offering a chance to fix those gaps before any harm occurs.
- Business managers should provide resources and training sessions on secure coding practices for their development teams. Arrange workshops or hire experts to demonstrate how proper encoding can prevent breaches and protect organisational data.
- Chief Information Officers (CIOs) must ensure policies are in place for developers to follow when encoding outputs in web applications. Develop guidelines that detail the accepted encoding techniques and routinely update them as new threats arise or technology evolves.
Audit / evidence tips
- AskThe web application’s development and security policies: Request documentation on encoding practices GoodIncludes clear, current policies with examples of applied encoding techniques
- AskRecords of recent security tests and audits of web applications: Request reports on tests checking for encoding errors GoodWould show regular testing schedules and evidence of resolved issues
- AskTo see training schedules and materials for developers on secure coding practices: Request training content relating to output encoding GoodWould include a regular training schedule with updated material reflecting current best practices
- AskA demonstration of the web application’s output encoding in action: Request access to a controlled application environment to see encoding at work GoodDemonstration shows consistent encoding application and no visible encoding errors
- AskCode review records that include output encoding checks: Request documentation from development cycles showing attention to encoding GoodRecord includes checklists with detailed notes and sign-offs from senior developers or security experts
Cross-framework mappings
How ISM-1241 relates to controls across ISO/IEC 27001, ISO/IEC 42001, Essential Eight, and ASD ISM.
ISO 27001
| Control | Notes | Details |
|---|---|---|
| layers Partially meets (2) expand_less | ||
| Annex A 8.25 | ISM-1241 addresses a specific secure development requirement: encoding all web application output to prevent unsafe interpretation in bro... | |
| Annex A 8.28 | ISM-1241 requires that output encoding is performed on all output produced by web applications to prevent injection-style client-side att... | |
These mappings show relationships between controls across frameworks. They do not imply full equivalence or certification.