OWASP Cheat Sheet Series

Ideas you will meet

Web Security

OWASP risks, XSS, CSRF, CORS and secure design.

What we already know about this

Our own lessons on these ideas. Read them here before you start, so it makes sense.

• Real-World Systems · 10 minSecuring a web APIAuthentication, authorisation, input handling and abuse protection: the layers that keep an API safe.Read hereClose

  Think in layers

  No single control secures an API. Defence comes from several layers, each assuming the others may fail.

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  Figure SDA-RWS-14-1 · API security layers · © forcsguy.com

  1. Transport

  Use HTTPS everywhere, with modern TLS and HSTS. Never accept credentials over plain HTTP.

  2. Authentication: who is calling?

  • Users: OAuth 2.0 / OpenID Connect access tokens, or secure session cookies for browsers (see the earlier lessons).
  • Machines: API keys or client credentials, stored as hashes, scoped, and rotatable.
  • Send credentials in the Authorization header, never in URLs.
  • Keep token lifetimes short, support revocation, and protect refresh tokens carefully.
  • Add multi-factor authentication for sensitive accounts, and protect login endpoints with rate limits.

  3. Authorisation: what may they do?

  This is where most real API breaches happen.

  • Check on every request, on the server, both the action (role or scope) and the specific object. "Is this user logged in?" is not enough; ask "may this user read order 123?" (broken object-level authorisation is the top API risk).
  • Apply least privilege: tokens carry only the scopes they need.
  • Never rely on the client hiding buttons.
  • Do not expose sequential ids as a security measure. Use authorisation checks (random ids only help a little).

  4. Input and output

  • Validate every input against a schema: types, lengths, ranges, allowed values. Reject unknown fields (prevents mass assignment, where a caller sets fields like isAdmin).
  • Use parameterised queries to prevent SQL injection, and encode output to prevent injection into other systems.
  • Limit request size and file types, and scan uploads.
  • Return only the fields needed (avoid excessive data exposure), and paginate lists.
  • Return generic error messages to clients and keep detail in logs, so errors do not leak internals.

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  Figure SDA-RWS-14-2 · Two ways a caller proves who it is · © forcsguy.com

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  Figure SDA-RWS-14-3 · Signing each request with a shared secret (HMAC) · © forcsguy.com

  Image: Spanish Coches · CC BY 2.0 · Source · Wikimedia Commons: Peaje - Toll - Péage (5020183265)

  5. Abuse protection

  • Rate limiting per user, key and IP, with stricter limits on expensive or sensitive endpoints.
  • Quotas and timeouts; protect against huge queries (depth limits in GraphQL, page size caps).
  • Bot and credential-stuffing defences: lockouts, delays, CAPTCHAs, breached-password checks.

  6. Webhooks and service-to-service calls

  • Sign outgoing webhooks with an HMAC over the body and a timestamp. Receivers verify the signature in constant time, reject old timestamps and handle events idempotently.
  • Between services use mutual TLS or signed tokens, and network policies, instead of trusting "internal" traffic.

  7. Browsers: CORS and cookies

  • Configure CORS with an explicit allow-list of origins. Never reflect any origin with credentials enabled.
  • Cookies: HttpOnly, Secure, SameSite. Protect state-changing requests from CSRF.

  An example worth knowing: in 2021 a flaw in a popular Java logging library (Log4Shell) let attackers run their own code simply by getting a crafted string written to a log.

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  Figure SDA-RWS-14-4 · How untrusted text became remote code execution · © forcsguy.com

  8. Secrets, data and visibility

  • Keep secrets in a secrets manager, not in code or images, and rotate them.
  • Encrypt sensitive data at rest, and minimise what you store.
  • Log authentication events and authorisation failures (never secrets), and alert on anomalies. Keep an inventory of your APIs so old versions and forgotten endpoints do not stay exposed.
  • Keep dependencies patched, and test: security tests, dependency scanning, and periodic penetration testing.

  More on this site

  Related reading

  • MDN: Cross-Origin Resource Sharing (CORS)MDN contributors · MozillaDocumentation
  • Introduction to JSON Web TokensAuth0 · jwt.ioDocumentation
  • RFC 6749: The OAuth 2.0 Authorization FrameworkIETFDocumentation
  • AWS Security BlogAmazon Web Services engineers · Amazon Web ServicesBlogs

  Show 6 moreShow fewer

  • OWASP Cheat Sheet SeriesOWASP · OWASP FoundationDocumentation
  • Google Security BlogGoogle engineers · GoogleBlogs
  • How we built rate limiting capable of scaling to millions of domainsCloudflare engineering · CloudflareCase Studies
  • OWASP Top 10OWASP · OWASP FoundationDocumentation
  • Rate LimitingKeycdn engineers · KeycdnCase Studies
  • Scalability and Authentication at TwitchTwitch engineers · TwitchCase Studies

  Open this lesson with its quiz and exercise

• Networking and Web Security Deep Dive · 14 minWeb attacks: XSS, CSRF, injection, SSRF and clickjackingHow each major web attack works in plain terms and the few defences that stop most of them.Read hereClose

  Attacker mindset

  Most web attacks share one cause: the application trusts input, or trusts the browser, more than it should. The fixes are few and repeatable. This lesson covers five of the most important attacks; the full checklist is in the OWASP lesson. (Authentication and sessions are in the passwords and securing-an-API lessons.)

  1. Cross-site scripting (XSS)

  What: attacker-controlled text is put into a page and the browser runs it as script. The script runs as your site, so it can read the page, act as the user and steal tokens.

  Types: stored (saved in your database, such as a comment), reflected (echoed from a URL parameter), DOM-based (client-side code writes untrusted data into the page unsafely).

  // vulnerable: untrusted text inserted as HTML
  el.innerHTML = comment.text;             // <img src=x onerror=stealCookies()> runs!
  
  // safe: insert as text
  el.textContent = comment.text;
  

  Defences

  • Escape output for its context (HTML, attribute, JavaScript, URL). Modern frameworks (React, Vue and others) escape by default; avoid raw-HTML escape hatches such as dangerouslySetInnerHTML or v-html with untrusted data.
  • If you must allow rich text, sanitise with a well-maintained library and an allow-list.
  • A Content Security Policy header limits where scripts can come from, reducing damage if something slips through.
  • Mark session cookies HttpOnly so scripts cannot read them.

  2. Cross-site request forgery (CSRF)

  What: a malicious site makes the victim's browser send a request to your site, and the browser automatically attaches the victim's cookies. If the user is logged in, the action happens (change email, transfer money).

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  Figure SDA-NSD-03-1 · How CSRF works and how a token stops it · © forcsguy.com

  Defences

  • SameSite cookies (Lax or Strict) stop browsers sending cookies on most cross-site requests; now the browser default for many cases but set it explicitly.
  • Anti-CSRF tokens: a random value tied to the session that must accompany state-changing requests.
  • Never change state with GET; check the Origin header; require re-authentication for sensitive actions.
  • APIs that use an Authorization header (not cookies) are not vulnerable in the same way, because the browser does not attach it automatically.

  3. Injection (SQL, command, and others)

  What: input is concatenated into an interpreter's command, so the input changes the command's meaning.

  // vulnerable
  db.query(`SELECT * FROM users WHERE email = '${email}'`);   // email = ' OR '1'='1  returns everyone
  
  // safe: a parameterised query keeps data and command separate
  db.query("SELECT * FROM users WHERE email = $1", [email]);
  

  Defences

  • Parameterised queries / prepared statements for SQL; ORMs do this for normal calls (beware raw-SQL escape hatches).
  • Avoid building shell commands from input; use library calls with argument arrays, not strings.
  • Validate input against an allow-list of expected formats, and apply least privilege to database accounts (the web app should not be able to drop tables).
  • The same pattern applies to NoSQL queries, LDAP, XML and template engines. Rule: never build code or queries by joining strings with untrusted data.

  4. Server-side request forgery (SSRF)

  What: the app fetches a URL supplied by the user (a preview, an image import, a webhook test). The attacker supplies an internal address; the server, which sits inside your network, fetches it and may return secrets, such as cloud instance credentials from the metadata endpoint, or probe internal services.

  Defences

  • Prefer an allow-list of destinations; if not possible, block private, loopback and link-local ranges after resolving the name, and re-check on every redirect (and beware of DNS tricks that change the answer between check and use).
  • Allow only http and https; set short timeouts and size limits.
  • Run fetchers in an isolated network with no route to sensitive services; use cloud settings that require a session token for metadata access.
  • Do not return the raw response to the user.

  5. Clickjacking

  What: your page is loaded in a hidden or disguised frame on an attacker's site, and the user is tricked into clicking buttons they cannot see (like "delete account").

  Defences

  • Send Content-Security-Policy: frame-ancestors 'none' (or 'self'), and the older X-Frame-Options: DENY.
  • Require confirmation for sensitive actions.

  One table to remember

  Attack	Root cause	Main defence
  XSS	Untrusted data becomes script in a page	Output escaping, CSP, HttpOnly cookies
  CSRF	Browser sends cookies on forged requests	SameSite cookies, anti-CSRF tokens
  Injection	Data joined into a command	Parameterised queries, validation, least privilege
  SSRF	Server fetches attacker-chosen URLs	Allow-lists, block internal ranges, isolation
  Clickjacking	Page framed by another site	frame-ancestors / X-Frame-Options

  Habits

  • Defence in depth: assume one layer will fail.
  • Validate on the server always; client checks are only for user experience.
  • Use framework defaults and keep dependencies updated.
  • Log and alert on suspicious input patterns and blocked requests.
  • Test: automated scanners, code review checklists and, for important systems, independent penetration tests.

  Real-world reading

  Related reading

  • MDN: Cross-Origin Resource Sharing (CORS)MDN contributors · MozillaDocumentation
  • OWASP Cheat Sheet SeriesOWASP · OWASP FoundationDocumentation
  • OWASP Top 10OWASP · OWASP FoundationDocumentation
  • Introduction to JSON Web TokensAuth0 · jwt.ioDocumentation

  Show 6 moreShow fewer

  • High Performance Browser NetworkingIlya Grigorik · O'ReillyBooks
  • RFC 9114: HTTP/3IETFDocumentation
  • Building Secure and Reliable SystemsGoogle engineers · O'Reilly / GoogleBooks
  • RFC 6749: The OAuth 2.0 Authorization FrameworkIETFDocumentation
  • Syscall Auditing at Scale at SlackScale at Slack engineers · Scale at SlackCase Studies
  • MySQL High Availability at GitHubGitHub Engineering · GitHubCase Studies

  Open this lesson with its quiz and exercise

• Networking and Web Security Deep Dive · 13 minThe OWASP Top 10 and secure designA map of the most common web application risks, what each means in practice and a short review checklist you can apply to any design.Read hereClose

  What the Top 10 is

  OWASP (the Open Worldwide Application Security Project) publishes an awareness list of the most critical web application risks, updated every few years from real data. Treat it as a checklist of categories to think about, not a complete standard. The names and order change between editions, so below the themes are described in stable terms with the current style of names in brackets.

  The recurring risk categories

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  Figure SDA-NSD-05-1 · The risk categories as a review map · © forcsguy.com

  Category	In plain words	Typical prevention
  Broken access control	Users can act outside their permissions (view others' data, call admin features)	Check authorisation on the server for every request and object; deny by default; test with two users
  Cryptographic failures	Sensitive data exposed (no TLS, weak or home-made crypto, plaintext storage)	TLS everywhere, strong standard algorithms, hash passwords with a slow salted hash, manage keys properly
  Injection	Input interpreted as code or commands	Parameterised queries, validation, least privilege (see the attacks lesson)
  Insecure design	The design itself lacks controls	Threat modelling, abuse cases, secure patterns, limits on resources
  Security misconfiguration	Default passwords, open storage buckets, verbose errors, missing headers	Hardened baselines, infrastructure as code, regular scans
  Vulnerable and outdated components	Known flaws in libraries or platforms	Inventory dependencies, update, use automated advisories
  Identification and authentication failures	Weak passwords, no MFA, bad session handling	MFA, rate limiting, safe recovery, strong session management
  Software and data integrity failures	Untrusted updates, insecure CI/CD, unsafe deserialisation	Signed artefacts, protected pipelines, verify dependencies
  Security logging and monitoring failures	Attacks go unnoticed	Log security events, alert, practise incident response
  Server-side request forgery	The server fetches attacker-chosen URLs	Allow-lists, block internal ranges, isolation

  Authentication, authorisation, encryption, hashing

  • Authentication answers "who are you?"; authorisation answers "what may you do?". Do both on the server, every request.
  • Encryption is reversible with a key (protect data in transit and at rest). Hashing is one-way (verify without storing the original): for passwords use a slow, salted algorithm such as Argon2, bcrypt or scrypt, never plain SHA of the password. A salt is a unique random value per password that defeats precomputed tables. (Details in the passwords lesson.)
  • Never invent your own cryptography.

  Secure design in practice

  1. Threat model early. Draw the data flows, list assets, ask "who might attack this and how?" (a simple method: for each component consider spoofing, tampering, repudiation, information disclosure, denial of service and elevation of privilege).
  2. Least privilege everywhere: users, services, database accounts, cloud roles.
  3. Defence in depth: several independent controls.
  4. Secure defaults: deny first; features off until needed.
  5. Fail safe: on error, do not grant access; do not leak details.
  6. Minimise data: do not collect or keep what you do not need.
  7. Automate: dependency scanning, static analysis, secret scanning, container scanning in CI.
  8. Test: unit tests for authorisation rules, automated scans, periodic penetration tests.
  9. Prepare for incidents: logs, alerts, runbooks, key rotation, backups.

  A short review checklist for any feature

  • Who is allowed to do this, and where is that checked?
  • What input comes from users, and how is it validated and encoded?
  • What secrets and personal data are involved, and how are they protected and logged?
  • What can an attacker abuse in volume (rate limits, size limits, cost)?
  • What third-party code or services does it add, and are they maintained?
  • How would we notice if it were abused?

  Where this fits

  Security is a property of the whole system: network edge (previous lessons), application code (attacks lesson), identity (SSO and passwords lessons), data (encryption) and operations (monitoring and incident response). Make it a routine part of design reviews, not a final gate.

  Real-world reading

  Related reading

  • OWASP Cheat Sheet SeriesOWASP · OWASP FoundationDocumentation
  • AWS Security BlogAmazon Web Services engineers · Amazon Web ServicesBlogs
  • Google Security BlogGoogle engineers · GoogleBlogs
  • OWASP Top 10OWASP · OWASP FoundationDocumentation

  Show 6 moreShow fewer

  • Building Secure and Reliable SystemsGoogle engineers · O'Reilly / GoogleBooks
  • Athenz: Fine-Grained, Role-Based Access Control at YahooYahoo engineers · YahooCase Studies
  • Scalable IAM Architecture to Secure Access to 100 AWS Accounts at SegmentSegment engineers · SegmentCase Studies
  • Security Development Lifecycle at SlackSlack engineers · SlackCase Studies
  • Centralized Application Logging at eBayeBay engineers · eBayCase Studies
  • Enterprise Key Management at SlackSlack engineers · SlackCase Studies

  Open this lesson with its quiz and exercise