> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/web/xss/reflected-xss-with-custom-tags-only.md).
# XSS Reflected with Only Custom Tags
XSS Reflected with Only Custom Tags notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Reflected XSS Into HTML Context with All Tags Blocked Except Custom Ones
This application blocks all "classic" HTML tags; Only custom elements are allowed. The goal of the lab is to inject a custom tag that automatically executes `alert(document.cookie)` (here you want to steal the session cookie via the alert).
Observations and approach (reworded)
* As all standard tags are filtered, the technique is to define/insert a custom tag that the browser will accept.
* A custom tag may contain event managers — but some events only occur if the element can receive focus.
* Custom items are not focalisable by default; It is therefore necessary to force the focus by adding `tabindex=1` in order for the related event to run when interacting (e.g. by clicking or tabulating).
1. Simple tag — non-focusable (does not execute alert):
```javascript
HELLO
```
2. Same tag, made focalisable with `tabindex` — `onfocus` works:
```javascript
HELLO
```
Final Proof of Concept (exfiltration via redirection)
* We build a URL that injects the custom tag with a `onfocus` calling `alert(document.cookie)` and force the focus by `tabindex=1`.
* Example payload to send to the victim (injection URL-encoded in the `search` parameter):
{% code overflow="wrap" %}
```javascript
```
{% endcode %}
---
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