CyberChef: Prototype pollution in Series Chart operation

On 5 June 2026 CyberChef received a security vulnerability report from @hyuunnn detailing a vulnerability in the Series Chart operation, where malicious input could result in prototype pollution of the data structures outputted from the operation. Other operations following Series Chart could have their behaviour manipulated by the attacker-controlled prototype, for example, injecting malicious content into their HTML output.

In this case, a demonstration was provided that chained the Series Chart operation with the Parse UDP operation to cause attacker controlled JavaScript to be executed.

Details

When the Series Chart operation parses user-supplied CSV, it accepts __proto__ as a key and uses it to pollute the object. The objToTbale function in src/core/lib/Protocol.mjs, used in a number of IP parsing operations, renders data items within <td> tags without escaping. These bugs can be chained to produce an XSS vulnerability.

Proof of Concept https://gchq.github.io/CyberChef/#recipe=Series_chart('CRLF','Comma','',1,'')Parse_UDP('Raw')&input=X19wcm90b19fLDxpbWcgc3JjPXggb25lcnJvcj1hbGVydChkb2N1bWVudC5kb21haW4pPiwxParse_UDP('Raw')&input=X19wcm90b19fLDxpbWcgc3JjPXggb25lcnJvcj1hbGVydChkb2N1bWVudC5kb21haW4pPiwx)

Encoded input data is:

__proto__,<img src=x onerror=alert(document.domain)>,1

Impact

Objects with attacker controlled prototypes could be processed by CyberChef operations. This enabled control of the output of some operations, and in particular, the ability to insert malicious JavaScript into the output of the Parse UDP operation.

Patches

This has been patched in release v11.2.0.

Workarounds

No workarounds are available. Users must upgrade to CyberChef v11.2.0.