GHSA-72qq-p3r5-f7wq: XSS

Summary

The openUrl function in @a2ui/webcore passes an agent-controlled URL directly to window.open() without validating the URI scheme. A malicious agent can supply a javascript: URI as the url argument of a Button component's functionCall action. When the user clicks the rendered button, arbitrary JavaScript executes in the victim application's browser origin, constituting a stored/reflected XSS. No non-default configuration is required; the Basic Catalog is enabled by default.

Details

The vulnerability exists in the openUrl function implementation within the Basic Catalog of @a2ui/webcore (commit 23a003248abbf59da6c376ea64ace91d82d209ff).

Sink — renderers/webcore/src/v09/basiccatalog/functions/basicfunctions.ts:428-430:

ts export const OpenUrlImplementation = createFunctionImplementation(OpenUrlApi, args => { if (args.url && typeof window !== 'undefined' && window.open) { window.open(args.url, 'blank'); } });

window.open is called unconditionally with the agent-supplied args.url value. No scheme allowlist or blocklist is applied.

Insufficient schema validation — renderers/webcore/src/v09/basiccatalog/functions/basicfunctionsapi.ts:453-458:

ts export const OpenUrlApi = { name: 'openUrl' as const, returnType: 'void' as const, schema: z.object({ url: z.preprocess(v => (v === undefined ? undefined : String(v)), z.string()), }), };

The Zod schema only requires a string; javascript: URIs pass validation without any rejection.

Full source-to-sink data flow:

1. renderers/webcore/src/v09/basiccatalog/components/basiccomponents.ts:356 — ButtonApi accepts action: ActionSchema (entry point). 2. renderers/webcore/src/v09/schema/common-types.ts:126-130 — ActionSchema permits { functionCall: FunctionCallSchema }. 3. renderers/webcore/src/v09/rendering/generic-binder.ts:243-255 — on click, bound action calls resolveDeepSync then dispatchAction. 4. renderers/webcore/src/v09/rendering/data-context.ts:93-105 — resolveDynamicValue detects the call key and invokes the named function. 5. renderers/webcore/src/v09/catalog/types.ts:177-186 — catalog invoker runs fn.schema.parse(rawArgs) then fn.execute(). 6. renderers/webcore/src/v09/basiccatalog/functions/basicfunctionsapi.ts:453-458 — OpenUrlApi validates url as z.string() only (no scheme check). 7. renderers/webcore/src/v09/basiccatalog/functions/basicfunctions.ts:428-430 — sink: window.open(args.url, 'blank') executes the javascript: URI.

All three renderers implemented in the A2UI repository are affected:

- React: renderers/react/src/v09/catalog/basic/components/Button.tsx:33 — onClick={props.action} - Lit: renderers/lit/src/v09/catalogs/basic/components/Button.ts:112 — @click=${() => props.action()} - Angular: renderers/angular/src/v09/catalog/basic/button.component.ts:103-110 — handleClick() → dataContext.resolveAction → dispatchAction

Any other A2UI renderer which depends on webcore and uses its basic catalog implementation is also affected.

Remediation

The issue was fixed in https://github.com/a2ui-project/a2ui/pull/1707 and released in web\core version 0.10.2, by blocking URLs that do not use the HTTP or HTTPS schemes, or those that are invalid.

Please fix this issue in your project by depending on a @a2ui/web\core version equal or greater than 0.10.2.