Mobile Bridge

The Mobile Bridge lets the Paylead WebApp (MFP) running inside your native WebView invoke native device capabilities: opening a URL in the system browser, writing to the clipboard, downloading a file. It uses the JSON-RPC 2.0 protocol over postMessage. Your native app receives requests from the WebApp and responds by calling back into JavaScript.

The bridge JavaScript is versioned (v1, v2, …). The machine-readable contract for each version is published as a schema.json. See the Methods reference below for the current version, and the Versions & download page to download a schema or browse older versions.

Transport

iOS (WKWebView)
Android (WebView)

WebApp → NativeThe WebApp posts to a message handler; WKWebView auto-deserializes the JSON, so your handler receives a [String: Any] dictionary.

WebApp

window.webkit.messageHandlers.payleadBridge.postMessage(msg)

Native → WebAppCall back with argument binding; never interpolate the JSON into the source.

Native

webView.callAsyncJavaScript(
  "window.payleadBridge.response(jsonString)",
  arguments: ["jsonString": jsonStr],
  ...
)

WebApp → NativeThe WebApp posts a JSON string that you parse yourself, because Android’s @JavascriptInterface only accepts primitives.

WebApp

window.PayleadBridge.postMessage(JSON.stringify(msg))

Native → WebAppQuote the payload with JSONObject.quote(); never interpolate the JSON into the source.

Native

webView.evaluateJavascript(
  "window.payleadBridge.response(" + JSONObject.quote(jsonStr) + ")",
  null
)

Never interpolate JSON strings into JavaScript source. Use argument binding (callAsyncJavaScript on iOS) or JSONObject.quote() (Android). Direct interpolation such as "response('${json}')" creates a JS injection vector. This rule applies every time you call back into the WebApp.

Message format

Request (WebApp → Native)

Your handler receives JSON-RPC 2.0 request objects:

{
  "jsonrpc": "2.0",
  "method": "browser.open",
  "params": { "url": "https://example.com" },
  "id": "89d89a45-4f57-4e68-9f61-dc71434b2f26"
}

Success response (Native → WebApp)

Call window.payleadBridge.response(jsonString) with a serialized JSON-RPC 2.0 result:

{ "jsonrpc": "2.0", "result": {}, "id": "89d89a45-4f57-4e68-9f61-dc71434b2f26" }

Error response (Native → WebApp)

If the action fails, respond with a JSON-RPC 2.0 error (see Error codes):

{
  "jsonrpc": "2.0",
  "error": { "code": -32000, "message": "Download failed" },
  "id": "89d89a45-4f57-4e68-9f61-dc71434b2f26"
}

Envelope constraints

jsonrpc must equal "2.0". Reject any other value with error code -32600. If the envelope cannot be parsed at all, respond with "id": null and error code -32600.
id must be a UUID v4 string. The WebApp always sends a randomly generated UUID v4; non-string ids are silently dropped by the WebApp.
The id in the response must match the id from the request.
result and error are mutually exclusive. On success include result and omit error; on failure include error and omit result. If the WebApp receives both, error takes precedence and the call rejects.
Timeouts. The WebApp rejects requests that don’t receive a response within 5 seconds for browser.open and clipboard.write, and 30 seconds for file.download. For file.download, respond once the download has been triggered (consent granted, transfer started), not once it completes.

Methods

`browser.open`

Opens a URL in the device’s native browser (outside the WebView).

url

string

required

URL to open. Must use the https:// scheme.Pattern ^https:// · max length 2048

{
  "jsonrpc": "2.0",
  "method": "browser.open",
  "params": {
    "url": "https://www.paylead.fr"
  },
  "id": 1
}

{
  "jsonrpc": "2.0",
  "result": {},
  "id": 1
}

`clipboard.write`

Copies text to the device clipboard.

text

string

required

Text to copy to clipboard.max length 4096

{
  "jsonrpc": "2.0",
  "method": "clipboard.write",
  "params": {
    "text": "voucher:89d89a45-4f57-4e68-9f61-dc71434b2f26"
  },
  "id": 1
}

{
  "jsonrpc": "2.0",
  "result": {},
  "id": 1
}

`file.download`

Downloads a file and presents it to the user.

url

string

required

URL of the file to download. Must use the https:// scheme.Pattern ^https:// · max length 200

filename

string

Suggested filename. Allowed characters: letters, digits, underscore, hyphen, dot. Must start with a letter or digit. The native app must still sanitise before writing to disk.Pattern ^[a-zA-Z0-9][a-zA-Z0-9_\-.]*$ · max length 255

mimeType

string

MIME type hint (e.g. application/pdf).one of application/pdf, text/csv, image/jpeg, image/jpg, image/png

{
  "jsonrpc": "2.0",
  "method": "file.download",
  "params": {
    "url": "https://api.paylead.fr/voucher/89d89a45-4f57-4e68-9f61-dc71434b2f26.pdf",
    "filename": "89d89a45-4f57-4e68-9f61-dc71434b2f26.pdf",
    "mimeType": "application/pdf"
  },
  "id": 1
}

{
  "jsonrpc": "2.0",
  "result": {},
  "id": 1
}

Error codes

When a method fails, the native app responds with a JSON-RPC 2.0 error object carrying one of these codes in error.code. error.message must never contain stack traces, filesystem paths, or other sensitive information.

Code	Description
`-32600`	Invalid JSON-RPC 2.0 request
`-32601`	Method not found
`-32602`	Invalid or missing params
`-32000`	Runtime error (scheme blocked, download failed, etc.) — error.message must not contain stack traces or filesystem paths
`-32001`	Permission denied (location, etc.) — error.message must not contain stack traces or filesystem paths

Integration guide

iOS (Swift / WKWebView)
Android (Kotlin / WebView)

Receive the request

Receive JSON-RPC requests in userContentController(_:didReceive:).

Execute and respond

Execute the action and call back with callAsyncJavaScript using named argument binding.

let config = WKWebViewConfiguration()
config.userContentController.add(self, name: "payleadBridge")
// Recommended (iOS 14+): restrict navigation to your app's domains
config.limitsNavigationsToAppBoundDomains = true

// Hosts allowed to reach the bridge: any sub-domain of paylead.fr / .tech / .eu
let allowedHost = #"\.paylead\.(fr|tech|eu)$"#

func userContentController(_ controller: WKUserContentController,
                           didReceive message: WKScriptMessage) {
    // Reject messages from sub-frames or unknown origins
    guard message.frameInfo.isMainFrame,
          let host = message.frameInfo.request.url?.host,
          host.range(of: allowedHost, options: .regularExpression) != nil else { return }

    guard message.name == "payleadBridge",
          let body = message.body as? [String: Any],
          let jsonrpc = body["jsonrpc"] as? String, jsonrpc == "2.0",
          let method = body["method"] as? String,
          let id = body["id"] as? String else { return }

    let params = body["params"] as? [String: Any] ?? [:]
    // Validate params against the method's JSON Schema (schema.json).
    // On failure respond with error code -32602 (Invalid params).
    guard validateParams(method: method, params: params) else { return }

    handleBridgeMethod(method, params: params) { result in
        let response: [String: Any] = ["jsonrpc": "2.0", "result": result, "id": id]
        guard let data = try? JSONSerialization.data(withJSONObject: response),
              let jsonStr = String(data: data, encoding: .utf8) else { return }
        // Safe: jsonStr is passed as a named argument, no interpolation into JS
        webView.callAsyncJavaScript(
            "window.payleadBridge.response(jsonString)",
            arguments: ["jsonString": jsonStr],
            in: nil, in: .defaultClient, completionHandler: nil
        )
    }
}

Check message.frameInfo.isMainFrame and verify the host against your allowlist before processing any message. Cross-origin iframes inside the WebView otherwise share access to window.webkit.messageHandlers.payleadBridge.
Use limitsNavigationsToAppBoundDomains = true and declare your domains under WKAppBoundDomains in Info.plist (iOS 14+).
Block navigations to hosts outside your allowlist in decidePolicyFor.

Android’s addJavascriptInterface only takes effect on the next page load and, once registered, is exposed to every frame until removed and reloaded. The safest approach is to block navigations to untrusted origins entirely, register the interface once, and re-check the origin on every call as defense in depth.

// Hosts allowed to reach the bridge: any sub-domain of paylead.fr / .tech / .eu
val allowedHost = Regex("""\.paylead\.(fr|tech|eu)$""")

webView.addJavascriptInterface(PayleadBridgeInterface(webView), "PayleadBridge")

webView.webViewClient = object : WebViewClient() {
    override fun shouldOverrideUrlLoading(
        view: WebView, request: WebResourceRequest
    ): Boolean {
        val host = request.url.host ?: return true
        return !allowedHost.containsMatchIn(host) // true = navigation blocked
    }
}

class PayleadBridgeInterface(private val webView: WebView) {
    @JavascriptInterface
    fun postMessage(json: String) {
        webView.post {
            // Defense in depth: re-check the current URL on every call
            val host = Uri.parse(webView.url).host ?: return@post
            if (!allowedHost.containsMatchIn(host)) return@post

            val request = try { JSONObject(json) } catch (_: Exception) { return@post }
            if (request.optString("jsonrpc") != "2.0") return@post
            val method = request.optString("method").ifEmpty { return@post }
            val id = request.optString("id").ifEmpty { return@post }
            val params = request.optJSONObject("params") ?: JSONObject()
            // Validate params against the method's JSON Schema (schema.json).
            // On failure respond with error code -32602 (Invalid params).
            if (!validateParams(method, params)) return@post

            handleBridgeMethod(method, params) { result ->
                val response = JSONObject().apply {
                    put("jsonrpc", "2.0"); put("result", result); put("id", id)
                }
                webView.post {
                    // Safe: JSONObject.quote() properly escapes the JSON string
                    webView.evaluateJavascript(
                        "window.payleadBridge.response(${JSONObject.quote(response.toString())})", null
                    )
                }
            }
        }
    }
}

Block navigations outside your allowlist in shouldOverrideUrlLoading. Removing the interface in onPageStarted/onPageFinished does not take effect until the next reload.
Re-check the current URL from inside postMessage itself.
Disable file access: setAllowFileAccess(false), setAllowFileAccessFromFileURLs(false), setAllowUniversalAccessFromFileURLs(false), setAllowContentAccess(false).
Enforce HTTPS only: setMixedContentMode(WebSettings.MIXED_CONTENT_NEVER_ALLOW).

Security checklist for partners

Before shipping your integration, verify each item:

HTTPS only: every URL received from the bridge is validated to start with https:// before opening or downloading. javascript:, data:, file:, and intent:// schemes are explicitly rejected.
Filename sanitisation: the schema restricts filenames to a safe character set (^[a-zA-Z0-9][a-zA-Z0-9_\-.]*$); as defense in depth, the native app re-validates and strips null bytes / OS-reserved names before writing to disk.
Origin allowlist: the bridge is only reachable from pages whose host matches your trusted allowlist (*.paylead.fr, *.paylead.tech, *.paylead.eu). Navigations outside the allowlist are blocked.
User consent: file.download triggers a native prompt so the user can confirm or cancel.
App-bound domains (iOS 14+): limitsNavigationsToAppBoundDomains = true and WKAppBoundDomains are configured in Info.plist.

General recommendations

Back/forward gestures: disable allowsBackForwardNavigationGestures (iOS) and handle the back button explicitly (Android) to avoid accidental off-domain navigations.
Loading indicators: file.download can take up to 30 seconds; surface progress in your native UI.
Correlation logging: when logging bridge errors natively, include the request id so failures can be cross-referenced with the WebApp’s client-side logs.

Transport

Message format

Request (WebApp → Native)

Success response (Native → WebApp)

Error response (Native → WebApp)

Envelope constraints

Methods

`browser.open`

`clipboard.write`

`file.download`

Error codes

Integration guide

Security checklist for partners

General recommendations

What’s next

Page URLs

Troubleshooting

​Transport

​Message format

​Request (WebApp → Native)

​Success response (Native → WebApp)

​Error response (Native → WebApp)

​Envelope constraints

​Methods

​browser.open

​clipboard.write

​file.download

​Error codes

​Integration guide

​Security checklist for partners

​General recommendations

​What’s next

Page URLs

Troubleshooting

Transport

Message format

Request (WebApp → Native)

Success response (Native → WebApp)

Error response (Native → WebApp)

Envelope constraints

Methods

`browser.open`

`clipboard.write`

`file.download`

Error codes

Integration guide

Security checklist for partners

General recommendations

What’s next