fix(security): base64-encode survey WebView payload to prevent script injection (ENG-1813)

[pandeymangg]

What & why

ENG-1813 — script injection via the survey WebView payload.

The survey payload was spliced into a JavaScript template literal in the WebView HTML:

const json = `{{WEBVIEW_DATA}}`;   // {{WEBVIEW_DATA}} = raw survey JSON

Survey content is server/admin-authored (headlines, subheaders, choice labels, styling, an HTML question type…). A value containing a backtick — or ${…}, which a template literal evaluates eagerly — breaks out of the string and runs as arbitrary JS in the WebView, which also holds the FormbricksJavascript bridge. This is the Android counterpart of the iOS issue (formbricks/ios#51).

The existing #→%23 and \"→' replacements were partial workarounds for the same class of problem: they did not stop backtick/${} breakout, and they corrupted legitimate survey content (every " became ', and hex colors like #FF0000 were percent-mangled).

Fix

Mirror the iOS remediation — never let survey content be parsed as JS source:

• Base64-encode the payload in Kotlin (Base64.NO_WRAP) and decode it in the WebView via atob + TextDecoder, then JSON.parse. Base64 output is [A-Za-z0-9+/=] only, so backticks, ${…}, quotes and </script> can no longer appear in the string and break out.
• Removed the #→%23 and \"→' workarounds — unnecessary once base64'd, and they were corrupting valid survey text (bonus data-integrity fix).
• Switched the single loadData call to loadDataWithBaseURL(null, …) so the base64 (which may contain +, /, =) isn't mangled by loadData's data:-URL percent-decoding. This is the analogue of iOS's loadHTMLString(_, baseURL: nil).

Verification

• ./gradlew :android:compileReleaseKotlin — passes.
• Existing FormbricksViewModelInstrumentedTest (getJson) is unaffected — its assertions don't depend on the removed workarounds.
• Published via publishToMavenLocal and smoke-tested in a host app — surveys render correctly.

Notes / scope

• No automated regression test yet: the SDK's tests are all instrumented (need an emulator). Tracked as a follow-up — the intended guard feeds hostile content (backtick / ${} / </script>) through payload generation and asserts it appears only inside the base64 blob and round-trips intact.
• Part of a cross-SDK sweep of the iOS survey-WebView findings. iOS: fix: harden survey webview — TLS, script injection, URL scheme, inspectable (ENG-1810/1811/1812/1813) ios#51. React Native equivalent: fix(security): escape survey WebView payload to prevent </script> injection (ENG-1813) react-native#72. Flutter was already safe. TLS (MITM), WebView-debug-in-release, and external-URL-scheme checks were also audited on Android and are already safe.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 69b03a2f-3b03-4c15-a63d-fbfb4e8dac5d

📥 Commits

Reviewing files that changed from the base of the PR and between 367db37 and c2266bc.

📒 Files selected for processing (1)

• android/src/main/java/com/formbricks/android/webview/FormbricksViewModel.kt

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Walkthrough

FormbricksViewModel now Base64-encodes UTF-8 survey JSON before embedding it in the WebView HTML. The HTML template decodes the payload with atob and TextDecoder before rendering. JSON escaping workarounds were removed, and the WebView binding adapter now uses loadDataWithBaseURL.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Description check	❓ Inconclusive	No pull request description was provided, so there's not enough text to judge its relevance.	Add a brief description of the WebView payload fix and the issue it addresses.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title matches the main change: fixing the JS payload bug in the WebView flow.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud