In early 2025, security researchers uncovered a surprising logic bug in Android’s HtmlToSpannedParser.java—tracked as CVE-2025-26443. This bug could let malicious apps sidestep the Android OS’s barriers and install new apps without the user ever enabling "Install from unknown sources." This post breaks down how the vulnerability works, shows where the code goes wrong, and links you to more official details.

What is CVE-2025-26443?

CVE-2025-26443 targets the Android platform’s HTML processing—specifically, how parseHtml() in HtmlToSpannedParser.java treats input. Apps can exploit this to bypass one of the most fundamental Android protections: prohibiting app installs unless the user specifically enables "unknown sources." This flaw can lead to a local privilege escalation, meaning a normal app could gain ability to install new apps without extra permissions.

How Does the Flaw Work?

When an Android app displays HTML content—for example, in an SMS, email, or in-app message—it often uses Android’s Html.fromHtml() function, which in turn uses HtmlToSpannedParser. This parser processes HTML tags and can generate click events (like a link).

The problem in parseHtml() is a logic error that confuses the context or origin of certain HTML links.

Here’s an illustrative snippet showing the core of the problem in simplified form

// HtmlToSpannedParser.java

if ("a".equals(name)) {
    String href = getAttribute("href");
    if (href != null) {
        // Should validate or sanitize href source
        ClickableSpan span = new ClickableSpan() {
            @Override
            public void onClick(View widget) {
                // BAD: Directly triggers Intent to href, even if it's an APK install link
                Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(href));
                widget.getContext().startActivity(intent);
            }
        };
        // Attach the span to the output
        output.setSpan(span, start, end, SPAN_EXCLUSIVE_EXCLUSIVE);
    }
}

What’s wrong?

- No check on the link’s scheme: If the link is to an APK file, it triggers a package installer intent.
- No check if "unknown sources" is enabled: The code blindly launches whatever the link is, sidestepping the system’s normal blocks.

Let’s say a malicious messaging app sends you an HTML-formatted message

<a href="file:///sdcard/malware.apk">Install Cool Game</a>

When you tap the link, Android’s package installer pops up—even if your system blocks unknown sources. A flaw in how the parser triggers this activity lets the install UI appear, without any permission checks.

No "unknown sources" warning appears.

Important: User interaction is still required. The user has to tap the link and confirm install. But regular apps should *not* be able to present the installer UI unless the user has chosen to trust unknown sources.

Proof-of-Concept Code

Here’s a simplified example showing how a rogue app could exploit it (for instructional purposes only):

// Given an HTML message string with a malicious APK link
String htmlMessage = "<a href=\"file:///sdcard/malware.apk\">Click to Install</a>";

// Parse and display in a TextView
TextView textView = findViewById(R.id.yourTextView);
textView.setText(Html.fromHtml(htmlMessage));
textView.setMovementMethod(LinkMovementMethod.getInstance());

If the device has the vulnerable version of Android, tapping this link opens the APK installer—regardless of "unknown sources" settings.

Mitigation

- Official fix: Google plans to add context checks and enforce that APK install attempts always honor the "unknown source" system setting.

References

- NIST CVE-2025-26443 entry
- Android Security Bulletins
- Android Issue Tracker (if available)
- Html.fromHtml() documentation)

Conclusion

CVE-2025-26443 is a reminder that even familiar system code can hide risky logic errors. If you’re a developer, always validate what’s being triggered by user actions—especially when users trust your app. If you’re a user, stay updated and be suspicious of links asking you to install something you didn’t expect.

Timeline

Published on: 09/04/2025 18:15:43 UTC
Last modified on: 09/08/2025 14:14:00 UTC