Fail closed when CSPRNG is unavailable during nonce generation

[dknauss]

Summary

This PR replaces the weak fallback in create_login_nonce() with a hard failure.

The existing fallback uses wp_hash() with inputs including $user_id, wp_rand(), and microtime(). That defensive path dates back to the PHP 5.x / random_compat era. On the plugin’s current minimum PHP version (7.2+), random_bytes() should use the OS CSPRNG and should not fail under normal conditions.

If secure randomness is unavailable, failing closed is safer than generating a weaker nonce. Both existing call sites already handle a false return with wp_die().

Change

try {
    $login_nonce['key'] = bin2hex( random_bytes( 32 ) );
} catch ( Exception $ex ) {
-   $login_nonce['key'] = wp_hash( $user_id . wp_rand() . microtime(), 'nonce' );
+   return false;
}

[github-actions]

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Co-authored-by: dknauss <dpknauss@git.wordpress.org>
Co-authored-by: masteradhoc <masteradhoc@git.wordpress.org>

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[Copilot]

Pull request overview

This PR hardens Two_Factor_Core::create_login_nonce() by removing the weak entropy fallback used when random_bytes() fails and instead returning false, ensuring nonce generation fails closed when a CSPRNG is unavailable.

Changes:

• Replace the wp_hash( $user_id . wp_rand() . microtime(), 'nonce' ) fallback with return false when random_bytes() throws.
• Preserve existing caller behavior: both in-file call sites already treat a false return as fatal via wp_die().

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