プラグイン名	WordPress Responsive Blocks Plugin
脆弱性の種類	オープンリダイレクト
CVE番号	CVE-2026-6675
緊急	低い
CVE公開日	2026-04-21
ソースURL	CVE-2026-6675

Security Advisory: Unauthenticated Open Email Relay / Open Redirection in Responsive Blocks plugin (CVE-2026-6675) — What WordPress Site Owners Must Do Now

著者： WP-Firewall セキュリティチーム
日付： 2026-04-21
タグ: WordPress, security, WAF, vulnerability, plugin, responsive-blocks, CVE-2026-6675

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まとめ： A low-severity but exploitable vulnerability (CVE-2026-6675) impacts the Responsive Blocks WordPress plugin (versions ≤ 2.2.0). An unauthenticated REST API parameter named email_to can be abused to create an open email relay or enable open redirection behavior. Update to version 2.2.1 immediately. If you cannot update right away, apply temporary mitigations described below.

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目次

• 何が起こったか
• 影響を受けるバージョンとタイムライン
• 脆弱性の技術的概要
• 現実の影響と攻撃シナリオ
• Detection: how to tell if you were targeted or abused
• Immediate fixes (recommended order of operations)
• Temporary mitigations and virtual patch examples
• Hardening guidance for plugin authors and site operators
• How WP‑Firewall helps and free plan information
• 最終的な注意事項とさらなる読み物

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何が起こったか

On 21 April 2026 a vulnerability affecting the Responsive Blocks WordPress plugin was published and assigned CVE-2026-6675. The root cause is improper validation and authorization around a REST API parameter (email_to) exposed by the plugin. An unauthenticated actor can use that parameter to relay email or trigger an unvalidated redirect path — effectively enabling open email relay and open redirection behaviors.

The plugin author released a patch in version 2.2.1 that corrects the issue. Administrators running versions 2.2.0 or older should update as soon as possible.

なぜあなたが気にすべきか： even low-severity vulnerabilities can be weaponized at scale. Open email relays permit bulk spam or phishing campaigns from your domain, which can lead to blacklisting, deliverability issues, or reputational damage. Open redirection can facilitate phishing and social‑engineering attacks that steer your users to malicious sites.

影響を受けるバージョンとタイムライン

• Affected: Responsive Blocks plugin — versions ≤ 2.2.0
• Patched: 2.2.1 (upgrade provided by the plugin developer)
• CVE: CVE-2026-6675
• 必要な特権: なし (認証されていない)
• Risk rating (reported): Low (Patchstack reported CVSS 5.3; classification: Open Redirection / Insecure Design)

注記： “Low” severity in CVSS does not mean “no action required.” Unauthenticated vectors in public-facing sites can be exploited en masse, so mitigate quickly.

脆弱性の技術的概要

At a high level, the plugin exposes a REST API route which accepts an email_to parameter and performs either of the following actions (depending on the plugin internals):

• Sends e-mails based directly on the email_to value without proper validation and authorization (open email relay behavior), or
• Uses the email_to or companion parameters to produce a redirect URL that is not validated against an allowlist (open redirection).

なぜこれが技術的に重要なのか:

• REST API endpoints in WordPress are reachable by anyone unless they implement proper capability checks. If a route does not require authentication and passes user-supplied parameters into email-sending or redirect functions, attackers can abuse it.
• Lack of validation means an attacker can specify arbitrary targets (email addresses or redirect hosts). In the case of email relay, the site becomes an SMTP-like vector for spam; for open redirection, attackers can lure users to the site (legitimate URL) and then redirect them to phishing/malware domains.

Exploit example (conceptual)

• An attacker issues a POST request to the plugin’s REST endpoint with an email_to parameter set to a target address or with a redirect URL that points to a malicious host.
• Because the endpoint did not validate email_to (e.g., via is_email() and domain/whitelist checks) and required no authentication, the request succeeds.
• Outcome: email is sent from your domain to third parties, or visitors are redirected to attacker-controlled domains.

重要： the exact REST route path and payload structure differ based on the plugin’s internal implementation. Regardless, the vector is the same: unauthenticated input passed directly to email/redirect logic.

現実の影響と攻撃シナリオ

Although classified as “low”, the practical outcomes can be quite harmful:

1. Spam and bulk phishing
   Attackers use your site to send large volumes of email to third parties (spam, phishing). Because emails originate from your server/domain, they appear more trusted, increasing click rates and potential damage.
2. Domain reputation and blacklisting
   ISP and anti-spam providers may blacklist your IP or domain after detecting outbound spam. Recovery is time-consuming and can disrupt legitimate email operations (transactional emails, newsletters, user notifications).
3. Redirect-based phishing
   Open redirections let attackers craft URLs using your legitimate domain to mask malicious payloads. Users see your domain in addresses (increasing trust) and are redirected to credential-harvesting pages.
4. ソーシャルエンジニアリングの増幅
   Using your domain increases trust in phishing campaigns — attackers can email victims appearing to be from a trusted source, or share links on social channels that begin at your domain.
5. SEO and user-trust damage
   Malicious redirects and spam can harm SEO rankings and user trust; cleaning that up can be costly.

Detection: how to tell if you were targeted or abused

Check the following quickly:

• Web server & access logs:
  • Look for unauthenticated POST/GET requests to REST API endpoints with parameters named email_to, リダイレクト, に, recipient, or other email-like fields.
  • Note frequency and origin IPs. Mass requests from many IPs may indicate automated scanning/exploitation.
• Mail logs:
  • Inspect mail logs (exim, postfix, sendmail logs or logs from your managed mail provider) for sudden increases in outbound mail volume, or messages with unusual subjects/content related to plugin behavior.
  • Check bounce notifications and out-of-office replies that indicate mass sending.
• Hosting/SMTP quotas:
  • Alerts about email-sending limits being reached or banned by the host.
  • Emails flagged as spam or rejected by large providers.
• Google Search Console / Search and security tools:
  • Messages about harmful content, phishing, or manual actions.
• Blacklist lookup:
  • Check if your sending IP or domain appears on common RBL/blacklists (Spamhaus, etc.).
• Content on site:
  • Look for injected redirection code or unexpected pages that perform meta-refresh or JavaScript redirects.

Immediate fixes (recommended order of operations)

1. Upgrade the plugin (best and fastest)
   Update Responsive Blocks to version 2.2.1 or later immediately. This is the official fix and should be applied first unless you have a compatibility blocker.
2. If you cannot update immediately, isolate the risk
   Temporarily disable the plugin from the WordPress admin or via wp‑cli:
   wp plugin deactivate responsive-blocks
   Or disable the plugin by renaming its directory via SFTP/SSH.
3. Block the problematic REST route with your firewall/WAF
   Block any requests that contain suspicious email_to values or patterns at the web server or upstream firewall before they hit WordPress.
   Example WAF rules are below.
4. Monitor email and web logs
   While applying mitigations, monitor logs for further attempts and clean up any outbound spam that was sent.
5. 利害関係者への通知
   Inform your hosting provider / in-house ops team. If abuse occurred, you may need to coordinate delisting or provide evidence to mail providers.
6. If abuse was confirmed, reset relevant credentials and update email settings
   Update any SMTP credentials used by the site, rotate API keys, and confirm no other plugins/themes were altered.

Temporary mitigations and virtual patch examples

If you need to keep the plugin active for business reasons and cannot upgrade immediately, you can apply temporary measures (virtual patches) to block the exploitation vector. Two approaches are useful:

A. Server-level blocking (recommended for immediate mitigation)

14. 認証されていないクライアントによって行われたリクエストをブロックします。 email_to= or suspicious payloads at the webserver or CDN edge (Cloudflare, etc.):

nginx example (reject requests containing email_to parameter):

# Block query-strings containing email_to=
if ($query_string ~* "email_to=") {
    return 403;
}
# Also block POST bodies containing "email_to="
# (For blocking POST bodies you need to use the nginx mod_security or WAF layer)

Apache（.htaccess）例：

<IfModule mod_rewrite.c>
  RewriteEngine On
  RewriteCond %{QUERY_STRING} (?:^|&)email_to= [NC]
  RewriteRule .* - [F]
</IfModule>

注記： blocking query strings may impact legitimate functionality if you use a compatible feature; test carefully.

B. WordPress-level virtual patch (MU-plugin)

Place the following PHP snippet as a must-use plugin (drop into wp-content/mu-plugins/virtual-patch-block-email_to.php). This forces early rejection of requests that include the email_to parameter in REST requests.

<?php
/*
Plugin Name: Virtual Patch - Block email_to REST abuse
Description: Temporary block for REST requests containing the email_to parameter.
*/

add_filter( 'rest_pre_dispatch', 'wpfw_block_emailto_rest_abuse', 10, 3 );

function wpfw_block_emailto_rest_abuse( $result, $server, $request ) {
    // only handle if this is a public REST request (not authenticated)
    // If you want to allow admins to use it, check current_user_can()
    $params = $request->get_params();

    if ( isset( $params['email_to'] ) ) {
        // Optional: validate that email_to is a safe email using is_email()
        $email = $params['email_to'];
        if ( ! is_email( $email ) ) {
            // invalid email, block request
            return new WP_Error( 'rest_forbidden', 'Forbidden', array( 'status' => 403 ) );
        }

        // If you want to completely block until you can upgrade:
        return new WP_Error( 'rest_forbidden', 'Forbidden', array( 'status' => 403 ) );

        // Alternative: implement allowlist logic:
        // $allowed = array( '[email protected]' );
        // if ( ! in_array( $email, $allowed, true ) ) {
        //     return new WP_Error( 'rest_forbidden', 'Forbidden', array( 'status' => 403 ) );
        // }
    }

    return $result;
}

注:

• This is a temporary mitigation. Replace or remove the mu-plugin after you update to the patched plugin version.
• Carefully test this in a staging environment before applying on a production server, especially if you use REST endpoints for legitimate workflows.

C. WAF rule examples (conceptual)

Block POST requests to any route that contain email_to= with email patterns or redirect parameters:
Regular expression examples for WAF rule engines (adjust for your WAF syntax):

• Block if POST body or query contains:
  (email_to=.+@.+\..+)
• もし リダイレクト parameter contains an external host:
  redirect=(?:https?://)(?!yourdomain\.com)

交換 yourdomain.com with your canonical domain(s). Use caution: overly broad rules may break legitimate third-party integrations.

Hardening guidance for plugin authors and site operators

If you develop or maintain WordPress plugins, or you manage WordPress sites, follow these best practices to avoid similar issues:

1. Apply strict input validation
   • Validate email addresses with is_email() before using them in wp_mail or other sending logic.
   • Validate URLs with esc_url_raw() and check hosts against an allowlist for redirects.
2. 適切な認可を強制する
   • REST endpoints should check user capabilities with 現在のユーザーができる() or use permission callbacks when registering routes via レジスタレストルート(). Do not allow unauthenticated requests to perform actions that can send email or perform redirects.
3. Avoid creating mail-relay-like features
   • Never accept arbitrary に addresses from unauthenticated users. If a user-facing contact form is required, restrict the recipient to a fixed mailbox or to a set of pre-approved addresses.
4. 使用 wp_safe_redirect for redirects
   • When redirecting, prefer wp_safe_redirect() and maintain an allowlist of domains or only redirect to internal paths.
5. Apply secure defaults
   • Default plugin behavior should be conservative: fail closed when inputs are invalid; require minimal privileges for potentially destructive actions.
6. 14. – 疑わしいリクエストをログに記録し、敏感なエンドポイントに対して内部レート制限を実装します。
   • Log suspicious activity and add throttling/rate-limiting on endpoints that send email or trigger redirects.
7. Provide a vulnerability disclosure and rapid update path
   • Quick fixes, security advisories, and a responsible disclosure contact make it easier for site owners to mitigate issues quickly.

WP‑Firewallの助けになる方法

As a provider of WordPress firewall and security services, our aim is to help site owners respond quickly to plugin vulnerabilities like this one. WP‑Firewall provides several layers of protection that are useful immediately:

• Managed WAF rules updated by our security team to block new exploitation vectors
• Virtual patching that protects endpoints without modifying plugin code
• Malware scanning to detect outbound abuse or injected redirectors
• Monitoring and alerting for suspicious REST API activity
• Guidance and support to coordinate remediation

Start with our free Basic plan to get essential protection and reduce exposure right away.

今日あなたのサイトを保護しましょう — WP‑Firewallの無料プランから始めましょう

We understand the pressure site owners face when vulnerabilities are published. If you want immediate, managed protection while you test and apply plugin updates, our Basic (Free) plan gives you essential defenses without cost. The free plan includes a managed firewall, unlimited bandwidth, a Web Application Firewall (WAF) tuned for WordPress, a malware scanner, and mitigation for OWASP Top 10 risks — exactly the layers that help stop unauthenticated REST API abuse in its tracks.

無料プランを探検し、こちらでサインアップしてください： https://my.wp-firewall.com/buy/wp-firewall-free-plan/

If you need more features, we also offer Standard and Pro tiers with automatic malware removal, IP blacklisting/whitelisting, auto vulnerability virtual patching, monthly security reports, and premium add-ons like a dedicated account manager and managed security services. These options are designed to support teams that want deeper visibility and proactive protection.

(Why this works: the WAF + virtual patching combination blocks exploitation early, while the malware scanner helps you confirm whether misuse already happened.)

Recommended long-term steps after remediation

1. プラグイン、テーマ、およびWordPressコアを最新の状態に保つ
   Regular updates are the single best defense against known vulnerabilities.
2. Implement host-level mail policies
   Configure authenticated SMTP and restrict outgoing mail rates. Use provider-level controls to prevent automated abuse.
3. Review your plugin inventory
   Remove unused plugins. Fewer plugins mean fewer potential vulnerabilities.
4. Deploy a staging environment for testing
   Test plugin updates and virtual patches in staging before rollout.
5. インシデントレスポンス計画を確立する
   Define roles, contacts (hosting, security vendor), and steps to take when a vulnerability is found.
6. Review and tighten REST API exposure
   Audit routes registered on your site (plugins and themes) and verify permission callbacks.

Detailed checklist for site administrators

Urgent (0–24 hours):

• Update Responsive Blocks to 2.2.1.
• If update is not possible immediately, disable the plugin.
• Put a WAF rule in place blocking requests containing email_to パターンを探します。.
• Monitor mail logs for sudden increases or anomalies.

Short term (24–72 hours):

• Place the MU-plugin virtual patch if you need to keep functionality running.
• Review web server logs for exploitation indicators.
• Notify your email provider/host if suspicious mail activity occurred.

中期（1〜2週間）：

• Review other installed plugins for similar REST API endpoints lacking permission checks.
• Harden mail flow and configure SPF/DKIM/DMARC correctly to minimize impact from spoofed email and to maintain deliverability.

長期（継続中）：

• Implement continuous monitoring and managed WAF rules.
• Keep an inventory and adopt a plugin-vetting policy before installing third-party plugins.

Sample log indicators to hunt for

• Repeated requests to REST endpoints containing email_to= or suspicious email addresses.
• Burst of POST requests to rarely-used endpoints shortly after public disclosure.
• Outbound SMTP sessions with high volume and identical payload patterns.
• Bouncebacks for large volumes of messages within a short time window.

What to do if you discover abuse

1. Stop the vector: disable the plugin or apply the temporary virtual patch/WAF rule.
2. Preserve logs: copy and save server logs, mail logs, and any suspicious payloads.
3. Inform hosting and mail providers: they may help block further abuse and start delisting processes.
4. Clean up any injected content and remove malicious pages/redirects.
5. Rotate credentials: SMTP, admin accounts, and any API keys used on the site.
6. Consider a professional security review if you see signs of a deeper compromise.

WP‑Firewallからの締めくくりの考え

This vulnerability is a reminder that even functionality which seems routine — sending email or handling redirects — can be abused if not implemented securely. The good news: a patch is available, and rapid mitigation steps exist. Prioritize the plugin update first. If you manage many sites, apply the virtual patch/WAF rules across your estate until updates are rolled out.

If you’d like help applying mitigations, setting up WAF rules, or adding managed virtual patching so you’re protected while you coordinate upgrades, WP‑Firewall’s team is ready to assist. Remember that combining strong WAF rules with secure plugin development practices dramatically reduces exposure to unauthenticated abuse.

Further reading and references

• Plugin author patch notes and changelog (check your plugin page)
• Your host or mail provider’s documentation for outbound mail logs and rate limits
• WordPress developer documentation: REST API best practices, permission callbacks, and data validation functions
• Public vulnerability advisory (CVE-2026-6675) for timeline and patch references

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If you want a short, prioritized remediation checklist emailed to you (one page, plain English), reply to this post or sign up for WP‑Firewall’s free protection plan at:
https://my.wp-firewall.com/buy/wp-firewall-free-plan/

Stay safe, and remember — timely updates plus layered defenses protect both your site and your users.
— WP‑ファイアウォールセキュリティチーム