The WordPress ecosystem is vast, with thousands of plugins extending its core functionality. However, the flexibility of these plugins can come at the cost of security if developers don’t adhere to strict input sanitization and output escaping practices. One such vulnerability was discovered in the popular Newsletter plugin, which is installed on over 300,000 websites. The issue, now identified as CVE-2025-3583, allows for Stored Cross-Site Scripting (XSS) that can be weaponized into a JavaScript backdoor, enabling attackers to hijack administrator accounts and compromise the entire site.
| CVE | CVE-2025-3583 |
| Newsletter – Send awesome emails from WordPress < 8.7.1 | |
| Critical | High |
| All Time | 30 838 157 |
| Active installations | 300 000+ |
| Publicly Published | May 30, 2025 |
| Last Updated | May 30, 2025 |
| Researcher | Dmitrii Ignatyev |
| OWASP TOP-10 | A7: Cross-Site Scripting (XSS) |
| PoC | Yes |
| Exploit | No |
| Reference | https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2025-3583 https://wpscan.com/vulnerability/a6582e14-e21e-48e7-9b4c-0044fb199825/ |
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| Logo of the plugin |  |
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Timeline
| February 17, 2025 | Plugin testing and vulnerability detection in the Newsletter – Send awesome emails from WordPress have been completed |
| February 17, 2025 | I contacted the author of the plugin and provided a vulnerability PoC with a description and recommendations for fixing |
| May 30, 2025 | Registered CVE-2025-3583 |
Discovery of the Vulnerability
While auditing the Newsletter plugin, we observed improper sanitization in the “Custom styles” field under plugin settings. This input is later rendered on the frontend page where the newsletter form is embedded, making it a perfect candidate for stored XSS. Importantly, this injection point is accessible to users with Editor permissions, which typically have access to visual customization but should not be allowed to inject raw scripts into global settings.
Understanding of XSS attack’s
XSS is one of the most prevalent vulnerabilities in WordPress plugins due to the dynamic content model and reliance on user-generated input. In WordPress, roles such as Editor or Author can be dangerous if unfiltered_html is not properly managed, as they can potentially input script-based payloads. Real-world scenarios show that such stored XSS vulnerabilities are often exploited for malicious purposes such as stealing cookies, injecting keyloggers, or creating admin backdoors by manipulating WordPress REST APIs.
Exploiting the XSS Vulnerability
To exploit CVE-2025-3583, an attacker with editor+ privileges:
POC:
1) Create a new page/post with form of this plugin 2) Go to settings 127.0.0.1/wordpress/wp-admin/admin.php?page=newsletter_main_main 3) Change "Custom styles" to </style><img src=x onerror=alert(1)> 4) To trigger XSS tou should go to the page from first step____
If exploited, this vulnerability could lead to:
- Administrator Account Takeover: Injected JS can hijack sessions or perform credential theft via fake login prompts.
- Persistence Backdoors: JavaScript can trigger REST calls that create new admin users or inject rogue plugins.
- Phishing or Clickjacking: Attacks against site visitors through injected UI elements.
This is especially dangerous considering the plugin’s large user base (300k+), where many installations may have Editors actively using design customization features.
Recommendations for Improved Security
To mitigate this vulnerability and protect against similar issues:
- Strictly Sanitize Admin Inputs: Even fields meant for CSS should undergo tag stripping and disallowed character filtering.
- Enforce Role Capability Checks: Prevent Editors or non-admins from updating plugin-wide settings unless explicitly intended.
- Escape All Output: Render inputs using escaping functions such as
esc_html()orwp_kses()before outputting on the frontend. - Disable
unfiltered_htmlfor non-admins where feasible. - Keep Plugins Updated: Always use the latest security-patched versions.
To prevent this type of attacks vendor used our methods of prevention.
By taking proactive measures to address Stored XSS vulnerabilities like CVE-2025-3583, WordPress website owners can enhance their security posture and safeguard against potential exploitation. Stay vigilant, stay secure.
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Dmitrii I.