CVE-2025-55735 is a stored Cross-Site Scripting (XSS) vulnerability affecting the FlaskBlog application version 2.8.0 and earlier. FlaskBlog is a blogging platform built using the Flask web framework. The vulnerability arises due to improper neutralization of user input during web page generation, specifically in the handling of the 'postContent' variable when creating blog posts. The application uses the Jinja2 template engine with the '| safe' filter applied to the post content when rendering it in the template/routes.html file. This filter disables automatic escaping of HTML entities, allowing malicious scripts embedded in post content to be executed in the context of users viewing the post. Since there is no validation or sanitization of the post content before storage or rendering, an attacker can inject arbitrary JavaScript code that will be persistently stored and executed whenever the post is viewed. The vulnerability is classified under CWE-79 (Improper Neutralization of Input During Web Page Generation) and CWE-807 (Reliance on Untrusted Inputs in a Security Decision). The CVSS v4.0 base score is 5.3 (medium severity), reflecting that the attack vector is network-based, requires no privileges, and no authentication, but does require user interaction (victim must view the malicious post). The impact primarily affects confidentiality and integrity of user data within the application context, with limited availability impact. No known exploits are currently reported in the wild, and no official patches are linked yet. However, the vulnerability poses a risk of session hijacking, credential theft, or defacement for users interacting with the vulnerable FlaskBlog instance.

For European organizations using FlaskBlog, especially those hosting public or internal blogging platforms, this vulnerability could lead to unauthorized script execution in users' browsers. This can result in theft of session cookies, user impersonation, defacement of content, or redirection to malicious sites. Organizations handling sensitive or regulated data through such platforms may face data confidentiality breaches and reputational damage. Given the medium severity and the requirement for user interaction, the risk is moderate but non-negligible. Attackers could exploit this vulnerability to target employees or customers, potentially leading to broader network compromise if credentials are stolen. Additionally, compliance with GDPR and other European data protection regulations could be impacted if personal data is exposed or manipulated due to this vulnerability. The lack of authentication requirement means any external attacker can submit malicious posts if the blog allows public posting or if an attacker compromises a user account with posting privileges.

To mitigate this vulnerability, organizations should immediately review and update their FlaskBlog installations to a version that properly escapes or sanitizes user input before rendering. If no official patch is available, developers should remove the use of the '| safe' filter on untrusted user content and instead apply proper HTML escaping or use a robust sanitization library such as Bleach to allow only safe HTML tags. Input validation should be implemented to restrict or sanitize post content at creation time. Additionally, implementing Content Security Policy (CSP) headers can reduce the impact of XSS by restricting script execution sources. Organizations should also audit user permissions to limit who can create posts and monitor for suspicious content submissions. Regular security training for users to recognize suspicious links and scripts can reduce the risk of exploitation. Finally, logging and monitoring of web application activity should be enhanced to detect potential exploitation attempts.