CVE-2025-55735: Dogukanurker Flaskblog XSS Vulnerability

CVE-2025-55735 Overview

CVE-2025-55735 is a stored Cross-Site Scripting (XSS) vulnerability [CWE-79] in FlaskBlog, an open-source blog application built with Flask. The flaw exists in versions 2.8.0 and earlier. The application accepts post content through the postContent variable without validation or sanitization. The rendering template in template/routes.html then uses Jinja2's | safe filter, which disables HTML escaping. An unauthenticated attacker who can create posts can inject arbitrary JavaScript that executes in the browser of any user viewing the affected post.

Critical Impact
Attackers can persist malicious JavaScript in blog posts to hijack sessions, deface content, or perform actions on behalf of authenticated readers.

Affected Products

Dogukanurker FlaskBlog versions 2.8.0 and earlier
The template/routes.html rendering component
Any deployment of FlaskBlog exposing post creation functionality

Discovery Timeline

2025-08-19 - CVE-2025-55735 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in the NVD database

Technical Details for CVE-2025-55735

Vulnerability Analysis

The vulnerability is a stored XSS issue that arises from two combined weaknesses. First, FlaskBlog does not validate or sanitize the postContent field submitted during post creation. Second, the template layer renders that content through Jinja2's | safe filter, instructing the engine to output the string as raw HTML.

Attackers can submit posts containing <script> tags, event handler attributes, or other HTML that browsers will parse and execute. Because the payload is persisted in the blog's data store, every subsequent viewer of the post triggers the injected code. This enables session token theft, credential harvesting via injected forms, forced actions against the site, and drive-by redirection.

Root Cause

The root cause is misuse of Jinja2's autoescape bypass. By default, Flask escapes template variables to prevent HTML injection. Applying | safe to user-controlled input in template/routes.html removes that protection. Combined with the absence of server-side input validation on postContent, the application trusts arbitrary user data as safe HTML.

Attack Vector

Exploitation requires network access to the post creation endpoint and user interaction from a victim who visits the malicious post. An attacker submits a post whose content contains an XSS payload. The payload is stored server-side and rendered without escaping on the post view page. When any authenticated or anonymous reader opens the post, the browser executes the attacker-supplied script within the origin of the FlaskBlog site.

Refer to the GitHub Security Advisory GHSA-gj9v-qhc3-gcfx for maintainer-provided technical details.

Detection Methods for CVE-2025-55735

Indicators of Compromise

Stored posts containing <script>, <iframe>, javascript: URIs, or inline event handlers such as onerror= and onload=
Outbound HTTP requests from user browsers to unknown domains shortly after loading a blog post
Unexpected cookie or session token exfiltration in web proxy or DNS logs
Anomalous administrative actions traced to sessions that recently viewed a specific post

Detection Strategies

Query the FlaskBlog post database for HTML tags or JavaScript keywords in the postContent column
Deploy Content Security Policy (CSP) reporting to identify inline script violations on post pages
Inspect web server logs for POST requests to the post creation endpoint containing encoded script payloads
Correlate template rendering paths that invoke the | safe filter with user-controlled variables

Monitoring Recommendations

Alert on new posts whose content matches regex patterns for HTML tags or JavaScript event handlers
Monitor web application firewall (WAF) logs for XSS signatures targeting the FlaskBlog post endpoints
Track outbound connections from visitor browsers to domains not on an allowlist

How to Mitigate CVE-2025-55735

Immediate Actions Required

Upgrade FlaskBlog to a version later than 2.8.0 once the maintainer publishes a patched release
Audit template/routes.html and remove the | safe filter from any user-controlled variable
Review existing posts for stored payloads and purge or sanitize suspicious content
Rotate session secrets and force re-authentication if exploitation is suspected

Patch Information

No fixed version is listed in the NVD entry at the time of publication. Consult the GitHub Security Advisory GHSA-gj9v-qhc3-gcfx for the maintainer's remediation guidance and monitor the upstream repository for a fixed release.

Workarounds

Remove the | safe filter from postContent rendering in template/routes.html to restore Jinja2 autoescaping
Sanitize post content server-side using a library such as bleach with a strict tag and attribute allowlist
Deploy a strict Content Security Policy that blocks inline scripts and restricts script sources to trusted origins
Restrict post creation to trusted, authenticated users and require moderator review before publication

bash

# Example: sanitize post content server-side with bleach before storage
pip install bleach

# In the Flask route handling post creation:
# import bleach
# ALLOWED_TAGS = ['p', 'br', 'strong', 'em', 'ul', 'ol', 'li', 'a', 'code']
# ALLOWED_ATTRS = {'a': ['href', 'title']}
# postContent = bleach.clean(request.form['postContent'],
#                            tags=ALLOWED_TAGS,
#                            attributes=ALLOWED_ATTRS,
#                            strip=True)

# In template/routes.html, remove the | safe filter:
# {{ post.content }}   instead of   {{ post.content | safe }}