CVE-2026-79663: Ech0 Stored XSS Vulnerability in RSS Feed

CVE-2026-79663 Overview

CVE-2026-79663 is a stored cross-site scripting (XSS) vulnerability in Ech0 versions before 4.7.3. The public RSS feed renders tag names and markdown content without HTML escaping. An attacker with administrator privileges can inject malicious tag names or raw HTML into echo content. That payload executes as JavaScript in RSS readers that render HTML-type summaries. The impact reaches anonymous subscribers and other users consuming the feed. The weakness is tracked under CWE-79.

Critical Impact
Stored XSS in the Ech0 RSS feed allows an authenticated administrator to execute arbitrary JavaScript in the context of downstream RSS readers, exposing anonymous subscribers to script execution.

Affected Products

Ech0 versions prior to 4.7.3
RSS readers that render type="html" summary content from Ech0 feeds
Public-facing Ech0 deployments exposing the RSS endpoint

Discovery Timeline

2026-08-25 - CVE-2026-79663 published to the National Vulnerability Database
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-79663

Vulnerability Analysis

Ech0 generates a public RSS feed that includes user-supplied tag names and markdown-rendered echo content. Before version 4.7.3, the feed generator emits these fields directly into RSS <description> and related summary elements without HTML entity encoding. Because the summary is served as HTML-type content, downstream RSS readers parse embedded script tags, event handlers, and inline HTML as executable markup.

The vulnerability requires administrator authentication to plant the payload, but the execution surface is anonymous. Any subscriber pulling the feed inherits the script context of their reader. Web-based aggregators are particularly exposed, since embedded JavaScript can act on the origin rendering the feed content.

Root Cause

The root cause is missing output encoding on server-side RSS generation. Tag names and markdown output are concatenated into feed XML without escaping characters such as <, >, ", and &. Markdown processing may additionally pass through raw HTML fragments, expanding the attack surface beyond simple tag name injection. This is a classic instance of CWE-79: Improper Neutralization of Input During Web Page Generation applied to a syndication feed.

Attack Vector

An administrator, or an attacker who has obtained administrator credentials, creates or edits an echo with a tag name or markdown body containing HTML or JavaScript payloads. The malicious content is stored in the Ech0 backend and later serialized into the public RSS feed. When a subscriber's RSS reader fetches the feed and renders the HTML-type summary, the injected script executes in that reader's context. See the GitHub Security Advisory GHSA-3v85-fqvh-7rxf and the VulnCheck Stored XSS Advisory for additional technical detail.

No verified public proof-of-concept code is available. The advisory describes the vector in prose only.

Detection Methods for CVE-2026-79663

Indicators of Compromise

Tag names in the Ech0 database containing <script>, onerror=, onload=, or javascript: substrings
RSS feed output where <description> elements contain unescaped HTML markup or event handlers
Administrator account activity creating or editing echoes with unusual HTML fragments in markdown bodies
Outbound requests from RSS reader hosts to attacker-controlled domains referenced in stored tag content

Detection Strategies

Query the Ech0 datastore for tag records matching common XSS payload patterns and review recent echo edits by admin accounts
Inspect the rendered /rss endpoint response for HTML entities that should have been encoded, such as raw angle brackets inside <description> blocks
Correlate administrator login events with subsequent tag creation or echo edits to identify anomalous authoring activity

Monitoring Recommendations

Log and alert on all administrative writes to Ech0 tag and echo tables, including source IP and user agent
Monitor web server access logs for scraping of the RSS endpoint from unexpected clients following admin write events
Track outbound DNS from workstations running web-based RSS readers for domains that appear in stored Ech0 content

How to Mitigate CVE-2026-79663

Immediate Actions Required

Upgrade Ech0 to version 4.7.3 or later, which introduces HTML escaping for RSS feed tag names and markdown content
Audit existing tag names and echo bodies for stored HTML or script payloads and sanitize or remove offending records
Rotate administrator credentials and review admin session logs for unauthorized authoring activity
Restrict administrator account access using strong authentication and network-level controls on the admin interface

Patch Information

The fix is included in Ech0 4.7.3. Refer to GitHub Security Advisory GHSA-3v85-fqvh-7rxf for release notes and the corresponding commits.

Workarounds

If upgrading is not immediately possible, disable the public RSS feed endpoint at the reverse proxy or application configuration level
Restrict RSS feed access to authenticated internal users while remediation is planned
Enforce a Content Security Policy on any web-based RSS reader used to consume Ech0 feeds to limit inline script execution
Manually strip HTML tags from tag names and echo markdown at the storage layer as a temporary hardening measure

bash

# Configuration example: block public RSS access at nginx until patched
location /rss {
    allow 10.0.0.0/8;
    deny all;
    proxy_pass http://ech0_backend;
}