CVE-2026-21885: Miniflux 2 SSRF Vulnerability

CVE-2026-21885 Overview

CVE-2026-21885 is a Server-Side Request Forgery (SSRF) vulnerability affecting Miniflux 2, an open source feed reader. Prior to version 2.2.16, the media proxy endpoint (GET /proxy/{encodedDigest}/{encodedURL}) can be abused by authenticated users to force the server to make HTTP requests to arbitrary internal or external destinations. An attacker can embed malicious URLs in feed entry content, causing Miniflux to generate signed proxy URLs that, when requested, make the server fetch and return responses from internal network resources.

Critical Impact
Authenticated attackers can access internal network resources including localhost services, private RFC1918 IP ranges, and cloud metadata endpoints (e.g., 169.254.169.254), potentially exposing sensitive configuration data, credentials, or enabling lateral movement within the network.

Affected Products

Miniflux 2 versions prior to 2.2.16

Discovery Timeline

January 8, 2026 - CVE-2026-21885 published to NVD
January 8, 2026 - Last updated in NVD database

Technical Details for CVE-2026-21885

Vulnerability Analysis

This SSRF vulnerability (CWE-918) resides in Miniflux's media proxy functionality, which is designed to proxy external media content through the Miniflux server for privacy and security purposes. The proxy endpoint accepts an encoded digest and encoded URL, validates the signature, and fetches the requested resource on behalf of the user.

The vulnerability arises because the proxy implementation fails to properly validate or restrict the destination URLs before making outbound requests. When an attacker crafts feed content containing references to internal network addresses, Miniflux generates valid signed proxy URLs for these malicious destinations. The signature mechanism, intended to prevent tampering, inadvertently legitimizes attacker-controlled URLs once they are processed through the feed content pipeline.

The attack requires authentication, meaning an attacker must have valid credentials to a Miniflux instance. However, once authenticated, the attacker can leverage the proxy to scan internal networks, access cloud provider metadata services, or interact with internal APIs that should not be accessible from the internet.

Root Cause

The root cause is insufficient URL validation in the media proxy endpoint. The application generates signed proxy URLs for any media URL found in feed content without checking whether the destination is a private, reserved, or otherwise restricted IP address space. The proxy then blindly follows these URLs, treating internal addresses the same as legitimate external media resources.

Attack Vector

The attack is network-based and requires low-privilege authentication. An attacker exploits this vulnerability by:

Creating or subscribing to a malicious RSS/Atom feed containing entries with media URLs pointing to internal addresses (e.g., http://169.254.169.254/latest/meta-data/ for cloud metadata, http://localhost:8080/admin, or http://192.168.1.1/config)
When Miniflux processes the feed, it generates signed proxy URLs for these internal targets
The attacker requests the generated /proxy/... URLs through their authenticated session
Miniflux's server fetches the internal resource and returns the response to the attacker

This allows extraction of sensitive information from internal services, cloud metadata endpoints, and potentially enables further attacks against internal infrastructure.

Detection Methods for CVE-2026-21885

Indicators of Compromise

Unusual outbound requests from Miniflux server to internal IP addresses (127.0.0.0/8, 10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16)
Requests to cloud metadata endpoints (169.254.169.254) originating from the Miniflux application
Proxy endpoint requests with encoded URLs resolving to non-public IP addresses
Feed subscriptions containing suspicious media URLs pointing to internal resources

Detection Strategies

Monitor Miniflux server egress traffic for connections to private IP ranges and link-local addresses
Implement network segmentation rules that alert on internal service access from web application servers
Review proxy endpoint access logs for patterns indicating systematic internal network scanning
Deploy web application firewall (WAF) rules to detect SSRF payloads in feed content

Monitoring Recommendations

Enable detailed logging for the /proxy/ endpoint including decoded destination URLs
Configure alerts for Miniflux server connections to metadata service endpoints (169.254.169.254)
Monitor for unusual response sizes or content types from the proxy endpoint that may indicate internal data exfiltration
Implement DNS query logging to detect resolution of internal hostnames from the Miniflux server

How to Mitigate CVE-2026-21885

Immediate Actions Required

Upgrade Miniflux 2 to version 2.2.16 or later immediately
Review proxy endpoint logs for evidence of exploitation attempts against internal resources
Temporarily disable the media proxy feature if upgrade is not immediately possible
Implement network-level controls to prevent the Miniflux server from accessing internal services

Patch Information

Version 2.2.16 of Miniflux 2 addresses this vulnerability by implementing proper URL validation in the media proxy endpoint. For additional details, refer to the GitHub Security Advisory.

Workarounds

Configure firewall rules on the Miniflux server to block outbound connections to private IP ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16, 127.0.0.0/8)
Block access to cloud metadata endpoints (169.254.169.254) from the application server
Deploy a reverse proxy with SSRF protection that validates outbound request destinations
Restrict Miniflux to only allow connections to explicitly whitelisted external domains if media proxying is required

bash

# Example iptables rules to block SSRF to internal networks from Miniflux server
# Block localhost
iptables -A OUTPUT -d 127.0.0.0/8 -m owner --uid-owner miniflux -j DROP
# Block private networks
iptables -A OUTPUT -d 10.0.0.0/8 -m owner --uid-owner miniflux -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -m owner --uid-owner miniflux -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -m owner --uid-owner miniflux -j DROP
# Block cloud metadata endpoint
iptables -A OUTPUT -d 169.254.169.254 -m owner --uid-owner miniflux -j DROP