CVE-2026-34576: Postiz AI Social Media Tool SSRF Vulnerability

CVE-2026-34576 Overview

CVE-2026-34576 is a Server-Side Request Forgery (SSRF) vulnerability in Postiz, an AI-powered social media scheduling tool. Prior to version 2.21.3, the POST /public/v1/upload-from-url endpoint accepts user-supplied URLs and fetches them server-side using axios.get() without implementing adequate SSRF protections. The only validation in place is a file extension check (.png, .jpg, etc.) which can be trivially bypassed by appending an image extension to any URL path. An authenticated API user can exploit this vulnerability to fetch internal network resources, cloud instance metadata, and other internal services, with the response data uploaded to storage and returned to the attacker.

Critical Impact

Authenticated attackers can access internal network resources, cloud provider metadata services (such as AWS IMDS), and sensitive internal service data through this SSRF vulnerability.

Affected Products

• Postiz versions prior to 2.21.3

Discovery Timeline

• April 2, 2026 - CVE-2026-34576 published to NVD
• April 2, 2026 - Last updated in NVD database

Technical Details for CVE-2026-34576

Vulnerability Analysis

This vulnerability is classified as CWE-918 (Server-Side Request Forgery). The core issue stems from the application's image upload functionality which allows users to provide a URL for server-side fetching. When a user submits a URL to the /public/v1/upload-from-url endpoint, the server makes an HTTP request using axios.get() to retrieve the content at that URL. The fetched content is then uploaded to storage and made available to the user.

The insufficient validation allows attackers to manipulate the application into making requests to arbitrary internal or external destinations. This can lead to exposure of sensitive data from cloud metadata services, internal APIs, configuration endpoints, and other resources that should not be externally accessible.

Root Cause

The root cause is the lack of proper SSRF protections on the URL fetching functionality. The only implemented security measure is a file extension check that verifies the URL ends with an image extension like .png or .jpg. This validation is fundamentally flawed because:

1. URL path components can be arbitrarily constructed with query parameters or fragments
2. Attackers can append image extensions to any URL (e.g., http://169.254.169.254/latest/meta-data/iam/security-credentials/.png)
3. The server follows the request regardless of the actual content type returned
4. No allowlist or blocklist validation is performed on target IP addresses or hostnames

Attack Vector

The attack vector is network-based and requires low-privilege authentication to access the API endpoint. An attacker with valid API credentials can craft malicious URLs targeting:

• Cloud Metadata Services: Access AWS, GCP, or Azure instance metadata (e.g., http://169.254.169.254/latest/meta-data/)
• Internal Services: Probe and access internal network services not exposed to the internet
• Local Resources: Target localhost services running on the same machine
• Port Scanning: Enumerate open ports and services within the internal network

The attack flow involves sending a POST request to /public/v1/upload-from-url with a crafted URL. The server fetches the specified resource, stores the response, and returns the content to the attacker, effectively acting as a proxy to bypass network segmentation.

Detection Methods for CVE-2026-34576

Indicators of Compromise

• Unusual outbound requests from the Postiz application server to internal IP ranges (10.x.x.x, 172.16.x.x-172.31.x.x, 192.168.x.x)
• Requests to cloud metadata IP addresses (169.254.169.254 for AWS/GCP, 169.254.169.123 for Azure)
• Abnormal patterns in the /public/v1/upload-from-url endpoint access logs
• Requests containing URLs with localhost, internal hostnames, or suspicious path patterns with appended image extensions

Detection Strategies

• Monitor application server egress traffic for connections to non-public IP ranges or metadata service IPs
• Implement Web Application Firewall (WAF) rules to detect and block SSRF patterns in URL parameters
• Review access logs for the /public/v1/upload-from-url endpoint for unusual URL submissions
• Deploy network-level monitoring to alert on unexpected internal service access from application servers

Monitoring Recommendations

• Enable detailed logging for all URL fetch operations in the Postiz application
• Set up alerts for any requests to RFC 1918 private IP ranges or link-local addresses
• Monitor for unusual data exfiltration patterns from the application's storage system
• Implement egress filtering and log all outbound HTTP requests from application servers

How to Mitigate CVE-2026-34576

Immediate Actions Required

• Upgrade Postiz to version 2.21.3 or later immediately
• Review access logs for potential exploitation attempts prior to patching
• Audit any credentials or sensitive data that may have been exposed through cloud metadata access
• Consider temporarily disabling the URL upload functionality if immediate patching is not possible

Patch Information

The vulnerability has been patched in Postiz version 2.21.3. Organizations should upgrade to this version or later as soon as possible. Additional details can be found in the GitHub Security Advisory GHSA-89vp-m2qw-7v34 and the GitHub Release v2.21.3.

Workarounds

• Implement network-level egress filtering to block application server access to internal IP ranges and metadata services
• Deploy a WAF rule to inspect and block requests to the vulnerable endpoint containing suspicious URL patterns
• Use network segmentation to limit the application server's access to internal resources
• If running on cloud infrastructure, disable or restrict IMDS access using instance metadata service version 2 (IMDSv2) or equivalent controls

# Example iptables rules to block metadata service access (AWS/GCP)
iptables -A OUTPUT -d 169.254.169.254 -j DROP
iptables -A OUTPUT -d 169.254.169.123 -j DROP

# Block common internal ranges from application server
iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -d 192.168.0.0/16 -j DROP