CVE-2023-27162: Openapi Generator SSRF Vulnerability

CVE-2023-27162 Overview

OpenAPI Generator up to version 6.4.0 contains a Server-Side Request Forgery (SSRF) vulnerability in the /api/gen/clients/{language} component. This vulnerability allows attackers to access network resources and sensitive information via a crafted API request. SSRF vulnerabilities enable malicious actors to make requests from the vulnerable server to internal services, potentially exposing sensitive data or enabling further attacks against internal infrastructure.

Critical Impact

This SSRF vulnerability allows unauthenticated attackers to access internal network resources and sensitive information through the OpenAPI Generator API endpoint, potentially leading to data exfiltration and internal network reconnaissance.

Affected Products

• OpenAPI Generator versions up to and including 6.4.0
• Applications using the vulnerable /api/gen/clients/{language} endpoint
• Self-hosted OpenAPI Generator instances with exposed API endpoints

Discovery Timeline

• 2023-03-31 - CVE-2023-27162 published to NVD
• 2025-02-18 - Last updated in NVD database

Technical Details for CVE-2023-27162

Vulnerability Analysis

This Server-Side Request Forgery (SSRF) vulnerability exists in the OpenAPI Generator's client generation API endpoint. The vulnerable component /api/gen/clients/{language} fails to properly validate and sanitize URL parameters in API requests, allowing attackers to specify arbitrary URLs that the server will then request on their behalf.

When the OpenAPI Generator processes a client generation request, it accepts an OpenAPI specification URL as input. The application retrieves the specification from the provided URL without adequate validation, enabling attackers to redirect these requests to internal network resources, cloud metadata endpoints, or other sensitive services that should not be externally accessible.

The vulnerability is classified under CWE-918 (Server-Side Request Forgery), which describes flaws where an attacker can make a server perform requests to unintended locations. In this case, the impact includes potential access to confidential data and the ability to interact with internal services.

Root Cause

The root cause of this vulnerability is insufficient input validation and URL sanitization in the OpenAPI Generator's API endpoint. The application does not implement proper allowlisting or blocklisting mechanisms for URLs provided in client generation requests. This allows attackers to specify internal IP addresses, localhost references, cloud metadata URLs (such as http://169.254.169.254/), or other internal network resources as the OpenAPI specification source.

Attack Vector

An attacker exploits this vulnerability by sending a specially crafted API request to the /api/gen/clients/{language} endpoint with a malicious URL in the openAPIUrl parameter. The server-side application then makes a request to this attacker-controlled URL, potentially exposing internal services or retrieving sensitive information.

The attack requires network access to the vulnerable OpenAPI Generator instance. No authentication or user interaction is required to exploit this vulnerability. Attackers can leverage this to:

• Access internal network services not exposed to the internet
• Retrieve cloud instance metadata containing credentials
• Scan internal networks for available services
• Exfiltrate sensitive configuration data

Technical details and proof-of-concept information are available in the GitHub Gist Code Snippet and the SJTU Notes Resource.

Detection Methods for CVE-2023-27162

Indicators of Compromise

• Unusual outbound requests from the OpenAPI Generator server to internal IP ranges (e.g., 10.x.x.x, 172.16.x.x, 192.168.x.x)
• Requests to cloud metadata endpoints such as 169.254.169.254 originating from the application server
• API calls to /api/gen/clients/{language} containing suspicious URL parameters pointing to internal resources
• Unexpected network traffic from the server to localhost or loopback addresses

Detection Strategies

• Monitor API access logs for requests to /api/gen/clients/ endpoints with URL parameters containing internal IP addresses or metadata service URLs
• Implement network monitoring to detect outbound connections from the OpenAPI Generator server to internal network ranges
• Deploy web application firewall (WAF) rules to detect and block SSRF patterns in incoming requests
• Use intrusion detection systems to alert on requests containing common SSRF payloads

Monitoring Recommendations

• Enable detailed logging for all API requests to the OpenAPI Generator service, including full request parameters
• Configure network segmentation alerts to detect when the application server attempts to access restricted internal resources
• Implement DNS query logging to identify resolution attempts for internal hostnames from the vulnerable application
• Set up automated alerting for requests containing suspicious URL schemes (e.g., file://, gopher://, dict://)

How to Mitigate CVE-2023-27162

Immediate Actions Required

• Upgrade OpenAPI Generator to a version newer than 6.4.0 that includes the security fix
• If immediate upgrade is not possible, restrict network access to the /api/gen/clients/ API endpoints
• Implement network-level controls to prevent the OpenAPI Generator server from accessing internal resources
• Review logs for evidence of exploitation attempts targeting this vulnerability

Patch Information

Organizations should upgrade to the latest version of OpenAPI Generator available from the GitHub OpenAPI Generator Repository. Review the release notes for security fixes addressing this SSRF vulnerability. The OpenAPI Generator Homepage provides additional documentation and upgrade guidance.

Workarounds

• Implement URL allowlisting at the application or proxy level to restrict which external URLs the OpenAPI Generator can access
• Deploy a web application firewall (WAF) with rules to block requests containing internal IP addresses or metadata service URLs in the openAPIUrl parameter
• Use network segmentation to isolate the OpenAPI Generator server and prevent it from accessing sensitive internal services
• Disable or remove the /api/gen/clients/ endpoint if client generation functionality is not required

# Example network restriction using iptables to block internal network access
# Block access to common internal ranges from the 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
iptables -A OUTPUT -d 169.254.169.254 -j DROP