Technical Analysis of "CVE‑2020‑15823: Server-Side Request Forgery (SSRF) in JetBrains YouTrack"

Overview:
The write-up by the MITM Lab details a critical Server-Side Request Forgery (SSRF) vulnerability in JetBrains YouTrack, a popular issue tracking tool. The SSRF vulnerability allowed attackers to abuse the server’s backend to make unauthorized HTTP requests, potentially accessing internal services and sensitive data.

Key Technical Details:

1. Understanding SSRF:
   SSRF is a type of web security vulnerability where an attacker can trick the server into making unintended requests to arbitrary locations. This can lead to:
2. Retrieval of internal data not intended to be publicly accessible.

3. Interaction with internal services, potentially leading to further compromise.

4. Vulnerable Feature in YouTrack:
   The vulnerability was found in a specific endpoint used for importing issues from external sources. YouTrack supports importing issues from JIRA and other platforms via configured URLs.

5. Import URL Handling: The mechanism to fetch issue data did not properly validate the URLs provided by users. Any URL could be supplied, leading to SSRF.

6. Exploit Mechanism:
   The endpoint /rest/import/jira/settings was identified as vulnerable. By sending a malicious request to this endpoint with a crafted URL, the server could be manipulated to fetch data from internal endpoints or external resources controlled by the attacker.

```http POST /rest/import/jira/settings Content-Type: application/json

{ "url": "http://attacker.com" } ```

• Response Handling: The response from the server included the content from the supplied URL, thus confirming the SSRF.

• Impact:

• Internal Network Access: The SSRF could be used to access otherwise internal-facing services, exposing potentially sensitive internal resources.
• Leakage of Sensitive Information: If the internal endpoints returned sensitive data without adequate authentication, it could be exfiltrated.

• Proxy for Attacks: The server could act as a proxy, masking the attacker's origin and leading to further exploitation or scanning of internal services.

• Mitigation and Fixes:

• Input Validation: Proper validation and sanitization of the URLs supplied by users should be enforced. This includes blacklisting internal IP ranges and only permitting whitelisted domains.
• Network Segmentation: Limiting the ability of the application server to communicate with internal services not required for its operation (e.g., through network firewall rules).

• SSRFPatch: As a temporary measure, libraries like SSRFPatch can be used to prevent such exploits by checking URLs against known patterns likely to be used for malicious purposes.

• Validity Check:

• To validate the presence of SSRF, security researchers often probe with URLs pointing to external servers under their control, enabling them to observe if the server makes unexpected outbound connections.

Key Takeaways:

• Importance of URL Validation: Proper validation and sanitization of user inputs, especially URLs, is crucial to prevent SSRF and other injection-based attacks.
• Dependency on Internal Services: Exposing internal services to external inputs can lead to significant security risks. Segregation and minimal exposure are key defense strategies.
• Dynamic Security Testing: Regular security testing, including Dynamic Application Security Testing (DAST) and application code reviews, can help identify and mitigate such vulnerabilities before they are exploited.

Conclusion:

The SSRF vulnerability in JetBrains YouTrack highlights the importance of rigorous input validation, especially for user-supplied URLs. The implications of SSRF can be severe, potentially exposing sensitive internal services and data. Mitigation involves a combination of input sanitization, network segmentation, and continuous security assessments to safeguard against such vulnerabilities.

For detailed information and the original write-up, visit MITM Lab's post.