CVE-2018-17452: Exploring the Server-Side Request Forgery (SSRF) vulnerability in GitLab Community and Enterprise Edition

GitLab is a popular platform for source code management, project planning, and continuous integration. However, security researchers have discovered an issue in GitLab Community and Enterprise Edition before 11.1.7, 11.2.x before 11.2.4, and 11.3.x before 11.3.1. This vulnerability, labeled as CVE-2018-17452, is a Server-Side Request Forgery (SSRF) attack that allows attackers to send unauthorized requests through a loopback address. In this long-read post, we will delve into the details of this exploit, review the code snippets, and provide links to original references.

Exploit Details

The vulnerability stems from a flaw in the validate_localhost function in the url_blocker.rb file. This function is responsible for verifying whether a given URL is a loopback address, which typically points to the same machine or server where the request originates. An attacker can exploit this by sending unauthorized requests through a loopback address, bypassing the intended security measures in place.

The following code snippet demonstrates the issue in the validate_localhost function

def validate_localhost(url)
  host = Addressable::URI.parse(url).host
  lo_ipv4 = IPAddr.new('127...1/8')
  lo_ipv6 = IPAddr.new('::1')
  ip = IPAddr.new(host)
  [lo_ipv4, lo_ipv6].any? { |addr| addr.include?(ip.to_s) }
rescue Addressable::URI::InvalidURIError, IPAddr::Error
  false
end

The above function aims to verify whether the host part of a given URL is a loopback address (either IPv4 or IPv6). However, it fails to account for some non-canonical representations of loopback addresses. As a result, an attacker can bypass the intended security mechanism.

Mitigation

To fix this issue, GitLab released an update that corrects the validate_localhost function to properly validate all possible representations of loopback addresses. The updated function looks like this:

def validate_localhost(url)
  host = Addressable::URI.parse(url).host
  lo_ipv4 = IPAddr.new('.../8')
  lo_ipv6 = IPAddr.new('::1')
  ip = IPAddr.new(host)
  [lo_ipv4, lo_ipv6].none? { |addr| addr.include?(ip.to_s) }
rescue Addressable::URI::InvalidURIError, IPAddr::Error
  false
end

Notice the change from 127...1/8 to .../8. This updated function now properly accounts for all non-canonical representations of loopback addresses and mitigates the SSRF vulnerability.

For more information about the CVE-2018-17452, check out the following resources

1. GitLab Security Advisory
2. A detailed report on the vulnerability
3. National Vulnerability Database entry for CVE-2018-17452

Conclusion

The CVE-2018-17452 demonstrated the importance of proper input validation and the need for continuous security testing and updates. By understanding this vulnerability and staying informed on the latest security developments, developers and users can better protect their systems and applications. As always, it is crucial to keep your software up-to-date and apply patches as soon as possible to reduce the risk of exploitation.

Timeline

Published on: 04/15/2023 23:15:00 UTC
Last modified on: 04/25/2023 19:10:00 UTC