PYSEC-2026-4083

Summary

The SSRF protection for header-based authentication uses a validate-then-use pattern vulnerable to DNS rebinding. validate_url_for_ssrf resolves the hostname via DNS and checks that the resolved IP is globally routable. However, the actual HTTP request happens later, during which the DNS record may have changed to point to an internal IP (127.0.0.1, 169.254.169.254, etc.). The SSRF redirect hook only validates redirect targets, not the initial connection.

Details

The vulnerability spans main.py (validation) and dependencies.py (use).

Step 1 -- URL validation at middleware time:

In src/mcp_atlassian/servers/main.py:524-531, the URL from X-Atlassian-Jira-Url is validated:

if jira_url_str:
    ssrf_error = validate_url_for_ssrf(jira_url_str)
    if ssrf_error:
        # blocked

validate_url_for_ssrf (src/mcp_atlassian/utils/urls.py:113-116) resolves DNS and checks is_global:

dns_error = _check_dns_resolution(hostname)
if dns_error:
    return dns_error

Step 2 -- HTTP request happens later with a separate DNS resolution:

In src/mcp_atlassian/servers/dependencies.py:544-562, a JiraFetcher is created with the attacker URL and makes HTTP requests to it. The SSRF redirect hook (attach_ssrf_hook=True) is applied but only checks redirect Location headers, not the initial connection target.

DNS rebinding timeline:

1. Attacker sets up rebind.attacker.com with short TTL (1 second)
2. First DNS resolution (validate_url_for_ssrf): returns 1.2.3.4 (public) -- passes
3. TTL expires
4. Second DNS resolution (requests.get): returns 169.254.169.254 (metadata)
5. HTTP request reaches internal IP, bypassing the SSRF check

PoC

Use a DNS rebinding service like rbndr.us. Send a request with X-Atlassian-Jira-Url set to a rebinding hostname (e.g., rebind-169.254.169.254-1.2.3.4.rbndr.us) and X-Atlassian-Jira-Personal-Token set to any value. With approximately 50% probability per attempt, the request reaches the internal metadata service.

Impact

• Cloud metadata access: Steal IAM credentials via 169.254.169.254
• Internal network scanning: Proxy requests to internal services
• AC:H: DNS rebinding is probabilistic and requires multiple attempts
• Limited to header_pat branch: Only affects deployments accepting header-based PAT authentication

Recommended Fix

Pin DNS resolution: resolve the hostname once during validation, then connect to the resolved IP directly (with the original hostname in the Host header). This eliminates the TOCTOU gap between validation and use.