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The vulnerability is an incomplete fix for the previously published GHSA-7r34-79r5-rcc9 (CVE-2026-27826). The original patch introduced `validate_url_for_ssrf` in utils/urls.py, which resolves the attacker‑controlled `X-Atlassian-{Jira,Confluence}-Url` header hostname once at middleware time, checks that all resolved IPs are global (public), and returns a verdict string (or `None` if valid). However, it does not return or pin the validated IP address. Later, in servers/dependencies.py, the fetcher is built using the raw hostname from the header (not the validated IP). When the actual HTTP connection is made, the hostname is re‑resolved via `getaddrinfo` at connect time. This creates a Time‑Of‑Check Time‑Of‑Use (TOCTOU) race: an attacker can control a DNS name that returns a public IP during the guard’s lookup (passing validation) but, within the short window before the connection, switches to an internal IP (e.g., `169.254.169.254` for cloud metadata) during the second resolution. The socket then connects to the internal address, resulting in an unauthenticated Server‑Side Request Forgery (SSRF) against cloud metadata services or internal network resources. The fix is present in PyPI `mcp-atlassian` versions ≥0.17.0, but because the validated IP is not pinned to the connection, all patched builds remain vulnerable. The PoC provided in the advisory loads the real urls.py, drives the genuine validate_url_for_ssrf, and simulates the two `getaddrinfo` calls to prove that the guard passes a public IP while the connection would resolve to 169.254.169.254. The advisory explicitly notes that a live end‑to‑end exploit additionally requires an attacker‑controlled fast‑rebinding authoritative DNS responder to win the race window, but the structural flaw is confirmed.
DailyCVE Form:
Platform: mcp-atlassian
Version: >=0.17.0
Vulnerability: SSRF via DNS-rebind
Severity: High
date: 2026-06-27
Prediction: TBD
What Undercode Say:
Verify no existing fix covers connect-time re-resolution
gh search prs issues --repo mcp-atlassian --match rebind TOCTOU getaddrinfo pin
Check the vulnerable code at HEAD ba72540
curl -s https://raw.githubusercontent.com/mcp-atlassian/mcp-atlassian/ba72540/src/mcp_atlassian/utils/urls.py | grep -A 20 "def validate_url_for_ssrf"
curl -s https://raw.githubusercontent.com/mcp-atlassian/mcp-atlassian/ba72540/src/mcp_atlassian/servers/dependencies.py | grep -A 10 "url = "
PoC boundary demonstration (Python)
python3 - <<'EOF'
import importlib.util, socket, sys
spec = importlib.util.spec_from_file_location("urls", "src/mcp_atlassian/utils/urls.py")
urls = importlib.util.module_from_spec(spec); spec.loader.exec_module(urls)
Simulate guard resolution
guard_ip = socket.getaddrinfo("rebind.attacker.example", 80)[bash][4][bash]
print(f"GUARD: {guard_ip}") 93.184.216.34 (public)
Simulate connect-time resolution
conn_ip = socket.getaddrinfo("rebind.attacker.example", 80)[bash][4][bash]
print(f"CONNECT: {conn_ip}") 169.254.169.254 (internal)
Call real validator
result = urls.validate_url_for_ssrf("http://rebind.attacker.example")
print(f"validate_url_for_ssrf returned: {result}") None (pass)
EOF
Exploit:
An unauthenticated attacker sends a request with the `X-Atlassian-{Jira,Confluence}-Url` header set to a domain they control. The domain’s DNS is configured with a very short TTL and a malicious authoritative server that returns a public IP (e.g., 93.184.216.34) for the initial resolution performed by validate_url_for_ssrf. Immediately after the guard passes, the attacker’s DNS server switches to returning `169.254.169.254` (or another internal IP) before the fetcher performs its connect‑time getaddrinfo. The outbound HTTP request is then made to the internal IP, allowing the attacker to read cloud instance metadata (IAM credentials, user data) or probe internal services. The race window is narrow, but with a fast‑responding DNS server and repeated attempts, it can be reliably won.
Protection:
Pin the connection to the IP address that was validated by validate_url_for_ssrf. This can be achieved by:
– Modifying `validate_url_for_ssrf` to return the validated IP address (or a list of allowed IPs) instead of a mere verdict.
– In servers/dependencies.py, when building the fetcher, use a custom requests adapter that overrides DNS resolution with a cached `getaddrinfo` result, or pass the validated IP directly in the URL while preserving the original `Host` header.
– Alternatively, implement a dedicated resolver that caches the validated IP for the lifetime of the request and forces the connection to use that cached address, eliminating the second resolution.
Impact:
Successful exploitation grants an unauthenticated attacker the ability to read cloud metadata endpoints (e.g., AWS IMDSv1 at 169.254.169.254/latest/meta-data/iam/security-credentials/), potentially exposing IAM credentials that could lead to privilege escalation and lateral movement within the cloud environment. It also allows probing internal services that are not exposed to the public internet, which may disclose sensitive configuration, internal API endpoints, or other vulnerable internal applications. The impact is identical to the original GHSA-7r34 advisory, as this bypass re‑introduces the same SSRF primitive on the “patched” version.
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Sources:
Reported By: github.com
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