Summary
MCP Atlassian: SSRF via DNS Rebinding in Header-Based Authentication Flow
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:
- Attacker sets up rebind.attacker.com with short TTL (1 second)
- First DNS resolution (validate_url_for_ssrf): returns 1.2.3.4 (public) -- passes
- TTL expires
- Second DNS resolution (requests.get): returns 169.254.169.254 (metadata)
- 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
Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside. Typical impact: access to internal metadata services, internal APIs, or cloud credentials.
CVE-2026-77265 has a CVSS score of 5.9 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.22.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
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.
Frequently Asked Questions
- What is CVE-2026-77265? CVE-2026-77265 is a medium-severity server-side request forgery (SSRF) vulnerability in mcp-atlassian (pip), affecting versions < 0.22.0. It is fixed in 0.22.0. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
- How severe is CVE-2026-77265? CVE-2026-77265 has a CVSS score of 5.9 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of mcp-atlassian are affected by CVE-2026-77265? mcp-atlassian (pip) versions < 0.22.0 is affected.
- Is there a fix for CVE-2026-77265? Yes. CVE-2026-77265 is fixed in 0.22.0. Upgrade to this version or later.
- Is CVE-2026-77265 exploitable, and should I be worried? Whether CVE-2026-77265 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-77265 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-77265? Upgrade
mcp-atlassianto 0.22.0 or later.