CVE-2026-77244: [mcp-atlassian] Authentication bypass in HTTP transport: AtlassianOpaqueTokenVerifier accepts any non-empty token

Description

mcp-atlassian deploys in two common patterns:

Pattern A (single-user, server-side credentials): operator sets JIRAUSERNAME + JIRAAPITOKEN (or CONFLUENCEUSERNAME + CONFLUENCEAPITOKEN) in environment variables. Server uses these to call Jira/Confluence. This is the documented quickstart pattern.

Pattern B (multi-user, OAuth or per-request PAT): operator sets up OAuth proxy or accepts per-user tokens via Authorization or service headers.

The authentication mechanism in HTTP transport has two issues that combine to permit unauthenticated access to Pattern A deployments:

1. AtlassianOpaqueTokenVerifier.verifytoken() at src/mcpatlassian/utils/tokenverifier.py accepts any non-empty string as a valid token:

async def verifytoken(self, token: str) -> AccessToken | None: if not token: return None scopes = self.requiredscopes or [] return AccessToken( token=token, clientid="atlassian", scopes=scopes, expiresat=int(time.time()) + 86400 30, )

The docstring documents this: "we accept non-empty tokens and attach the required scopes."

2. The default deployment does NOT enable the OAuth proxy auth provider (OAUTHPROXYENABLEENV defaults to false; main.py:726). When buildauthprovider() returns None, FastMCP HTTP transport accepts requests with no authentication challenge.

3. UserTokenMiddleware.parseauthheader (main.py:601-664) extracts tokens from Authorization headers and stores them in scope state. If NO Authorization header is present (main.py:584-595), the middleware does not reject the request — it simply does not populate useratlassiantoken.

4. JiraFetcher / ConfluenceFetcher fall back to JiraConfig.fromenv() when no user-supplied token is in scope state. fromenv() reads JIRAAPITOKEN and JIRAUSERNAME from environment and uses them as the API credentials.

Composition: an attacker who reaches the HTTP transport (e.g., server exposed on a port reachable from attacker — direct bind, Docker port mapping, reverse proxy without auth, container in a network the attacker joined) can:

- Send no Authorization header at all, OR - Send any garbage Bearer token

Either request reaches tool handlers. The tool handlers, finding no user-supplied token, use the server's env-var credentials to call Jira / Confluence. The attacker has full operator-level access to the operator's Atlassian instance.

This is the same vulnerability class as CVE-2026-27825 (Arctic Wolf, unauthenticated RCE+SSRF in Atlassian MCP). The previous CVE was for a different code path; this report concerns the auth verifier and middleware behavior present in the current main branch. Steps to Reproduce

Source-level demonstration:

1. Verify the verifier accepts arbitrary tokens:

cd src/ python -c " import asyncio from mcpatlassian.utils.tokenverifier import AtlassianOpaqueTokenVerifier v = AtlassianOpaqueTokenVerifier(requiredscopes=['read:jira-work']) result = asyncio.run(v.verifytoken('anything-at-all')) print('Accepted:', result is not None) print('Token stored:', result.token if result else None) print('Scopes granted:', result.scopes if result else None) "

Expected: Accepted: True Token stored: anything-at-all Scopes granted: ['read:jira-work']

End-to-end (researcher's own Atlassian sandbox):

1. Start mcp-atlassian in HTTP mode against a researcher-owned Atlassian Cloud instance with JIRAAPITOKEN configured:

export JIRAURL=https://researcher.atlassian.net export JIRAUSERNAME=researcher@example.com export JIRAAPITOKEN=<researcher's-real-token> export MCPTRANSPORT=streamable-http export PORT=3000 # Do NOT set OAUTHPROXYENABLEENV — leave it default (false) mcp-atlassian

2. From another machine (or curl on localhost), with no auth:

curl -X POST http://localhost:3000/mcp \ -H "content-type: application/json" \ -H "accept: application/json, text/event-stream" \ -d '{ "jsonrpc":"2.0", "id":1, "method":"tools/call", "params":{ "name":"jiragetissue", "arguments":{"issuekey":"PROJ-1"} } }'

Expected: returns the Jira issue payload — using the server's JIRAAPITOKEN to authenticate to Atlassian. No client-side token provided.

3. Optional: same call with a garbage Bearer for completeness:

curl ... -H "Authorization: Bearer anything-at-all" ...

Same result. Impact: Attacker profile: any party with network reach to the HTTP transport. No credentials, no prior account, no privileged position required.

Typical deployment patterns at risk:

- Docker compose with port exposed (very common in mcp-atlassian's docs and community deployments) - Cloud-deployed MCP server behind a load balancer where the LB doesn't enforce auth (delegates to the application) - Internal corporate network where any employee can reach the server - Misconfigured Kubernetes ingress - Tunneled MCP server via ngrok / Cloudflare Tunnel for development that gets left exposed

Security impact after exploitation:

1. Full Jira read access. Every project, every issue, every comment, every attachment, every user — using the operator's API token.

2. Full Jira write access. Create, edit, delete issues. Add comments under the operator's identity. Move issues across boards. Bulk-edit.

3. Full Confluence read/write access. Same surface — pages, spaces, attachments, permissions, restricted spaces visible to the operator's identity.

4. Audit trail names the operator. Every API call is signed with the operator's token. From Atlassian's logging side, the operator is the actor — covering the attacker's tracks and shifting blame.

5. Pivot. Attachments often contain credentials, infrastructure diagrams, customer data. Confluence pages often store secrets in plaintext under the assumption of access control.

6. Persistence. Attacker can create new Jira webhooks, automation rules, or Confluence integrations that survive beyond the MCP session.

CVE-2026-27825 (Arctic Wolf, May 2026) was scored CVSS 9.8 Critical for unauth RCE+SSRF in this same code surface. This report is the auth-bypass component of the same class against the current main branch.

Suggested Fix

The most direct fix is the standard MCP-server-with-env-creds pattern:

1. When OAUTHPROXYENABLEENV is not set, REFUSE to start the HTTP transport unless an explicit "single-user mode" flag is set:

SINGLEUSERMODE = isenvtruthy("MCPATLASSIANSINGLEUSER") if MCPTRANSPORT == "streamable-http" and not authprovider and not SINGLEUSERMODE: raise SystemExit( "HTTP transport requires either OAUTHPROXYENABLE=true " "or MCPATLASSIANSINGLEUSER=true (acknowledges that env " "credentials will be used for any incoming request)." )

2. Even with SINGLEUSERMODE, bind the HTTP transport to 127.0.0.1 by default unless the operator overrides with an explicit MCPATLASSIANBINDPUBLIC=true.

3. Document the multi-tenant pattern as requiring OAuth proxy or per-request user-token middleware with a verifier that actually verifies (not the opaque-accept-anything stub).

4. Replace AtlassianOpaqueTokenVerifier with a verifier that performs a token-info or whoami call to Atlassian. The fact that Atlassian tokens are opaque does not preclude verification — a /rest/api/3/myself call validates the token and returns the associated user, which the verifier can attach to the AccessToken's scopes and userid fields.

Defense in depth: the README quickstart should not encourage exposing the HTTP transport without auth. The docker-compose.yml in the repo should bind to 127.0.0.1 only by default.