CVE-2026-77250: MCP Atlassian: OAuth fallback token storage writes plaintext access and refresh tokens with group-readable permissions

Summary

When OAuth tokens are saved, MCP Atlassian always writes a plaintext fallback copy under ~/.mcp-atlassian/oauth-<clientid>.json. The fallback file is created with the process default umask rather than restrictive permissions. In this environment the file was created as mode 0664, exposing access and refresh tokens to same-group local users and any process that can read the home directory.

Details

OAuthConfig.savetokens() stores OAuth token data in keyring, but it also unconditionally maintains a plaintext file fallback for backwards compatibility in src/mcpatlassian/utils/oauth.py:350-386. If keyring saving fails it also falls back to the same file path in src/mcpatlassian/utils/oauth.py:387-391.

The fallback writer creates ~/.mcp-atlassian and then writes oauth-<clientid>.json with a normal open(tokenpath, "w") call in src/mcpatlassian/utils/oauth.py:392-420. No mode=0o600, os.open(..., 0o600), chmod, or owner-only directory permission is applied. The file contains both accesstoken and refreshtoken (src/mcpatlassian/utils/oauth.py:360-367) and is later loaded from the same plaintext path in src/mcpatlassian/utils/oauth.py:450-470.

The security policy warns that OAuth client credentials and secrets should not be exposed (SECURITY.md:39-44), but the current implementation creates a persistent plaintext token copy even when keyring succeeds.

PoC

The following safe local proof uses a temporary HOME and mocked keyring writes. It creates and deletes only temporary files.

bash uv run python - <<'PY' import json, os, shutil, stat, tempfile from pathlib import Path from unittest.mock import patch from mcpatlassian.utils.oauth import OAuthConfig

home = tempfile.mkdtemp(prefix='mcp-atlassian-oauth-poc-') oldhome = os.environ.get('HOME') os.environ['HOME'] = home try: cfg = OAuthConfig(clientid='poc-client', clientsecret='client-secret', redirecturi='http://localhost/callback', scope='offlineaccess', cloudid='cloud-id') cfg.accesstoken = 'poc-access-token' cfg.refreshtoken = 'poc-refresh-token' cfg.expiresat = 2000000000 with patch('keyring.setpassword', returnvalue=None): cfg.savetokens() tokenfile = Path(home) / '.mcp-atlassian' / 'oauth-poc-client.json' mode = stat.SIMODE(tokenfile.stat().stmode) data = json.loads(tokenfile.readtext()) print(json.dumps({ 'tokenfileexists': tokenfile.exists(), 'tokenfilemodeoctal': oct(mode), 'containsaccesstoken': data.get('accesstoken') == 'poc-access-token', 'containsrefreshtoken': data.get('refreshtoken') == 'poc-refresh-token', 'tokenfilepath': str(tokenfile), }, indent=2, sortkeys=True)) finally: if oldhome is not None: os.environ['HOME'] = oldhome else: os.environ.pop('HOME', None) shutil.rmtree(home) PY

Observed output from this environment:

json { "containsaccesstoken": true, "containsrefreshtoken": true, "tokenfileexists": true, "tokenfilemodeoctal": "0o664", "tokenfilepath": "/tmp/mcp-atlassian-oauth-poc-9m9wvktp/.mcp-atlassian/oauth-poc-client.json" }

The proof confirms that a plaintext file containing both access and refresh tokens is created and is not owner-only.

Impact

A local user, container sidecar, compromised dependency, backup job, or other process with filesystem read access to the account's home directory can recover OAuth access and refresh tokens. Refresh tokens can allow continued Atlassian API access until revoked or expired, depending on the OAuth app and token policy. In shared hosts, Kubernetes volumes, developer workstations, and CI runners, this can lead to persistent Atlassian account compromise.