CVE-2026-27826: DNS Rebinding TOCTOU Bypass in mcp-atlassian Server

The mcp-atlassian server is a Model Context Protocol (MCP) server used to interface with Atlassian applications, including Jira and Confluence. In versions prior to 0.17.0, the application exposes an HTTP/SSE interface that processes unauthenticated incoming requests. When managing connections, the server allows clients to dynamically specify target Atlassian instances via custom HTTP headers, creating an expansive attack surface.\n\nSpecifically, the server reads the target hostnames from the X-Atlassian-Jira-Url and X-Atlassian-Confluence-Url headers. Because these requests can be initiated by unauthenticated users, the design introduces risks of arbitrary outbound network requests. This vulnerability class falls under CWE-918 (Server-Side Request Forgery) combined with a CWE-367 (Time-of-Check to Time-of-Use) race condition, allowing adjacent network actors to target internal nodes.

The vulnerability arises from a flawed validation routine designed to block private IP addresses. In an attempt to secure custom URLs, the developers implemented validate_url_for_ssrf in src/mcp_atlassian/utils/urls.py. This utility resolves the client-supplied hostname and checks if any resolved IP address belongs to non-global ranges (e.g., private subnets, loopback, or link-local addresses).\n\nHowever, the code suffers from a classic Time-of-Check to Time-of-Use (TOCTOU) vulnerability. The validation step performs DNS resolution and, if successful, allows execution to proceed. Crucially, the validated IP address is discarded, and the application subsequently passes the original hostname string to the Python requests library. If an attacker controls the authoritative DNS server for the target domain, they can configure it to return a public IP during the validation phase, and then immediately return a private IP (such as 169.254.169.254 or 127.0.0.1) during the connection phase.\n\nAdditionally, the redirect protection implemented as a response hook is defective. The hook intercepts responses and checks response.is_redirect. However, in the standard requests library, redirects are followed internally by default, meaning that the response hook is only executed on the final resolved object. If an intermediate redirect leads to a non-redirect (e.g., a 200 OK on a private resource), the hook fails to trigger, leaving the system fully vulnerable to redirect-based bypasses.

Let us analyze the vulnerable validation logic from src/mcp_atlassian/utils/urls.py in detail. The original DNS resolution check loops through the resolved sockets, but never pins the resulting address:\n\npython\n# Vulnerable DNS validation routine\ndef _check_dns_resolution(hostname: str) -> str | None:\n try:\n results = socket.getaddrinfo(hostname, None)\n except socket.gaierror:\n return f\"DNS resolution failed for {hostname}\"\n\n for _family, _type, _proto, _canonname, sockaddr in results:\n ip_str = sockaddr[0]\n addr = ipaddress.ip_address(ip_str)\n if not addr.is_global:\n return f\"DNS for {hostname} resolves to non-global IP: {ip_str}\"\n return None\n\n\nEven if _check_dns_resolution returns None (indicating a safe IP), the application goes on to make an independent request using the unpinned URL. The diagram below illustrates this TOCTOU flow:\n\nmermaid\ngraph LR\n Client[Client Request] --> Validation[validate_url_for_ssrf]\n Validation -->|DNS Query 1: Public IP| Safe[Validation Passes]\n Safe --> Outbound[requests.get]\n Outbound -->|DNS Query 2: Private IP| Target[Private Resource / IMDS]\n\n\nFurthermore, the redirect validation mechanism utilizes a post-response hook that is fundamentally bypassed because intermediate redirects are processed natively inside requests prior to the hook's invocation.

An attacker can achieve full Server-Side Request Forgery (SSRF) bypass using standard DNS rebinding techniques. The attack requires deploying a custom DNS server that answers requests dynamically with a very low Time-to-Live (TTL) of 0 seconds. On the first query, the DNS server returns a benign global IP address (e.g., 104.192.141.1). On the second query, the DNS server returns a local or loopback address (e.g., 169.254.169.254).\n\nThe exploitation sequence is as follows:\n\n1. The attacker establishes an unauthenticated session with the MCP server by sending an initial handshake to /mcp.\n\n2. The attacker triggers a tool execution such as jira_get_issue while providing custom headers: X-Atlassian-Jira-Url: http://rebind.attacker.com and X-Atlassian-Jira-Personal-Token: testing.\n\n3. During the validation phase, the host machine queries rebind.attacker.com, receives 104.192.141.1, and validates the URL as safe.\n\n4. When mcp-atlassian initiates the outbound HTTP connection, it sends another DNS query due to the expired TTL, resolving rebind.attacker.com to 169.254.169.254.\n\n5. The server connects to the AWS Instance Metadata Service, retrieving sensitive credentials or IAM role policies.

The successful exploitation of CVE-2026-27826 allows unauthenticated, remote attackers to perform arbitrary HTTP requests from the context of the hosting server. Depending on where the mcp-atlassian server is deployed, this capability can lead to severe compromises.\n\nIn cloud environments (such as AWS, Google Cloud, or Azure), the attacker can access the cloud metadata service to retrieve temporary IAM role credentials, resulting in a full cloud account takeover. In local or enterprise networks, the vulnerability enables internal port scanning, discovery of microservices, and interactions with unauthenticated administrative endpoints (such as Redis, Consul, or local databases).\n\nThe CVSS v3.1 base score of 8.2 is classified as High severity. The score reflects a high confidentiality impact and low integrity impact, with the scope changed because the vulnerability in mcp-atlassian is leveraged to compromise adjacent systems on the internal network.

The primary remediation for this vulnerability is to upgrade mcp-atlassian to version 0.17.0 or higher, which properly addresses the DNS rebinding flaw and enforces strict validation.\n\nIf upgrading immediately is not possible, the following mitigations must be implemented:\n\n1. Define Allowed Domains: Set the environment variable MCP_ALLOWED_URL_DOMAINS to restrict the target domains to trusted Atlassian instances (e.g., yourorg.atlassian.net).\n\n2. Network Filtering: Implement host-level firewall rules using iptables to block the server process from initiating outgoing connections to local IP addresses and the link-local metadata address (169.254.169.254).\n\n3. Isolate Deployment: Place the MCP server in a private security group with no outbound access to other internal network resources.