CVE-2026-77242 in mcp-atlassian
Summary
by MITRE • 09/22/2026
MCP Atlassian is a Model Context Protocol (MCP) server for Atlassian products (Confluence and Jira). Prior to 0.22.0, validate_url_for_ssrf checks a hostname's resolved addresses, but Requests and urllib3 resolve the hostname again when connecting. A caller can use a short-lived DNS answer that is public during validation and private during connection, preserving unauthenticated access to internal or metadata endpoints despite the earlier CVE-2026-27826 remediation. The advisory traces the vulnerable input and processing flow through validate_url_for_ssrf, _check_dns_resolution, socket.getaddrinfo, and _make_ssrf_safe_hook, which identify the affected entry points, controls, and code paths. This issue is fixed in version 0.22.0.
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Analysis
by VulDB Data Team • 09/22/2026
The vulnerability identified in MCP Atlassian versions prior to 0.22.0 represents a sophisticated Time-of-Check-to-Time-of-Use (TOCTOU) race condition that undermines the effectiveness of previous security controls designed to prevent Server-Side Request Forgery attacks. The core issue lies in the discrepancy between how the application validates URLs and how the underlying HTTP libraries resolve hostnames during actual network connections. Specifically, the function validate_url_for_ssrf performs an initial check by resolving the hostname's IP addresses using socket.getaddrinfo to ensure they do not point to private or internal ranges. This validation step is intended to block requests targeting local infrastructure or cloud metadata endpoints, a critical defense against SSRF attacks that allow attackers to access sensitive internal resources from within a compromised application environment.
However, this security control is rendered ineffective due to the behavior of the Requests library and its dependency urllib3. When an HTTP connection is actually established, these libraries perform their own independent DNS resolution process rather than relying on the previously validated IP address. This architectural separation creates a race condition window where an attacker can manipulate Domain Name System responses between the initial validation phase and the actual connection establishment. By exploiting short-lived DNS records or leveraging fast-flux techniques, an adversary can provide a public-facing hostname that resolves to a safe external IP during the validation check but rapidly changes its resolution to point to a private internal address by the time the socket is opened for data transmission.
This flaw effectively bypasses the remediation implemented in CVE-2026-27826, which previously addressed similar SSRF vectors within the same codebase. The vulnerability allows unauthenticated actors or low-privileged users to redirect outbound requests from the Atlassian Confluence and Jira instances toward internal network segments that are typically isolated from external access. This includes cloud provider metadata endpoints such as AWS EC2 instance metadata service, which can expose sensitive configuration data, temporary credentials, and other secrets critical for further lateral movement within a compromised infrastructure. The attack path involves manipulating input processing flows through _check_dns_resolution and _make_ssrf_safe_hook to exploit the timing gap in DNS resolution consistency.
From an industry standard perspective, this vulnerability aligns with CWE-367, which describes Time-of-check Time-of-use (TOCTOU) race conditions, where security decisions are made based on state that may change before it is acted upon. It also maps directly to MITRE ATT&CK technique T1557, specifically Adversary-in-the-Middle or Lateral Tool Transfer variants involving SSRF for internal network reconnaissance and credential theft. The ability to pivot from an external-facing application into the internal network highlights a critical gap in defense-in-depth strategies that rely solely on input validation without ensuring state consistency across all layers of the request lifecycle.
To mitigate this risk, organizations running MCP Atlassian must immediately upgrade to version 0.22.0 or later, where the developers have addressed the race condition by aligning DNS resolution logic with the actual connection establishment process. Until an upgrade is feasible, administrators should implement strict network-level controls such as egress filtering at the firewall level to restrict outbound connections from Atlassian servers to only known and necessary external destinations. Additionally, deploying web application firewalls that inspect outgoing traffic for patterns indicative of SSRF attempts can provide a secondary layer of defense. It is also advisable to isolate Atlassian instances in dedicated network segments with limited internet access and ensure that internal metadata endpoints are not accessible via standard DNS resolution from the server's context where possible.