CVE-2026-77267: MCP Atlassian SSRF Vulnerability

CVE-2026-77267 Overview

CVE-2026-77267 is a Server-Side Request Forgery (SSRF) vulnerability in MCP Atlassian, a Model Context Protocol (MCP) server that bridges AI clients with Atlassian Confluence and Jira. Versions prior to 0.22.0 process the X-Atlassian-Jira-Url and X-Atlassian-Confluence-Url request headers inside _process_authentication_headers without invoking validate_url_for_ssrf. A caller who can set these headers can redirect outbound requests to internal services or cloud metadata endpoints. The flaw bypasses an incomplete remediation applied for CVE-2026-27826. The maintainer released a fix in version 0.22.0.

Critical Impact
Attackers with the ability to inject custom Atlassian URL headers can coerce the MCP server into fetching internal-only resources, including cloud instance metadata services, exposing credentials and internal application data [CWE-918].

Affected Products

MCP Atlassian versions prior to 0.22.0
Deployments exposing the MCP Atlassian server to callers who can set custom HTTP headers
AI/agent integrations that proxy user-controlled headers into the MCP server

Discovery Timeline

2026-09-22 - CVE-2026-77267 published to the National Vulnerability Database
2026-09-23 - Last updated in NVD database

Technical Details for CVE-2026-77267

Vulnerability Analysis

MCP Atlassian implements per-request URL overrides through the X-Atlassian-Jira-Url and X-Atlassian-Confluence-Url headers. The _process_authentication_headers function reads these values and constructs Atlassian API fetchers directly from them. The prior fix for CVE-2026-27826 introduced a validate_url_for_ssrf helper to reject internal targets, but that helper is not invoked on the header-supplied URLs. As a result, the server issues outbound HTTP requests to arbitrary destinations chosen by the caller.

The issue is classified as Server-Side Request Forgery [CWE-918]. Impact scope extends beyond the vulnerable process because the MCP server can be pivoted to reach adjacent systems.

Root Cause

The root cause is missing input validation on trusted-looking configuration headers. _process_authentication_headers treats header-supplied hostnames as authoritative and skips the SSRF guard that the codebase already implements for other URL inputs. The prior remediation for CVE-2026-27826 hardened some paths but did not cover this header-driven code path.

Attack Vector

An authenticated caller sends a request to the MCP Atlassian server with a crafted X-Atlassian-Jira-Url or X-Atlassian-Confluence-Url header pointing to an internal IP address, a link-local address such as 169.254.169.254, or an internal DNS name. The server constructs a fetcher for that URL and sends authenticated requests to it. Responses may be returned to the caller, allowing exfiltration of cloud instance metadata, internal API responses, or reachability information for internal network reconnaissance.

No public exploit code is referenced in the advisory. The security patch is available in the upstream repository.

python

# Illustrative patch context from the upstream commit
# Source: https://github.com/sooperset/mcp-atlassian/commit/b041733473f95119dd539542a43c280737a8e460
# The fix hardens path/URL validation across attachment, transport, SSRF,
# authorization, and filter code paths. Example from confluence/attachments.py:

# Before:
#   if not os.path.isabs(file_path):
#       file_path = os.path.abspath(file_path)

# After:
file_path = str(validate_safe_path(file_path))

Source: GitHub Commit b041733

Detection Methods for CVE-2026-77267

Indicators of Compromise

Inbound requests to the MCP Atlassian server containing X-Atlassian-Jira-Url or X-Atlassian-Confluence-Url headers referencing RFC1918 ranges, 127.0.0.0/8, or 169.254.169.254.
Outbound HTTP connections from the MCP Atlassian process to cloud metadata endpoints or internal-only hostnames.
Atlassian API fetchers issuing requests to hosts that do not match the configured Confluence or Jira base URL.

Detection Strategies

Parse reverse-proxy or ingress logs for the two Atlassian URL headers and alert when values resolve to non-public addresses.
Correlate MCP Atlassian process telemetry with egress network flows to detect requests to unexpected internal destinations.
Baseline legitimate Atlassian API endpoints and flag deviations at the network layer.

Monitoring Recommendations

Enable structured logging on the MCP Atlassian deployment and forward request headers and outbound URL targets to a central log store.
Monitor cloud instance metadata service access from workloads hosting MCP servers.
Alert on any authentication failures or 4xx/5xx bursts originating from the MCP process against unusual hosts.

How to Mitigate CVE-2026-77267

Immediate Actions Required

Upgrade MCP Atlassian to version 0.22.0 or later, which routes header-supplied URLs through validate_url_for_ssrf.
Restrict who can reach the MCP Atlassian server to trusted clients only, and terminate untrusted callers at an ingress proxy.
Strip X-Atlassian-Jira-Url and X-Atlassian-Confluence-Url headers at the reverse proxy unless explicitly required.

Patch Information

The fix is included in the MCP Atlassian v0.22.0 release, delivered through Pull Request #1448 and commit b041733. Full advisory details are in GHSA-5wf4-jqxh-8gm3.

Workarounds

Configure an egress firewall or network policy that blocks the MCP Atlassian workload from reaching RFC1918 ranges, 127.0.0.0/8, and 169.254.169.254.
Deploy the MCP server on cloud instances that enforce IMDSv2 with a hop limit of 1 to defeat metadata SSRF.
Front the MCP Atlassian server with a proxy that removes or overrides the two vulnerable headers before they reach the application.

bash

# Example NGINX snippet to strip caller-supplied Atlassian URL headers
location / {
    proxy_set_header X-Atlassian-Jira-Url "";
    proxy_set_header X-Atlassian-Confluence-Url "";
    proxy_pass http://mcp_atlassian_upstream;
}