{
  "guidislink": false,
  "id": "https://tailscale.com/security-bulletins/#ts-2026-009",
  "link": "https://tailscale.com/security-bulletins/#ts-2026-009",
  "links": [
    {
      "href": "https://tailscale.com/security-bulletins/#ts-2026-009",
      "rel": "alternate",
      "type": "text/html"
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  "published": "Mon, 13 Jul 2026 00:00:00 GMT",
  "summary": "\u003cp\u003e\u003cstrong\u003e\u003cem\u003eDescription\u003c/em\u003e\u003c/strong\u003e: Insecure command line argument handling in Tailscale SSH permitted \u003ccode\u003eroot\u003c/code\u003e user access in violation of ACLs.\u003c/p\u003e\n\u003ch4\u003eWhat happened?\u003c/h4\u003e\n\u003cp\u003e\u003ca href=\"https://tailscale.com/docs/features/tailscale-ssh\"\u003eTailscale SSH\u003c/a\u003e previously accepted usernames that contained a leading \u003ccode\u003e-\u003c/code\u003e character. On Linux platforms these usernames were passed as arguments to \u003ccode\u003egetent(1)\u003c/code\u003e to retrieve the corresponding \u003ccode\u003epasswd\u003c/code\u003e entry, where they were interpreted as flags permitting attacker-controlled behavior. Specifically, if a user connected with the username \u003ccode\u003e-i\u003c/code\u003e this would have been interpreted as \u003ccode\u003e--no-idn\u003c/code\u003e and \u003ccode\u003egetent\u003c/code\u003e would have printed the entire \u003ccode\u003epasswd\u003c/code\u003e file contents starting with the \u003ccode\u003eroot\u003c/code\u003e user, causing Tailscale to open an interactive \u003ccode\u003eroot\u003c/code\u003e session.\u003c/p\u003e\n\u003cp\u003eTailscale SSH now rejects usernames with leading dashes.\u003c/p\u003e\n\u003cp\u003eThis vulnerability is fixed in Tailscale version 1.98.9 or newer.\u003c/p\u003e\n\u003ch4\u003eWhat was the impact?\u003c/h4\u003e\n\u003cp\u003eA user with SSH access to a Linux node would have been able to obtain a \u003ccode\u003eroot\u003c/code\u003e session by connecting with the username \u003ccode\u003e-i\u003c/code\u003e, in violation of ACL policy.\u003c/p\u003e\n\u003ch4\u003eWho was affected?\u003c/h4\u003e\n\u003cp\u003eUsers of Tailscale SSH on Linux hosts that rely on \u003ccode\u003eautogroup:nonroot\u003c/code\u003e user restrictions in Tailscale ACLs.\u003c/p\u003e\n\u003ch4\u003eWhat do I need to do?\u003c/h4\u003e\n\u003cp\u003eIf you use Tailscale SSH, upgrade to Tailscale version 1.98.9 or newer.\u003c/p\u003e\n\u003ch4\u003eCredits\u003c/h4\u003e\n\u003cp\u003eWe would like to thank Anthropic and Ada Logics for reporting this issue.\u003c/p\u003e",
  "summary_detail": {
    "base": "https://tailscale.com/security-bulletins/index.xml",
    "language": null,
    "type": "text/html",
    "value": "\u003cp\u003e\u003cstrong\u003e\u003cem\u003eDescription\u003c/em\u003e\u003c/strong\u003e: Insecure command line argument handling in Tailscale SSH permitted \u003ccode\u003eroot\u003c/code\u003e user access in violation of ACLs.\u003c/p\u003e\n\u003ch4\u003eWhat happened?\u003c/h4\u003e\n\u003cp\u003e\u003ca href=\"https://tailscale.com/docs/features/tailscale-ssh\"\u003eTailscale SSH\u003c/a\u003e previously accepted usernames that contained a leading \u003ccode\u003e-\u003c/code\u003e character. On Linux platforms these usernames were passed as arguments to \u003ccode\u003egetent(1)\u003c/code\u003e to retrieve the corresponding \u003ccode\u003epasswd\u003c/code\u003e entry, where they were interpreted as flags permitting attacker-controlled behavior. Specifically, if a user connected with the username \u003ccode\u003e-i\u003c/code\u003e this would have been interpreted as \u003ccode\u003e--no-idn\u003c/code\u003e and \u003ccode\u003egetent\u003c/code\u003e would have printed the entire \u003ccode\u003epasswd\u003c/code\u003e file contents starting with the \u003ccode\u003eroot\u003c/code\u003e user, causing Tailscale to open an interactive \u003ccode\u003eroot\u003c/code\u003e session.\u003c/p\u003e\n\u003cp\u003eTailscale SSH now rejects usernames with leading dashes.\u003c/p\u003e\n\u003cp\u003eThis vulnerability is fixed in Tailscale version 1.98.9 or newer.\u003c/p\u003e\n\u003ch4\u003eWhat was the impact?\u003c/h4\u003e\n\u003cp\u003eA user with SSH access to a Linux node would have been able to obtain a \u003ccode\u003eroot\u003c/code\u003e session by connecting with the username \u003ccode\u003e-i\u003c/code\u003e, in violation of ACL policy.\u003c/p\u003e\n\u003ch4\u003eWho was affected?\u003c/h4\u003e\n\u003cp\u003eUsers of Tailscale SSH on Linux hosts that rely on \u003ccode\u003eautogroup:nonroot\u003c/code\u003e user restrictions in Tailscale ACLs.\u003c/p\u003e\n\u003ch4\u003eWhat do I need to do?\u003c/h4\u003e\n\u003cp\u003eIf you use Tailscale SSH, upgrade to Tailscale version 1.98.9 or newer.\u003c/p\u003e\n\u003ch4\u003eCredits\u003c/h4\u003e\n\u003cp\u003eWe would like to thank Anthropic and Ada Logics for reporting this issue.\u003c/p\u003e"
  },
  "title": "TS-2026-009",
  "title_detail": {
    "base": "https://tailscale.com/security-bulletins/index.xml",
    "language": null,
    "type": "text/plain",
    "value": "TS-2026-009"
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}