GHSA-J944-W549-3453

Vulnerability from github – Published: 2026-05-07 16:40 – Updated: 2026-06-08 23:47
VLAI
Summary
Cinny vulnerable to access token disclosure via invalidated emoji pack avatar URL in service worker
Details

Impact

A remote authenticated attacker who shares a room with a victim and has permissions to create room emotes (for example in a DM) can cause the victim's client to send their Matrix access token to an attacker-controlled server. This occurs when the victim opens the emoji or sticker picker for the room containing a malicious emote pack.

The root causes are:

(1) an incorrect fallback in EmojiBoard that uses untrusted pack.meta.avatar (user-controlled) without converting/validating it as an MXC URL, allowing arbitrary HTTP(S) URLs to be used; and

(2) the service worker attaching the user's Authorization bearer token to all outbound GET requests whose URL contains /_matrix/client/v1/media/download or /_matrix/client/v1/media/thumbnail without verifying the request host matches the configured homeserver origin. An attacker-controlled URL containing those path fragments and permissive CORS will receive the victim's Authorization header (access token).

Impacted users: anybody using affected Cinny web app versions who opens the emoji/sticker picker in a room containing a malicious emote pack and who is logged in (authenticated).

Patches

Version with fixes: https://github.com/cinnyapp/cinny/releases/tag/v4.10.3

Severity
7.1 (High)
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N
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{
  "affected": [
    {
      "package": {
        "ecosystem": "npm",
        "name": "cinny"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "4.10.3"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-42553"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-20"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-05-07T16:40:52Z",
    "nvd_published_at": "2026-05-27T18:16:22Z",
    "severity": "HIGH"
  },
  "details": "### Impact\nA remote authenticated attacker who shares a room with a victim and has permissions to create room emotes (for example in a DM) can cause the victim\u0027s client to send their Matrix access token to an attacker-controlled server. This occurs when the victim opens the emoji or sticker picker for the room containing a malicious emote pack. \n\nThe root causes are: \n\n(1) an incorrect fallback in EmojiBoard that uses untrusted `pack.meta.avatar` (user-controlled) without converting/validating it as an MXC URL, allowing arbitrary HTTP(S) URLs to be used; and \n\n(2) the service worker attaching the user\u0027s Authorization bearer token to all outbound GET requests whose URL contains `/_matrix/client/v1/media/download` or `/_matrix/client/v1/media/thumbnail` without verifying the request host matches the configured homeserver origin. An attacker-controlled URL containing those path fragments and permissive CORS will receive the victim\u0027s Authorization header (access token). \n\nImpacted users: anybody using affected Cinny web app versions who opens the emoji/sticker picker in a room containing a malicious emote pack and who is logged in (authenticated).\n\n### Patches\nVersion with fixes: https://github.com/cinnyapp/cinny/releases/tag/v4.10.3",
  "id": "GHSA-j944-w549-3453",
  "modified": "2026-06-08T23:47:42Z",
  "published": "2026-05-07T16:40:52Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/cinnyapp/cinny/security/advisories/GHSA-j944-w549-3453"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-42553"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/cinnyapp/cinny"
    },
    {
      "type": "WEB",
      "url": "https://github.com/cinnyapp/cinny/releases/tag/v4.10.3"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N",
      "type": "CVSS_V4"
    }
  ],
  "summary": "Cinny vulnerable to access token disclosure via invalidated emoji pack avatar URL in service worker"
}


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