{"cve":"CVE-2026-42553","nvd":{"published":"2026-05-27T18:16:22.857","modified":"2026-10-26T11:10:00.157","status":"Deferred","description":"Cinny is a Matrix client. Prior to 4.10.3, 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. This is caused by 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. Also, 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). This vulnerability is fixed in 4.10.3.","cvss_v4":{"vector":"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","score":7.1},"cvss_v3":null,"cwes":["CWE-20"],"references":["https://github.com/cinnyapp/cinny/releases/tag/v4.10.3","https://github.com/cinnyapp/cinny/security/advisories/GHSA-j944-w549-3453"],"products":[],"source":"nvd","synced_at":"2026-10-26T11:33:08.859761529+00:00"},"epss":{"cve":"CVE-2026-42553","score":0.00302,"percentile":0.2271,"date":"2026-10-25"},"kev":null,"osv":[{"id":"GHSA-j944-w549-3453","url":"https://osv.dev/vulnerability/GHSA-j944-w549-3453","summary":"Cinny vulnerable to access token disclosure via invalidated emoji pack avatar URL in service worker","modified":"2026-06-09T00:00:19.393740437Z","packages":[{"ecosystem":"npm","name":"cinny","purl":"pkg:npm/cinny","ranges":["<4.10.3"],"fixed":["4.10.3"]}]}],"attribution":["This product uses the NVD API but is not endorsed or certified by the NVD.","EPSS scores: FIRST Exploit Prediction Scoring System, https://www.first.org/epss","Known exploited vulnerabilities: CISA Known Exploited Vulnerabilities Catalog, https://www.cisa.gov/known-exploited-vulnerabilities-catalog","Package advisories: OSV (https://osv.dev), from the GitHub Advisory Database, PyPA, Go, RustSec and other ecosystem databases, under CC-BY 4.0 or CC0","CVE records: the CVE Program's CVE List (https://www.cve.org), under its terms of use (https://www.cve.org/Legal/TermsOfUse). CVE is a registered trademark of The MITRE Corporation; this product is not affiliated with the CVE Program."]}