dbcve.org
Improper Input ValidationWeakness · CWE-20

CVE-2026-42553

HIGH · 7.1 CVSS v4.0 Published 2026-05-27
Mitigation only
No fix yet — a mitigation exists. There is no fixed release. A documented workaround reduces exposure in the meantime.
See remediation →
77/100
Remediation priority · High
Remotely reachable Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
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.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

In Cinny Matrix client prior to 4.10.3, the EmojiBoard component uses untrusted pack.meta.avatar data from emote packs without validating it as an MXC URL, allowing arbitrary HTTP(S) URLs. Additionally, the service worker blindly attaches the user's Matrix access token to all GET requests containing /_matrix/client/v1/media/download or /_matrix/client/v1/media/thumbnail path fragments without verifying the target host matches the configured homeserver, enabling token theft via attacker-controlled URLs.

MitigationUpgrade to Cinny version 4.10.3 or later. There is no practical workaround; client-side fixes are required to properly validate MXC URLs and restrict token attachment to legitimate homeserver origins only.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
Low
Authentication
X
User interaction
None
Scope
X

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine Cinny version
    Locate the Cinny application and check its version number in the application metadata, about page, or package.json file
    Affected if Version is prior to 4.10.3 (versions 4.x less than 4.10.3, or any version below 4.x)
  2. Check for loaded emote packs
    Inspect the Cinny client storage (localStorage/sessionStorage or IndexedDB) for emote pack data entries, particularly examining pack.meta.avatar fields
    Affected if Emote packs with custom pack.meta.avatar URLs are loaded and the avatar field contains non-mxc:// URLs (arbitrary HTTP/HTTPS URLs)
  3. Verify MXC URL format in emote data
    If emote packs exist, examine stored avatar URLs - legitimate Matrix content URLs use mxc:// format (e.g., mxc://servername/mediaid)
    Affected if Any pack.meta.avatar value uses http:// or https:// instead of mxc:// protocol
  4. Check service worker behavior
    Review network requests made by the Cinny client when loading media via /_matrix/client/v1/media/download or /_matrix/client/v1/media/thumbnail endpoints
    Affected if The Matrix access token is being attached to requests destined for servers other than the configured homeserver

User is affected if running Cinny version below 4.10.3 AND either uses custom emote packs with arbitrary avatar URLs or has media requests that leak tokens to non-homeserver domains

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Mitigation available No clean upgrade yet — mitigate in the meantime
Mitigation

Upgrade to Cinny version 4.10.3 or later. There is no practical workaround; client-side fixes are required to properly validate MXC URLs and restrict token attachment to legitimate homeserver origins only.

Recommended fix High confidence

4.10.3

  1. Check the currently installed Cinny version (typically found in the application settings or package.json)
  2. Upgrade Cinny to version 4.10.3 or later by using your package manager (e.g., npm install [email protected], yarn add [email protected], or pip install for server deployments)
  3. If using a hosted/service instance, contact the hosting provider to ensure they have updated to version 4.10.3 or later
  4. Verify the upgrade was successful by checking the Cinny version in the application settings after update

Generated from the published advisory — verify against the referenced sources before acting.

Have this fixed Scoped from the published advisory
  • Consultation4.0 h
  • Implementation8.0 h
  • Testing6.0 h
  • Review / QA3.0 h
21.0 hours of engineering $3,680
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-42553 in production — separate from our analysis above.

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What this is

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What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
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