# CVE-2026-42553: Cinny: Access token disclosure via invalidated emoji pack avatar URL in service worker

> An attacker can steal a victim's Matrix access token by tricking them into opening a malicious emoji pack, leading to account takeover.

- **Severity:** High 7.1 (`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`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 May 2026
- **Updated:** 27 May 2026
- **Also known as:** GHSA-j944-w549-3453

## What is CVE-2026-42553?

CVE-2026-42553 is a high-severity vulnerability in cinnyapp cinny, published on 27 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official 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.

Weakness: CWE-20 (Improper Input Validation)

## Which versions are affected by CVE-2026-42553?

2 open source packages ship an affected version, including cinny.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `cinny` | npm | All versions | 4.10.3 |
| `cinny` | npm | All versions | 4.10.3 |

## How do I fix CVE-2026-42553?

Upgrade cinny to 4.10.3 or later.

## Is CVE-2026-42553 being exploited?

There is no known exploitation of CVE-2026-42553 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-42553 and fix it if it is affected.

What it is: CVE-2026-42553 is a high-severity vulnerability in cinnyapp cinny, published on 27 May 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-42553 in the wild and no public exploit code.

Affected packages and fixed versions:
- cinny (npm): affected all versions; fixed in 4.10.3
- cinny (npm): affected all versions; fixed in 4.10.3

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-42553/ (Markdown: https://insights.sbomify.com/CVE-2026-42553/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-42553
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-42553
- OSV: https://osv.dev/vulnerability/CVE-2026-42553

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-42553/
Summaries are generated and may contain mistakes; the official description above is the record.