# CVE-2026-66912: Stored Cross-Site Scripting in Zimbra


- **Severity:** Medium 5.3 (`CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:P/VC:N/VI:N/VA:N/SC:L/SI:L/SA:N`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 29 Oct 2026
- **Updated:** 29 Oct 2026
- **Also known as:** GHSA-rprc-q5mj-4mcw

## What is CVE-2026-66912?

CVE-2026-66912 is a medium-severity vulnerability in Synacor Zimbra Collaboration, published on 29 Oct 2026. 

### Official description

Zimbra is vulnerable to Stored Cross-Site Scripting (XSS) via the From email header during HTML reply composition in Classic UI. Due to insufficient sanitization of the From header value, a remote attacker can send a crafted email containing a malicious From header with an embedded JavaScript payload. When the victim opens the message in Classic UI and clicks Reply while composing in HTML mode with reply headers disabled, the unsanitized From header content is rendered as HTML and the JavaScript payload executes in the victim's browser, potentially allowing the attacker to steal session information or perform actions on behalf of the victim. This issue was fixed in version 10.1.21

Weakness: CWE-79 (Improper Neutralization of Input During Web Page Generation)

## Which versions are affected by CVE-2026-66912?


## How do I fix CVE-2026-66912?

No fix has been published for CVE-2026-66912 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-66912 being exploited?

There is no known exploitation of CVE-2026-66912 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-66912 and fix it if it is affected.

What it is: CVE-2026-66912 is a medium-severity vulnerability in Synacor Zimbra Collaboration, published on 29 Oct 2026. 
Exploitation: There is no known exploitation of CVE-2026-66912 in the wild and no public exploit code.

Affected product: Synacor Zimbra Collaboration. No package-level version data is available, so check the vendor advisory linked from the reference page.

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-2026-66912/ (Markdown: https://insights.sbomify.com/CVE-2026-66912/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-66912
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-66912
- OSV: https://osv.dev/vulnerability/CVE-2026-66912

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-66912/
Summaries are generated and may contain mistakes; the official description above is the record.