# CVE-2026-40203: Information disclosure in OX Dovecot CE

> An attacker can confirm if a message body matches a guess by observing IMAP traffic sizes when compression is enabled, disclosing sensitive information.

- **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 28 Aug 2026
- **Updated:** 28 Aug 2026
- **Also known as:** ALPINE-CVE-2026-40203, AZL-98274, DEBIAN-CVE-2026-40203, GHSA-cf77-42mj-h39j, UBUNTU-CVE-2026-40203

## What is CVE-2026-40203?

CVE-2026-40203 is a low-severity vulnerability in Open-Xchange GmbH OX Dovecot CE, published on 28 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

When IMAP compression is enabled, the same compression state is reused across responses in a session, so response sizes depend on both attacker-supplied mail and other mail in the same mailbox. An attacker that can send mail to a user and can also observe the sizes of that user's IMAP traffic can confirm whether the body of a small message matches a guessed text. Recovery of arbitrary unknown content was not demonstrated, but the attack can disclose whether a secret-like message body matches a candidate. Disable IMAP compression. Update to non-vulnerable version. No publicly available exploits are known.

Weakness: CWE-200 (Exposure of Sensitive Information to an Unauthorized Actor)

## Which versions are affected by CVE-2026-40203?


## How do I fix CVE-2026-40203?

Upgrade to a fixed version.

## Is CVE-2026-40203 being exploited?

There is no known exploitation of CVE-2026-40203 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-40203 and fix it if it is affected.

What it is: CVE-2026-40203 is a low-severity vulnerability in Open-Xchange GmbH OX Dovecot CE, published on 28 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-40203 in the wild and no public exploit code.

Affected product: Open-Xchange GmbH OX Dovecot CE. 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-2026-40203/ (Markdown: https://insights.sbomify.com/CVE-2026-40203/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-40203
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-40203
- OSV: https://osv.dev/vulnerability/CVE-2026-40203

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-40203/
Summaries are generated and may contain mistakes; the official description above is the record.