# CVE-2021-3657: A flaw was found in mbsync versions prior to 1.4.4.


- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Fix available:** Yes
- **Published:** 18 Feb 2022
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-3657, GHSA-gq8r-4g7c-69vw, UBUNTU-CVE-2021-3657

## What is CVE-2021-3657?

CVE-2021-3657 is a critical vulnerability in Debian Debian Linux, published on 18 Feb 2022. 

### Official description

A flaw was found in mbsync versions prior to 1.4.4. Due to inadequate handling of extremely large (>=2GiB) IMAP literals, malicious or compromised IMAP servers, and hypothetically even external email senders, could cause several different buffer overflows, which could conceivably be exploited for remote code execution.

Weakness: CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer)

## Which versions are affected by CVE-2021-3657?

It affects 4 products.


Products: Debian Debian Linux, Fedoraproject Fedora, Isync Project Isync, Redhat Enterprise Linux.

## How do I fix CVE-2021-3657?

Upgrade to a fixed version.

## Is CVE-2021-3657 being exploited?

There is no known exploitation of CVE-2021-3657 in the wild and no public exploit code.

## Other vulnerabilities in Debian Debian Linux

- [CVE-2015-3306](https://insights.sbomify.com/CVE-2015-3306/): ProFTPD Improper Access Control (critical 10.0, exploited in the wild)
- [CVE-2025-49113](https://insights.sbomify.com/CVE-2025-49113/): RoundCube Webmail Deserialization of Untrusted Data (critical 9.9, exploited in the wild)
- [CVE-2026-24061](https://insights.sbomify.com/CVE-2026-24061/): GNU InetUtils Argument Injection (critical 9.8, exploited in the wild)
- [CVE-2025-39682](https://insights.sbomify.com/CVE-2025-39682/): Linux Kernel Improper Check for Unusual or Exceptional Conditions (critical 9.8, exploited in the wild)
- [CVE-2024-9680](https://insights.sbomify.com/CVE-2024-9680/): Mozilla Firefox Use-After-Free (critical 9.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-3657 and fix it if it is affected.

What it is: CVE-2021-3657 is a critical vulnerability in Debian Debian Linux, published on 18 Feb 2022. 
Exploitation: There is no known exploitation of CVE-2021-3657 in the wild and no public exploit code.

Affected product: Debian Debian Linux. 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-2021-3657/ (Markdown: https://insights.sbomify.com/CVE-2021-3657/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-3657
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-3657
- OSV: https://osv.dev/vulnerability/CVE-2021-3657

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-3657/
Summaries are generated and may contain mistakes; the official description above is the record.