# CVE-2026-93563: Netty SmtpResponseDecoder unbounded memory accumulation

Source: https://junglewise.ai/threats/cve-2026-93563-a-flaw-was-found-in-netty-s-smtpresponsedecoder-component-a  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-18. Updated: 2026-09-26  
License: CC BY 4.0 with a link back

CVE-2026-93563 · Severity: high · CVSS 7.5 · Published 2026-09-18

Netty is a widely-used networking library for building scalable Java applications. A flaw in its SMTP response decoder allows a malicious SMTP server to send specially crafted messages that consume unlimited memory on the client, eventually crashing the application with an out-of-memory error and causing service denial.

Technologies: [Netty Project Netty](https://junglewise.ai/threats/technologies/netty-project-netty). Vendors: [Netty Project](https://junglewise.ai/threats/vendors/netty-project).

## Quick answers

**Is CVE-2026-93563 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-93563 being exploited in the wild.

**How severe is CVE-2026-93563?** High severity, with a CVSS score of 7.5, published 18 September 2026.

**Which products and versions are affected?** Netty Project Netty.

**Is there a fix for CVE-2026-93563?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-93563 attacked?** It is attacked over the network.

**Am I affected by CVE-2026-93563?** Check whether you run Netty Project Netty. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The SmtpResponseDecoder component fails to bound memory allocation when processing multi-line SMTP responses lacking proper terminators, allowing a remote attacker acting as an MITM or malicious SMTP server to trigger unbounded memory accumulation in the JVM heap. This leads to OutOfMemoryError and denial of service via process crash. The vulnerability requires network access to SMTP connections and no authentication.

## Affected products

- Netty Project Netty

## Timeline

- 2026-09-18: disclosed

## References

- [https://access.redhat.com/downloads/content/package-browser/](https://access.redhat.com/downloads/content/package-browser/)
- [https://access.redhat.com/jbossnetwork/restricted/listSoftware.html](https://access.redhat.com/jbossnetwork/restricted/listSoftware.html)

## Related threats

- [Netty incomplete hostname verification in QUIC certificate verification](https://junglewise.ai/threats/cve-2026-100665-netty-versions-from-4-2-11-final-before-4-2-18-final-contain-an)
- [Netty netty-codec-http request smuggling via chunk-size parsing](https://junglewise.ai/threats/cve-2026-93574-a-flaw-was-found-in-netty-s-netty-codec-http-component-a-remote)
- [Netty HTTP/2 header injection vulnerability](https://junglewise.ai/threats/cve-2026-93579-a-flaw-was-found-in-netty-s-http-2-stack-this-vulnerability)
- [Netty RedisArrayAggregator heap memory exhaustion](https://junglewise.ai/threats/cve-2026-93572-a-flaw-was-found-in-netty-s-redisarrayaggregator-component-a)
- [Netty memory leak in StompSubframeDecoder](https://junglewise.ai/threats/cve-2026-93494-a-flaw-was-found-in-netty-s-stompsubframedecoder-component-a)