CVE-2026-93563: Netty SmtpResponseDecoder DOS Vulnerability

CVE-2026-93563 Overview

CVE-2026-93563 is a denial-of-service vulnerability in Netty's SmtpResponseDecoder component. The decoder fails to enforce an upper bound on multi-line Simple Mail Transfer Protocol (SMTP) responses. A malicious or man-in-the-middle (MITM) SMTP server can send a crafted, unterminated multi-line response to a Netty-based client. The client accumulates response fragments in Java Virtual Machine (JVM) heap memory until an OutOfMemoryError triggers a process crash. The flaw is categorized under [CWE-1035] and exploitable over the network without authentication or user interaction.

Critical Impact
A remote attacker controlling or intercepting SMTP server traffic can crash Netty-based Java clients through unbounded memory consumption, causing service outage.

Affected Products

Netty SmtpResponseDecoder component
Java applications embedding Netty for SMTP client functionality
Downstream Red Hat products packaging vulnerable Netty releases

Discovery Timeline

2026-09-18 - CVE-2026-93563 published to the National Vulnerability Database (NVD)
2026-09-25 - Last updated in NVD database

Technical Details for CVE-2026-93563

Vulnerability Analysis

Netty is a widely deployed asynchronous event-driven network framework for Java. The SmtpResponseDecoder class parses SMTP server responses received over a channel pipeline. SMTP responses can span multiple lines, with each intermediate line prefixed by a hyphen and the final line prefixed by a space after the status code.

The decoder buffers incoming lines until it observes the terminating line. It does not enforce a maximum aggregate size on the accumulated buffer. An attacker who controls the server side of the connection can stream response lines indefinitely without ever sending the terminating sequence. Each fragment is retained in the client's heap.

The exhaustion pattern eventually raises an OutOfMemoryError in the JVM, terminating the affected process. Impact is limited to availability; confidentiality and integrity are not affected.

Root Cause

The root cause is missing input length validation during multi-line response aggregation in SmtpResponseDecoder. The component trusts remote peers to send well-formed, bounded responses and lacks a configurable ceiling on cumulative response size.

Attack Vector

Exploitation requires the victim to initiate or accept an SMTP session against an attacker-controlled endpoint. This can occur through direct connection to a hostile mail server or through MITM interception of an outbound SMTP session where TLS is absent, downgraded, or not verified. Once the session is established, the attacker sends a stream of multi-line response continuations and withholds the terminator until the client JVM crashes.

No verified public proof-of-concept code is available. Technical detail is described in the Red Hat CVE-2026-93563 Advisory and Red Hat Bug Tracker #2536976.

Detection Methods for CVE-2026-93563

Indicators of Compromise

Java process termination with java.lang.OutOfMemoryError: Java heap space referencing io.netty.handler.codec.smtp frames in the stack trace
Long-lived outbound SMTP connections showing sustained inbound data volume without session completion
Sudden heap growth in Netty-based services correlated with SMTP client activity

Detection Strategies

Instrument JVM heap and garbage collection metrics for Netty-based SMTP clients and alert on abnormal allocation rates
Inspect application logs for repeated OutOfMemoryError conditions originating in SMTP decoding paths
Deploy network monitoring rules that flag SMTP responses exceeding expected line counts or total byte size per session

Monitoring Recommendations

Capture SMTP session metadata including byte counts, line counts, and session duration for baseline comparison
Forward JVM crash dumps and application error logs to a centralized analytics platform for correlation
Track outbound SMTP destinations against known-good mail relay allowlists to detect anomalous endpoints

How to Mitigate CVE-2026-93563

Immediate Actions Required

Inventory Java applications that embed Netty and use SmtpResponseDecoder in their channel pipelines
Apply the fixed Netty release referenced by the Red Hat CVE-2026-93563 Advisory as soon as it is available for your distribution
Restrict outbound SMTP connections to trusted mail relays through egress firewall rules

Patch Information

Refer to the Red Hat CVE-2026-93563 Advisory for the list of affected packages and remediated versions across Red Hat products. Upstream Netty fix references are tracked in Red Hat Bug Tracker #2536976. Rebuild and redeploy Java applications after upgrading the Netty dependency.

Workarounds

Enforce mandatory Transport Layer Security (TLS) with strict certificate validation on all SMTP client connections to prevent MITM injection
Configure JVM heap limits and container memory ceilings so decoder exhaustion fails fast and is contained by orchestrator restart policies
Wrap the SMTP channel pipeline with a length-limiting frame decoder or session timeout to bound per-connection resource consumption

bash

# Example egress restriction limiting SMTP traffic to a trusted relay
iptables -A OUTPUT -p tcp --dport 25 -d trusted-relay.example.com -j ACCEPT
iptables -A OUTPUT -p tcp --dport 25 -j DROP
iptables -A OUTPUT -p tcp --dport 587 -d trusted-relay.example.com -j ACCEPT
iptables -A OUTPUT -p tcp --dport 587 -j DROP