CVE-2026-85786: Amazon ion-java Denial of Service Vulnerability

CVE-2026-85786 Overview

CVE-2026-85786 is a denial-of-service vulnerability in Amazon ion-java versions before 1.12.1. The flaw stems from improper handling of highly compressed data, allowing remote attackers to submit a crafted compressed Ion document that expands to an arbitrarily large size upon decompression. The vulnerability results from insufficient coverage of the GZIP auto-decompression opt-out originally introduced for CVE-2026-75936. Applications parsing untrusted Ion input can exhaust memory or CPU resources, disrupting service availability. Amazon has addressed the issue in ion-java version 1.12.1.

Critical Impact
Remote unauthenticated attackers can trigger service disruption in Java applications that decompress Ion documents from untrusted sources.

Affected Products

Amazon ion-java versions prior to 1.12.1
Java applications embedding vulnerable ion-java releases
AWS services and SDKs that depend on ion-java for Ion document parsing

Discovery Timeline

2026-09-04 - CVE-2026-85786 published to NVD
2026-09-08 - Last updated in NVD database

Technical Details for CVE-2026-85786

Vulnerability Analysis

The vulnerability is classified under [CWE-409]: Improper Handling of Highly Compressed Data (Data Amplification). Amazon Ion is a richly-typed, self-describing data serialization format that supports both binary and text encodings. The ion-java library accepts GZIP-compressed Ion streams and automatically decompresses them during parsing. A previous fix for CVE-2026-75936 introduced an opt-out mechanism intended to disable this auto-decompression behavior in security-sensitive contexts. That opt-out did not cover all decompression code paths, leaving a residual attack surface.

An attacker who submits a small, highly compressed Ion payload can force the parser to allocate memory proportional to the decompressed size rather than the wire size. The resulting amplification enables classic decompression-bomb behavior against any service that ingests attacker-controlled Ion documents.

Root Cause

The root cause is incomplete remediation of CVE-2026-75936. The GZIP auto-decompression opt-out did not apply consistently across every code path where ion-java reads compressed input. Callers that expected the opt-out to prevent expansion remained exposed to unbounded decompression, resulting in resource exhaustion under adversarial input.

Attack Vector

Exploitation requires no authentication or user interaction. An attacker sends a crafted GZIP-compressed Ion document over any network channel that ultimately reaches an application using vulnerable ion-java. Common exposure points include REST APIs, message queues, and file upload endpoints that accept Ion payloads. Decompression triggers memory or CPU exhaustion sufficient to degrade or halt the target process. No verified proof-of-concept code is publicly available at this time. Refer to the AWS Security Bulletin 2026-100 for vendor guidance.

Detection Methods for CVE-2026-85786

Indicators of Compromise

Sudden spikes in Java heap usage or garbage collection activity on services that parse Ion documents
OutOfMemoryError exceptions originating in ion-java classes such as IonReaderBuilder or GZIP input stream wrappers
Repeated small inbound requests followed by process unresponsiveness or container restarts

Detection Strategies

Inventory Java applications and dependencies using software composition analysis to identify ion-java versions below 1.12.1
Instrument Ion parsing code paths with size and time limits, and alert when decompression ratios exceed reasonable thresholds
Correlate application crash telemetry with inbound payload size to identify amplification patterns

Monitoring Recommendations

Track JVM memory metrics, GC pauses, and process restarts on services exposed to external Ion input
Log the compressed-to-decompressed size ratio for all inbound Ion documents and flag outliers
Monitor upstream load balancer 5xx rates and request latency for services parsing Ion payloads

How to Mitigate CVE-2026-85786

Immediate Actions Required

Upgrade ion-java to version 1.12.1 or later across all applications and container images
Audit transitive dependencies for vulnerable ion-java versions using tools such as mvn dependency:tree or gradle dependencies
Restrict inbound Ion document size at the network edge until patched artifacts are deployed

Patch Information

Amazon released the fix in ion-java v1.12.1. Update Maven or Gradle build files to pin the patched version and rebuild dependent applications. Consult the AWS Security Bulletin 2026-100 for the authoritative advisory and affected downstream services.

Workarounds

Reject compressed Ion input at the application boundary when feasible, and require callers to submit uncompressed payloads
Enforce strict maximum request body sizes on endpoints that accept Ion documents
Run Ion parsing in isolated processes or containers with hard memory limits to contain resource exhaustion

bash

# Maven dependency pin example
# <dependency>
#   <groupId>com.amazon.ion</groupId>
#   <artifactId>ion-java</artifactId>
#   <version>1.12.1</version>
# </dependency>

mvn versions:set-property -Dproperty=ion-java.version -DnewVersion=1.12.1
mvn dependency:tree | grep ion-java