CVE-2026-102496: Apache XmlSchema DOS Vulnerability

CVE-2026-102496 Overview

CVE-2026-102496 is a denial-of-service vulnerability in Apache XmlSchema. The library fails to enforce a depth limit when constructing its schema model from input XML Schema Definition (XSD) documents. A crafted schema with deeply nested structures forces the parser into unbounded recursion, exhausting the thread stack and triggering a StackOverflowError. Any application that accepts and parses untrusted XSD content using Apache XmlSchema is exposed. The maintainers have released version 2.3.3 to remediate the issue. The weakness is classified as uncontrolled recursion [CWE-674].

Critical Impact
A single malicious schema submitted over the network can crash the parsing thread or the entire hosting process, producing a reliable denial-of-service condition without authentication or user interaction.

Affected Products

Apache XmlSchema versions prior to 2.3.3
Java applications and web services that consume untrusted XSD input through Apache XmlSchema
Downstream projects that embed Apache XmlSchema for WSDL or SOAP schema processing

Discovery Timeline

2026-09-29 - CVE-2026-102496 published to the National Vulnerability Database
2026-09-29 - Last updated in NVD database

Technical Details for CVE-2026-102496

Vulnerability Analysis

Apache XmlSchema builds an in-memory representation of an XSD document by walking its element tree. Constructs such as xs:complexType, xs:sequence, xs:choice, and xs:group can contain further type or group references, and the library resolves these by recursive descent. Because the traversal does not track or cap nesting depth, an attacker who controls the schema can force the parser to recurse arbitrarily deep. Each recursive call consumes Java Virtual Machine (JVM) stack frames until the thread exhausts its stack and throws StackOverflowError. The failure typically terminates the parsing thread and can destabilize services that treat the error as unrecoverable.

Root Cause

The root cause is missing input validation on schema structural depth during model construction. Apache XmlSchema relies on the JVM stack for traversal state rather than an explicit iterative worklist or a depth counter. Without a bound, structural complexity in attacker-controlled input translates directly into recursion depth in the parser.

Attack Vector

An attacker delivers a malicious XSD document to any endpoint that parses schemas with the vulnerable library. Typical delivery paths include SOAP or WSDL processing, XML validation endpoints, document import features, and integration APIs that fetch remote schemas. Exploitation requires no privileges and no user interaction. The impact is limited to availability; confidentiality and integrity are not affected.

No verified public proof-of-concept code is available. Consult the Apache Mailing List Thread and the Openwall OSS Security Update for the maintainer discussion and remediation notes.

Detection Methods for CVE-2026-102496

Indicators of Compromise

Repeated java.lang.StackOverflowError entries in application logs with stack frames referencing org.apache.ws.commons.schema classes
Abrupt worker-thread termination or process crashes coinciding with XSD or WSDL ingest requests
Inbound requests carrying XSD payloads with anomalously deep nesting of complexType, sequence, choice, or group elements

Detection Strategies

Inventory Java services using Apache XmlSchema and flag any version earlier than 2.3.3 during software composition analysis scans
Add web application firewall or API gateway rules that measure XML element nesting depth and reject requests exceeding a sane threshold
Correlate StackOverflowError events in centralized logging with source IPs and request payload sizes to surface probing activity

Monitoring Recommendations

Track JVM thread crash rates and unhandled exception counters on services that parse external schemas
Alert on sudden spikes in XSD, WSDL, or SOAP request volume from a single client, which may indicate DoS probing
Monitor upstream load balancer 5xx rates on schema-processing endpoints as a leading indicator of successful exploitation

How to Mitigate CVE-2026-102496

Immediate Actions Required

Upgrade Apache XmlSchema to version 2.3.3 in all affected applications and rebuild dependent artifacts
Identify transitive dependencies pulling in older Apache XmlSchema releases and pin the fixed version in dependency management files
Restrict schema ingestion endpoints to authenticated clients where feasible until patching is complete

Patch Information

Apache XmlSchema 2.3.3 remediates the recursion issue. Update Maven or Gradle coordinates to reference the fixed release and redeploy all services. Review the Apache Mailing List Thread for the maintainer announcement.

Workarounds

Place a size and depth-limiting reverse proxy in front of endpoints that accept XSD or WSDL input
Reject externally supplied schemas and only process schemas from a trusted internal registry
Run schema parsing in an isolated worker process with restart-on-crash supervision to contain availability impact

bash

# Maven dependency update to the fixed release
# pom.xml
# <dependency>
#   <groupId>org.apache.ws.xmlschema</groupId>
#   <artifactId>xmlschema-core</artifactId>
#   <version>2.3.3</version>
# </dependency>

mvn versions:set-property -Dproperty=xmlschema.version -DnewVersion=2.3.3
mvn dependency:tree | grep xmlschema-core