Java Deserialization Proxy Class Filter Bypass

ZDRES-232: resolveProxyClass Not Overridden - acceptMatchers Filter Bypass via java.lang.reflect.Proxy Assessment: Fully addressed. When the serialised stream contains a TC_PROXYCLASSDESC (the marker for a java.lang.reflect.Proxy ), JDK’s ObjectInputStream.readProxyDesc() is dispatched. JDK then calls the default ObjectInputStream.resolveProxyClass(interfaces) implementation, which performs Class.forName(intf, false, latestUserDefinedLoader()) for EACH interface name and constructs the proxy class â€” bypassing the accepted classes list . ZDRES-233: Class.forName(name, initialize=true, classLoader) in readClassDescriptor Triggers Static Initialiser of Allow-Listed Classes Assessment: Fully addressed. For ANY class on the allow-list, deserialising a stream that names it triggers the class’s (static initialiser) BEFORE any instance is constructed. This means an attacker who supplies a class name on the allow-list (e.g., the developer wrote accept(“com.myapp.*") , att

Casky was already ahead

This CVE exploits attack patterns that Casky's 419matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

CVE-2026-47065 is a critical Java deserialization vulnerability (CVSS 9.8) that exploits unsafe handling of serialized Proxy objects in ObjectInputStream.readProxyDesc(). When a serialized stream contains a TC_PROXYCLASSDESC marker, the JDK dispatches to resolveProxyClass() without proper validation, allowing attackers to bypass acceptMatchers filters by dynamically constructing arbitrary Proxy classes through interface name resolution. This affects any Java application deserializing untrusted data—particularly enterprise systems handling remote communications, message queues, RPC frameworks, and data persistence layers that rely on Java serialization without robust input validation.

Casky.ai's 419 mapped security skills detect the attack patterns underlying this vulnerability across MITRE ATT&CK techniques: TA0001 (Initial Access) captures malicious serialized payloads injected into network streams or files; TA0043 (Reconnaissance) identifies probing for deserialization endpoints; and TA0002 (Execution) detects when arbitrary code runs during unsafe Proxy instantiation. Practitioners using Casky's Claude-powered analysis would identify suspicious patterns including unexpected java.lang.reflect.Proxy class instantiation in logs, ClassLoader invocations with unusual interface names, and bypass attempts against security filters—flagging these as high-priority execution threats requiring immediate patching or deserialization policy enforcement.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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419 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-47065.

analyzing-android-malware-with-apktool

malware analysis · medium

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analyzing-apt-group-with-mitre-navigator

threat intelligence · low

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analyzing-bootkit-and-rootkit-samples

malware analysis · medium

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MITRE ATT&CK Techniques

TA0001 TA0043 TA0002

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