Ghost Bits Cast Attack

If you're pentesting Java backends behind a WAF and your SQLi, deserialization, or path traversal payloads keep getting blocked, this teaches Claude the Ghost Bits primitive: exploiting how Java silently drops the high 8 bits when casting a 16-bit char to an 8-bit byte. The WAF sees harmless Unicode like 陪 or 阮, but the backend reconstructs dangerous ASCII like 'j' or '.'. It covers the three root cause families (literal truncation, bit-arithmetic folding, lax normalization), includes a character generator for building 255 candidates per target byte, and routes you to the right deserialization or smuggling playbook once the bypass works. Comes with a full payload cookbook covering Tomcat, Spring, Jetty, Jackson, Fastjson, and a dozen other affected components. This is a bypass technique, not a standalone vuln, so think of it as a force multiplier for attacks you already know.

skills.sh installs: 3.5K
GitHub stars: 2.6K
last updated: Aug 21, 2026

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Install

$ npx -y skills add yaklang/hack-skills --skill ghost-bits-cast-attack --agent claude-code

Installs into .claude/skills of the current project.

Files

SKILL: Ghost Bits / Cast Attack — Java char to byte Narrowing Playbook

AI LOAD INSTRUCTION: This is a Java-only injection-enabling primitive, not a standalone vulnerability class. Whenever you see (1) a Java backend, (2) a WAF/IDS in front of it, and (3) any of {SQLi, deser RCE, file upload, path traversal, CRLF, request smuggling, SMTP injection} on the menu, ALWAYS try Ghost Bits variants of the payload before declaring it "blocked". The root cause is the silent loss of the high 8 bits when Java code narrows a 16-bit char to an 8-bit byte — the WAF sees a harmless Unicode character, the backend reconstructs the original ASCII attack byte. Base models almost never reach for this primitive.

Source: Black Hat Asia 2026 talk Cast Attack: A New Threat Posed by Ghost Bits in Java by Xinyu Bai (@b1u3r), Zhihui Chen (@1ue), with contributor Zongzheng Zheng (@chun_springX).