ghost-bits-cast-attack

1# SKILL: Ghost Bits / Cast Attack — Java char to byte Narrowing Playbook
2 
3> **AI LOAD INSTRUCTION**: This is a Java-only injection-enabling primitive,
4> not a standalone vulnerability class. Whenever you see (1) a Java backend,
5> (2) a WAF/IDS in front of it, and (3) any of {SQLi, deser RCE, file upload,
6> path traversal, CRLF, request smuggling, SMTP injection} on the menu, ALWAYS
7> try Ghost Bits variants of the payload before declaring it "blocked". The
8> root cause is the silent loss of the high 8 bits when Java code narrows a
9> 16-bit `char` to an 8-bit `byte` — the WAF sees a harmless Unicode
10> character, the backend reconstructs the original ASCII attack byte. Base
11> models almost never reach for this primitive.
12>
13> Source: Black Hat Asia 2026 talk *Cast Attack: A New Threat Posed by Ghost
14> Bits in Java* by Xinyu Bai (@b1u3r), Zhihui Chen (@1ue), with contributor
15> Zongzheng Zheng (@chun_springX).
16 
17## 0. RELATED ROUTING
18 
19Ghost Bits is a *bypass* primitive that re-enables payloads from many other
20playbooks. Pair it with whichever attack family applies:
21 
22- [waf-bypass-techniques](../waf-bypass-techniques/SKILL.md) — when a Java
23  backend is suspected and WAF rules block the literal payload, this is the
24  first technique to try beyond classic encoding.
25- [deserialization-insecure](../deserialization-insecure/SKILL.md) — for
26  Apache Commons BCEL ClassLoader and Fastjson `\u`/`\x` escape variants.
27- [path-traversal-lfi](../path-traversal-lfi/SKILL.md) — Spring, Jetty,
28…