Extracción de Entitlements de Mach-O e Indexado de IPSW

Reading time: 8 minutes

Visión general

Esta página explica cómo extraer entitlements de binarios Mach-O de forma programática recorriendo LC_CODE_SIGNATURE y analizando el SuperBlob de la firma de código, y cómo escalar esto a través de firmwares IPSW de Apple montando e indexando su contenido para búsquedas/diffs forenses.

Si necesitas un repaso sobre el formato Mach-O y code signing, consulta también: macOS code signing and SuperBlob internals.

• Consulta macOS code signing details (SuperBlob, Code Directory, special slots): macOS Code Signing
• Consulta general Mach-O structures/load commands: Universal binaries & Mach-O Format

Entitlements en Mach-O: dónde residen

Los entitlements se almacenan dentro de los datos de la firma de código referenciados por el load command LC_CODE_SIGNATURE y ubicados en el segmento __LINKEDIT. La firma es un CS_SuperBlob que contiene múltiples blobs (code directory, requirements, entitlements, CMS, etc.). El blob de entitlements es un CS_GenericBlob cuyo contenido es un Apple Binary Property List (bplist00) que mapea claves de entitlements a valores.

Estructuras clave (de xnu):

/* mach-o/loader.h */
struct mach_header_64 {
uint32_t magic;
cpu_type_t cputype;
cpu_subtype_t cpusubtype;
uint32_t filetype;
uint32_t ncmds;
uint32_t sizeofcmds;
uint32_t flags;
uint32_t reserved;
};

struct load_command {
uint32_t cmd;
uint32_t cmdsize;
};

/* Entitlements live behind LC_CODE_SIGNATURE (cmd=0x1d) */
struct linkedit_data_command {
uint32_t cmd;        /* LC_CODE_SIGNATURE */
uint32_t cmdsize;    /* sizeof(struct linkedit_data_command) */
uint32_t dataoff;    /* file offset of data in __LINKEDIT */
uint32_t datasize;   /* file size of data in __LINKEDIT */
};

/* osfmk/kern/cs_blobs.h */
typedef struct __SC_SuperBlob {
uint32_t magic;   /* CSMAGIC_EMBEDDED_SIGNATURE = 0xfade0cc0 */
uint32_t length;
uint32_t count;
CS_BlobIndex index[];
} CS_SuperBlob;

typedef struct __BlobIndex {
uint32_t type;    /* e.g., CSMAGIC_EMBEDDED_ENTITLEMENTS = 0xfade7171 */
uint32_t offset;  /* offset of entry */
} CS_BlobIndex;

typedef struct __SC_GenericBlob {
uint32_t magic;   /* same as type when standalone */
uint32_t length;
char data[];      /* Apple Binary Plist containing entitlements */
} CS_GenericBlob;

Important constants:

• LC_CODE_SIGNATURE cmd = 0x1d
• CS SuperBlob magic = 0xfade0cc0
• Entitlements blob type (CSMAGIC_EMBEDDED_ENTITLEMENTS) = 0xfade7171
• DER entitlements may be present via special slot (e.g., -7), see the macOS Code Signing page for special slots and DER entitlements notes

Note: Multi-arch (fat) binaries contain multiple Mach-O slices. You must pick the slice for the architecture you want to inspect and then walk its load commands.

Extraction steps (generic, lossless-enough)

1. Parse Mach-O header; iterate ncmds worth of load_command records.
2. Locate LC_CODE_SIGNATURE; read linkedit_data_command.dataoff/datasize to map the Code Signing SuperBlob placed in __LINKEDIT.
3. Validate CS_SuperBlob.magic == 0xfade0cc0; iterate count entries of CS_BlobIndex.
4. Locate index.type == 0xfade7171 (embedded entitlements). Read the pointed CS_GenericBlob and parse its data as an Apple binary plist (bplist00) to key/value entitlements.

Implementation notes:

• Code signature structures use big-endian fields; swap byte order when parsing on little-endian hosts.
• The entitlements GenericBlob data itself is a binary plist (handled by standard plist libraries).
• Some iOS binaries may carry DER entitlements; also some stores/slots differ across platforms/versions. Cross-check both standard and DER entitlements as needed.
• For fat binaries, use the fat headers (FAT_MAGIC/FAT_MAGIC_64) to locate the correct slice and offset before walking Mach-O load commands.

Minimal parsing outline (Python)

The following is a compact outline showing the control flow to find and decode entitlements. It intentionally omits robust bounds checks and full fat binary support for brevity.

import plistlib, struct

LC_CODE_SIGNATURE = 0x1d
CSMAGIC_EMBEDDED_SIGNATURE = 0xfade0cc0
CSMAGIC_EMBEDDED_ENTITLEMENTS = 0xfade7171

# all code-signing integers are big-endian per cs_blobs.h
be32 = lambda b, off: struct.unpack_from(">I", b, off)[0]

def parse_entitlements(macho_bytes):
# assume already positioned at a single-arch Mach-O slice
magic, = struct.unpack_from("<I", macho_bytes, 0)
is64 = magic in (0xfeedfacf,)
if is64:
ncmds = struct.unpack_from("<I", macho_bytes, 0x10)[0]
sizeofcmds = struct.unpack_from("<I", macho_bytes, 0x14)[0]
off = 0x20
else:
# 32-bit not shown
return None

code_sig_off = code_sig_size = None
for _ in range(ncmds):
cmd, cmdsize = struct.unpack_from("<II", macho_bytes, off)
if cmd == LC_CODE_SIGNATURE:
# struct linkedit_data_command is little-endian in file
_, _, dataoff, datasize = struct.unpack_from("<IIII", macho_bytes, off)
code_sig_off, code_sig_size = dataoff, datasize
off += cmdsize

if code_sig_off is None:
return None

blob = macho_bytes[code_sig_off: code_sig_off + code_sig_size]
if be32(blob, 0x0) != CSMAGIC_EMBEDDED_SIGNATURE:
return None

count = be32(blob, 0x8)
# iterate BlobIndex entries (8 bytes each after 12-byte header)
for i in range(count):
idx_off = 12 + i*8
btype = be32(blob, idx_off)
boff  = be32(blob, idx_off+4)
if btype == CSMAGIC_EMBEDDED_ENTITLEMENTS:
# GenericBlob is big-endian header followed by bplist
glen = be32(blob, boff+4)
data = blob[boff+8: boff+glen]
return plistlib.loads(data)
return None

Usage tips:

• Para tratar binarios fat, primero lee struct fat_header/fat_arch, elige la slice de arquitectura deseada, luego pasa el subrango a parse_entitlements.
• En macOS puedes validar los resultados con: codesign -d --entitlements :- /path/to/binary

Ejemplos de hallazgos

Los binarios de plataforma privilegiada suelen solicitar entitlements sensibles como:

• com.apple.security.network.server = true
• com.apple.rootless.storage.early_boot_mount = true
• com.apple.private.kernel.system-override = true
• com.apple.private.pmap.load-trust-cache = ["cryptex1.boot.os", "cryptex1.boot.app", "cryptex1.safari-downlevel"]

Buscar estos a escala a través de imágenes de firmware es extremadamente valioso para mapear la superficie de ataque y comparar diferencias entre lanzamientos/dispositivos.

Escalado a través de IPSWs (montaje e indexación)

Para enumerar ejecutables y extraer entitlements a escala sin almacenar imágenes completas:

• Usa la herramienta ipsw de @blacktop para descargar y montar los sistemas de archivos de firmware. El montaje aprovecha apfs-fuse, por lo que puedes recorrer volúmenes APFS sin extracción completa.

# Download latest IPSW for iPhone11,2 (iPhone XS)
ipsw download ipsw -y --device iPhone11,2 --latest

# Mount IPSW filesystem (uses underlying apfs-fuse)
ipsw mount fs <IPSW_FILE>

• Recorrer volúmenes montados para localizar archivos Mach-O (comprobar magic y/o usar file/otool), luego analizar entitlements y frameworks importados.
• Persistir una vista normalizada en una base de datos relacional para evitar crecimiento lineal a través de miles de IPSWs:
• executables, operating_system_versions, entitlements, frameworks
• many-to-many: executable↔OS version, executable↔entitlement, executable↔framework

Ejemplo de consulta para listar todas las versiones del sistema operativo que contienen un nombre de ejecutable dado:

SELECT osv.version AS "Versions"
FROM device d
LEFT JOIN operating_system_version osv ON osv.device_id = d.id
LEFT JOIN executable_operating_system_version eosv ON eosv.operating_system_version_id = osv.id
LEFT JOIN executable e ON e.id = eosv.executable_id
WHERE e.name = "launchd";

Notas sobre la portabilidad de la DB (si implementas tu propio indexador):

• Usa una abstracción/ORM (por ejemplo, SeaORM) para mantener el código agnóstico a la DB (SQLite/PostgreSQL).
• SQLite requiere AUTOINCREMENT solo en un INTEGER PRIMARY KEY; si quieres PKs i64 en Rust, genera entidades como i32 y convierte los tipos, SQLite almacena INTEGER internamente como entero con signo de 8 bytes.

Herramientas de código abierto y referencias para entitlement hunting

• Firmware mount/download: https://github.com/blacktop/ipsw
• Entitlement databases and references:
• Jonathan Levin’s entitlement DB: https://newosxbook.com/ent.php
• entdb: https://github.com/ChiChou/entdb
• Large-scale indexer (Rust, self-hosted Web UI + OpenAPI): https://github.com/synacktiv/appledb_rs
• Apple headers for structures and constants:
• loader.h (Mach-O headers, load commands)
• cs_blobs.h (SuperBlob, GenericBlob, CodeDirectory)

Para más información sobre los detalles internos de code signing (Code Directory, special slots, DER entitlements), consulta: macOS Code Signing

Referencias

• appledb_rs: a research support tool for Apple platforms
• synacktiv/appledb_rs
• blacktop/ipsw
• Jonathan Levin’s entitlement DB
• ChiChou/entdb
• XNU cs_blobs.h
• XNU mach-o/loader.h
• SQLite Datatypes