SKILL: Heap Exploitation — Expert Attack Playbook

AI LOAD INSTRUCTION: Expert glibc heap exploitation techniques. Covers ptmalloc2 internals, bin structures, tcache mechanics, libc/heap leak methods, and attack selection by glibc version. Distilled from ctf-wiki heap sections, how2heap, and real-world exploitation. Base models often confuse glibc version constraints and miss safe-linking (PROTECT_PTR) introduced in 2.32.

0. RELATED ROUTING

• [stack-overflow-and-rop](../stack-overflow-and-rop/SKILL.md) — when the overflow is on the stack rather than the heap
• [format-string-exploitation](../format-string-exploitation/SKILL.md) — leak heap/libc addresses via format string
• [arbitrary-write-to-rce](../arbitrary-write-to-rce/SKILL.md) — convert heap arbitrary write into code execution
• [binary-protection-bypass](../binary-protection-bypass/SKILL.md) — bypass ASLR/RELRO to use heap write effectively

Advanced References

• [HOUSEOFTECHNIQUES.md](./HOUSEOFTECHNIQUES.md) — House of Force/Spirit/Orange/Einherjar/Roman/Pig/Banana/Cat/Apple and tcache attacks
• [IOFILEEXPLOITATION.md](./IOFILEEXPLOITATION.md) — IOFILE vtable hijack, FSOP, stdout/stdin abuse, exit flow exploitation

1. PTMALLOC2 STRUCTURE QUICK REFERENCE

malloc_chunk Layout (64-bit)

         chunk pointer (returned by malloc - 0x10)
         ┌──────────────────────────┐
    0x00 │  prev_size (if prev free)│
    0x08 │  size        | A | M | P │  ← P=PREV_INUSE, M=IS_MMAPPED, A=NON_MAIN_ARENA
         ├──────────────────────────┤  ← user data starts here (returned pointer)
    0x10 │  fd (if free)            │  ← forward pointer to next free chunk
    0x18 │  bk (if free)            │  ← backward pointer to prev free chunk
    0x20 │  fd_nextsize (large only)│
    0x28 │  bk_nextsize (large only)│
         └──────────────────────────┘

Bin Types

Key Global Structures

2. LEAK METHODS

Libc Base Leak

Heap Base Leak

Safe-Linking Decode (glibc ≥ 2.32)

# PROTECT_PTR: fd_stored = (chunk_addr >> 12) ^ real_fd
# To decode: real_fd = fd_stored ^ (chunk_addr >> 12)
# To encode: fd_stored = (chunk_addr >> 12) ^ target_addr

def deobfuscate(stored_fd, chunk_addr):
    return stored_fd ^ (chunk_addr >> 12)

def obfuscate(target, chunk_addr):
    return (chunk_addr >> 12) ^ target

3. ATTACK CATEGORIES BY GLIBC VERSION

glibc < 2.26 (No tcache)

glibc 2.26–2.28 (tcache, no key)

glibc 2.29–2.31 (tcache key introduced)

glibc 2.32–2.33 (safe-linking / PROTECT_PTR)

glibc ≥ 2.34 (hooks removed)

Post-2.34 targets: see [arbitrary-write-to-rce](../arbitrary-write-to-rce/SKILL.md) for IOFILE, exitfuncs, TLSdtorlist, dl_fini.

4. COMMON VULNERABILITY PATTERNS

5. TOOLS

# pwndbg heap inspection
pwndbg> heap                      # display all chunks
pwndbg> bins                      # show all bin contents
pwndbg> tcachebins                # tcache status
pwndbg> fastbins                  # fastbin status
pwndbg> unsortedbin               # unsortedbin content
pwndbg> vis_heap_chunks           # visual heap layout
pwndbg> find_fake_fast &__malloc_hook  # find nearby fake fastbin chunks

# how2heap — reference implementations
git clone https://github.com/shellphish/how2heap

# heapinspect
pip install heapinspect
heapinspect <pid>

# pwntools helpers
from pwn import *
libc = ELF('./libc.so.6')
print(hex(libc.symbols['__malloc_hook']))
print(hex(libc.symbols['__free_hook']))

6. DECISION TREE

Heap vulnerability identified
├── What is the primitive?
│   ├── UAF (read + write)
│   │   ├── Can read freed chunk? → Leak libc (unsortedbin) or heap (tcache fd)
│   │   └── Can write freed chunk? → Tcache poisoning / fastbin dup
│   ├── Double free
│   │   ├── glibc < 2.29 → direct tcache dup
│   │   ├── glibc 2.29-2.31 → corrupt tcache key first, or House of Botcake
│   │   └── glibc ≥ 2.32 → need heap leak for safe-linking encode
│   ├── Heap overflow (controlled size)
│   │   ├── Overwrite next chunk size → overlapping chunks → UAF
│   │   └── Overwrite fd directly → arbitrary allocation
│   ├── Off-by-one / off-by-null
│   │   ├── Null byte into size → House of Einherjar (backward consolidation)
│   │   └── One byte into size → shrink chunk, create overlap
│   └── Arbitrary write (from overlap or poisoned allocation)
│       ├── glibc < 2.34 → __malloc_hook / __free_hook → one_gadget
│       ├── glibc ≥ 2.34 → _IO_FILE vtable, exit_funcs, TLS_dtor_list
│       └── Partial RELRO → GOT overwrite
│
├── Need libc leak?
│   ├── Free chunk into unsortedbin (size > 0x410 or fill 7 tcache)
│   ├── Read fd/bk → main_arena offset → libc base
│   └── Alternative: stdout FILE partial overwrite for leak
│
└── Need heap leak? (glibc ≥ 2.32)
    ├── Read tcache fd from freed chunk
    └── Decode: real_addr = stored_fd ^ (chunk_addr >> 12)