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 — when the overflow is on the stack rather than the heap
• format-string-exploitation — leak heap/libc addresses via format string
• arbitrary-write-to-rce — convert heap arbitrary write into code execution
• binary-protection-bypass — bypass ASLR/RELRO to use heap write effectively

Advanced References

• HOUSE_OF_TECHNIQUES.md — House of Force/Spirit/Orange/Einherjar/Roman/Pig/Banana/Cat/Apple and tcache attacks
• IO_FILE_EXPLOITATION.md — _IO_FILE vtable hijack, FSOP, stdout/stdin abuse, exit flow exploitation

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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

Bin	Size Range (64-bit)	Structure	LIFO/FIFO
tcache (per-thread)	≤ 0x410 (7 entries per size)	Singly linked (next pointer)	LIFO
fastbin	≤ 0x80 (default)	Singly linked (fd)	LIFO
unsortedbin	Any freed size	Doubly linked circular	FIFO
smallbin	< 0x400	Doubly linked circular	FIFO
largebin	≥ 0x400	Doubly linked + size-sorted	Sorted

Key Global Structures

Structure	Location	Purpose
main_arena	libc .data segment	Contains bin heads, top chunk, system_mem
mp_	libc .data	malloc parameters (tcache settings, mmap threshold)
tcache_perthread_struct	Heap (first allocation)	Per-thread tcache bins and counts

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2. LEAK METHODS

Libc Base Leak

Method	Precondition	Technique
Unsortedbin fd/bk	Free a chunk > tcache range (or fill tcache)	fd/bk → main_arena + 0x60 (or +0x70 depending on version) → libc base
Smallbin fd/bk	Chunk moved from unsortedbin to smallbin	Same as unsortedbin leak
stdout FILE leak	Write to _IO_2_1_stdout_	Corrupt _IO_write_base to leak libc data (see IO_FILE)

Heap Base Leak

Method	Precondition	Technique
Tcache fd pointer	Free two tcache chunks, read first's fd	fd → heap address (XOR'd in ≥ 2.32)
Fastbin fd	Free two fastbin chunks	fd → heap address
UAF read	Use-after-free on freed chunk	Read fd/bk directly

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

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3. ATTACK CATEGORIES BY GLIBC VERSION

glibc < 2.26 (No tcache)

Attack	Primitive Needed	Result
Fastbin dup	Double free	Arbitrary allocation
Unsortedbin attack	Corrupt unsortedbin bk	Write main_arena addr to target (used for __malloc_hook nearby overwrite)
Unlink attack	Heap overflow into prev_size + fd/bk	Arbitrary write (with known heap pointer)
House of Force	Top chunk size overwrite	Arbitrary allocation
House of Spirit	Write fake chunk header	Fastbin allocation at fake chunk
Off-by-one null	Null byte overflow into next chunk size	Overlapping chunks

glibc 2.26–2.28 (tcache, no key)

Attack	Notes
Tcache poisoning	Overwrite tcache fd → arbitrary allocation, no size check
Tcache dup	Double free into tcache (no double-free detection yet)
All previous attacks	Still work, but chunks go to tcache first

glibc 2.29–2.31 (tcache key introduced)

Attack	Bypass for tcache key
Tcache dup	Corrupt key field (at chunk+0x18) before second free
House of Botcake	Double free: one in unsortedbin, one in tcache → overlapping
Tcache stashing unlink	Abuse smallbin→tcache refill to get arbitrary chunk

glibc 2.32–2.33 (safe-linking / PROTECT_PTR)

Attack	Adaptation
Tcache poisoning	Encode target with (chunk_addr >> 12) ^ target
Heap leak required	Need heap addr to decode/encode safe-linked pointers
Fastbin dup	Same encoding required

glibc ≥ 2.34 (hooks removed)

Change	Impact
__malloc_hook removed	Cannot overwrite hook for one_gadget
__free_hook removed	Cannot overwrite hook
__realloc_hook removed	Cannot use realloc trick for one_gadget constraints

Post-2.34 targets: see arbitrary-write-to-rce for _IO_FILE, exit_funcs, TLS_dtor_list, _dl_fini.

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4. COMMON VULNERABILITY PATTERNS

Vulnerability	Description	Exploitation Path
UAF (Use-After-Free)	Access chunk after free	Read: leak fd/bk; Write: corrupt fd for tcache poisoning
Double Free	free() same chunk twice	Tcache dup (bypass key) or fastbin dup
Heap Overflow	Write past chunk boundary	Corrupt next chunk's metadata (size, fd, bk)
Off-by-one	One byte overflow	Null byte → shrink next chunk size → overlapping chunks
Off-by-null	Specifically \x00 overflow	Clear PREV_INUSE → trigger backward consolidation
Uninitialized read	Read heap memory without clearing	Leak fd/bk from recycled chunk

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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']))

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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)