Heap Exploitation

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

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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 | 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

---

3. ATTACK CATEGORIES BY GLIBC VERSION

glibc > 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 onegadget | | __free_hook removed | Cannot overwrite hook | | __realloc_hook removed | Cannot use realloc trick for onegadget constraints |

Post-2.34 targets: see [arbitrary-write-to-rce](../arbitrary-write-to-rce/SKILL.md) 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 

# 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  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)

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: yaklang
• Source: yaklang/hack-skills
• License: MIT

Install and usage instructions live in the source repository linked above.