/heap-exploitation

name: heap-exploitation
description: >-
  Heap exploitation playbook. Use when targeting ptmalloc2/glibc heap vulnerabilities including UAF, double free, overflow, off-by-one/null, and leveraging tcache/fastbin/unsortedbin attacks for arbitrary write or code execution.

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

• [HOUSE_OF_TECHNIQUES.md](./HOUSE_OF_TECHNIQUES.md) — House of Force/Spirit/Orange/Einherjar/Roman/Pig/Banana/Cat/Apple and tcache attacks
• [IO_FILE_EXPLOITATION.md](./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

---

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](../arbitrary-write-to-rce/SKILL.md) for `_IO_FILE`, `exit_funcs`, `TLS_dtor_list`, `_dl_fini`.

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4. COMMON VULN