Unsafe Relocation Fixup Exploitation

A skill for analyzing and exploiting unsafe relocation fixup vulnerabilities in asset loaders and binary parsers.

Overview

Unsafe relocation fixups occur when asset loaders blindly trust attacker-controlled metadata to patch pointers within loaded data. This creates arbitrary read/write primitives that can be weaponized for RCE.

Vulnerability Pattern

The classic unsafe relocation handler:

int *GrannyGRNFixUp_0(DWORD RelocationCount,
                       Relocation *PointerFixupArray,
                       int *SectionArray,
                       char *destination)
{
  while (RelocationCount--) {
    int target_base = SectionArray[PointerFixupArray->SectionNumber]; // UNCHECKED
    int *patch_site = (int *)(destination + PointerFixupArray->SectionOffset); // UNCHECKED
    *patch_site = target_base;
    if (target_base)
      *patch_site = target_base + PointerFixupArray->Offset;
    ++PointerFixupArray;
  }
  return SectionArray;
}

Critical flaws:

• SectionNumber

  index is never bounds-checked

• SectionOffset

  is never validated against section size
• Negative offsets allow walking backwards into allocator metadata
• Oversized indices allow walking outside the section array

Exploitation Methodology

Stage 1: Write Backwards into Loader Metadata

Goal: Use negative offsets to corrupt the section pointer array itself.

Key insight: Custom allocators prepend headers to allocations. By calculating the exact distance between:

• Start of first section buffer
• Section pointer array (stored before first section)

You can craft relocations that write into the pointer array.

Calculation example:

0x20 (header) + 0x20 (section descriptors)
+ n * 1 (section types) + n * 1 (flags)
+ n * 4 (pointer table) = 0x4000

Solving for n gives the number of sections needed to position metadata at a specific offset.

Stage 2: Deterministic Heap Layout

Windows 10 NT Heap behavior:

• Allocations ≤ 0x4000 → randomized LFH (Large Free Heap)
• Allocations > 0x4000 → deterministic backend allocator

Strategy:

1. Inflate metadata (many empty sections) until allocation > 0x4000
2. Preload malicious assets to "prime" heap layout
3. Force predictable placement of:
   • Allocation #1: metadata + section pointer arrays
   • Allocation #2: section 0 contents
   • Allocation #3: section 1 contents

Stage 3: Convert to RCE

1. Corrupt SectionContentArray[1] using negative offset relocation
2. Recycle the corrupted pointer - Section 1's relocation table now uses your injected pointer
3. Hit reliable dispatchers - Overwrite function pointers that fire during normal operation:
   • Allocator callbacks (Malloc/Free)
   • Virtual tables
   • Animation dispatchers
   • Rendering callbacks

Analysis Checklist

When investigating potential unsafe relocation vulnerabilities:

1. Identify asset loaders that maintain SectionArray/relocation tables
2. Diff relocation handlers for missing bounds on indices/offsets
3. Measure allocator headers from both game wrapper and OS heap
4. Calculate backwards offsets precisely
5. Force deterministic placement by inflating metadata
6. Identify reliable targets - function pointers that will fire

Defensive Considerations

• Always bounds-check relocation indices
• Validate offsets against section sizes
• Use safe integer arithmetic
• Consider ASLR/DEP hardening
• Validate asset metadata before processing

References

• Synacktiv – Exploiting Anno 1404
• W. Yason – Windows 10 Segment Heap Internals