[CRITICAL] Metal API Memory Leak - Heap Memory Never Released to OS (CWE-400)
Security Classification
This issue constitutes a resource exhaustion vulnerability (CWE-400):
Aspect
Details
Type
Uncontrolled Resource Consumption
CWE
CWE-400
Vector
Local (any Metal application)
Impact
System instability, denial of service
User Control
None - no mitigation available
Recovery
Requires application restart
Summary
Metal heap allocations are never released back to macOS, even when the memory is entirely unused. This causes continuous, unbounded memory growth until system instability or crash. The issue affects any application using Metal API heap allocation.
This was discovered in Unreal Engine 5, but reproduces in a completely blank UE5 project with zero application code - confirming this is Metal framework behavior, not application-level.
Environment
OS: macOS Tahoe 26.2
Hardware: Apple Silicon M4 Max (also reproduced on M1, M2, M3)
API: Metal
Reproduction Steps
Run any Metal application that allocates and deallocates GPU buffers via Metal heaps
Open Activity Monitor and observe the application's memory usage
Let the application run idle (no user interaction required)
Observe memory growing continuously at ~1-2 MB per second
Memory never plateaus or stabilizes
Eventually system becomes unstable
For testing: Any Unreal Engine 5.4+ project on macOS will reproduce this. Even a blank project with no gameplay code exhibits the leak. (Tested on UE 5.7.1)
Observed Behavior
Memory Analysis
Using Unreal's memreport -full command, two reports taken 86 seconds apart:
Metric
Report 1 (183s)
Report 2 (269s)
Delta
Process Physical
4373.64 MB
4463.39 MB
+89.75 MB
Metal Heap Buffer
7168 MB
8192 MB
+1024 MB
Unused Heap
3453 MB
4477 MB
+1024 MB
Object Count
73,840
73,840
0 (no change)
Key Finding
Metal Heap grew by exactly 1 GB while "Unused Heap" also grew by 1 GB. This demonstrates:
Metal is allocating new heap blocks in ~1 GB increments
Previously allocated heap memory becomes "unused" but is never released
The unused memory accumulates indefinitely
No application-level objects are leaking (count remains constant)
Memory Growth Pattern
Continuous growth while idle (no user interaction)
Growth rate: approximately 1-2 MB per second
No plateau or stabilization occurs
Metal allocates new 1 GB heap blocks rather than reusing freed space
Eventually leads to system instability and crash
What is NOT Causing This
We verified the following are NOT the source:
Application objects - Object count remains constant
Application code - Blank project with no code reproduces the issue
Texture streaming - Disabling texture streaming had no effect
CPU garbage collection - Running GC has no effect (this is GPU memory)
Mitigations Attempted (None Worked)
setPurgeableState
Setting resources to purgeable state before release:
[buffer setPurgeableState:MTLPurgeableStateEmpty];
Result: Metal ignores this hint and does not reclaim heap memory.
Avoiding Heap Pooling
Forcing individual buffer allocations instead of heap-based pooling.
Result: Leak persists - Metal still manages underlying allocations.
Aggressive Buffer Compaction
Attempting to compact/defragment buffers within heaps every frame.
Result: Only moves data between existing heaps. Does NOT release heaps back to OS.
Reducing Pool Sizes
Minimizing all buffer pool sizes to force more frequent reuse.
Result: Slightly slows the leak rate but does not stop it.
Root Cause Analysis
How Metal Heap Allocation Appears to Work
Metal allocates GPU heap blocks in large chunks (~1 GB observed)
Application requests buffers from these heaps
When application releases buffers, memory becomes "unused" within the heap
Metal does NOT release heap blocks back to macOS, even when entirely unused
When fragmentation prevents reuse, Metal allocates new heap blocks
Result: Continuous memory growth with no upper bound
The Core Problem
There appears to be no Metal API to force heap memory release. The only way to reclaim this memory is to destroy the Metal device entirely, which requires restarting the application.
Expected Behavior
Metal should:
Release unused heaps - When a heap block is entirely unused, release it back to macOS
Respect purgeable hints - Honor setPurgeableState calls from applications
Compact allocations - Defragment heap allocations to reduce fragmentation
Provide control APIs - Allow applications to request heap compaction or release
Enforce limits - Have configurable maximum heap memory consumption
Security Implications
Local Denial of Service - Any Metal application can exhaust system memory, causing instability affecting all running applications
Memory Pressure Attack - Forces other applications to swap to disk, degrading system-wide performance
No Upper Bound - Memory consumption continues until system failure
Unmitigable - End users have no way to prevent or limit the leak
Affects All Metal Apps - Any application using Metal heaps is potentially affected
Impact
Applications become unstable after extended use
System-wide performance degrades as memory pressure increases
Users must periodically restart applications
Developers cannot work around this at the application level
Long-running applications (games, creative tools, servers) are particularly affected
Request
Investigate Metal heap memory management behavior
Implement heap release when blocks become entirely unused
Honor setPurgeableState hints from applications
Consider providing an API for applications to request heap compaction
Document any intended behavior or workarounds
Additional Notes
This issue has been observed across multiple Unreal Engine versions (5.4, 5.7) and multiple Apple Silicon generations (M1 through M4). The behavior is consistent and reproducible.
The Unreal Engine team has implemented various CVars to attempt mitigation (rhi.Metal.HeapBufferBytesToCompact, rhi.Metal.ResourcePurgeInPool, etc.) but none successfully address the issue because the root cause is at the Metal framework level.
Tested: January 2026
Platform: macOS Tahoe 26.2, Apple Silicon (M1/M2/M3/M4)
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