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● CVE-2024-30051 · CVSS 7.8[5], [1] · High
FlawOpen Security Research

CVE-2024-30051: Windows DWM Heap Overflow

CVE-2024-30051 is a locally exploitable Windows Desktop Window Manager (DWM) heap overflow. Reverse engineering found that DWM rounded an allocation size down, then copied the original, larger size; Microsoft fixed the flaw in its May 2024 security update.[5], [3], [6]

💡 Plain English Explainer (ELI5)

Imagine reserving space by counting only whole crates, rounding the remainder down, and then loading the entire shipment anyway. The loading plan is smaller than the amount actually moved, so the shipment crosses the space that was reserved. This is the allocation-versus-copy size mismatch described for CVE-2024-30051.[6]

Core Concepts & Subsystem Terms

Desktop Window Manager (DWM)
Windows component that manages desktop composition. Microsoft classifies CVE-2024-30051 as a DWM Core privilege-escalation vulnerability.[4], [6]
Allocation/copy size mismatch
The allocation used a rounded-down size, while the copy used the original size, allowing a heap write beyond the allocated buffer.[6]
CWE-122: Heap-based Buffer Overflow
A write exceeds the bounds of a heap-allocated buffer. The advisory maps this CVE to CWE-122.[5], [1], [2]
SYSTEM integrity level
A Windows integrity level associated with highly privileged processes. It is not itself proof that a process runs under the LocalSystem account.[6]

Framework mappings

Root Cause Analysis

Core Security’s reverse engineering identified the affected routine as CCommandBuffer::Initialize. It calculated an allocation as (size / 0x90) * 0x90, using integer division that rounds down to a multiple of 0x90, while a later memcpy used the original size. For some inputs, the copy therefore exceeded the heap allocation. Microsoft fixed CVE-2024-30051 in the May 14, 2024 security update. Core Security describes the resulting DWM process as running at SYSTEM integrity level, which is distinct from saying the process runs as the LocalSystem account. Kaspersky reported finding an exploit in mid-April 2024 and observing it alongside QakBot and other malware; those observations do not establish how the malware first reached a machine.[6], [4], [7]

Step-by-Step Attack Flow

Step 1

Local interaction with DWM

The issue is a local privilege-escalation flaw in Windows DWM; public advisories describe local attack requirements.[5], [4]

Step 2

Allocation size is rounded down

Reverse engineering identifies CCommandBuffer::Initialize calculating a buffer allocation in multiples of 0x90.[6]

Step 3

Copy retains the original size

The later copy uses the unrounded size. When it exceeds the allocation, the operation can write beyond the heap buffer.[6], [2]

Step 4

Patch and impact boundary

Microsoft addressed the flaw in the May 2024 update. Core Security describes the DWM context as SYSTEM integrity; published reporting does not establish a universal initial-access chain.[3], [6], [7]

Source Code: Flaw vs. Secure Implementation

These are representative educational patterns informed by the published analysis, not Microsoft source code or Microsoft’s patch. They do not reproduce the exploit.[6], [4]

UNPATCHED FLAW
// Representative unsafe pattern; not Microsoft source code.
size_t requested = get_command_buffer_size();
size_t allocation_size = (requested / allocation_unit) * allocation_unit;
BYTE *dst = HeapAlloc(heap, flags, allocation_size);
if (dst == NULL) return E_OUTOFMEMORY;
memcpy(dst, source, requested);
HARDENED SECURE PATCH
// Representative defensive pattern; not Microsoft's patch source.
size_t requested = get_command_buffer_size();
size_t source_len = get_source_length();
if (requested > source_len || requested > MAX_COMMAND_BUFFER_SIZE) return E_INVALIDARG;
BYTE *dst = HeapAlloc(heap, flags, requested);
if (dst == NULL) return E_OUTOFMEMORY;
memcpy(dst, source, requested);

Engineering & System Hardening Checklist

Sources