RIOT OS versions up to and including 2026.01-devel-317 contain a stack-based buffer overflow vulnerability in the ethos utility due to missing bounds checking when processing incoming serial frame data. The vulnerability occurs in the _handle_char() function, where incoming frame bytes are appended to a fixed-size stack buffer without verifying that the current write index remains within bounds. An attacker capable of sending crafted serial or TCP-framed input can cause the current write index to exceed the buffer size, resulting in a write past the end of the stack buffer. This condition leads to memory corruption and application crash.