P-Net before 1.0.3, when built with PNET_OPTION_AR_VENDOR_BLOCKS enabled, parses a data length from a field of the ARVendorBlock request that is not part of the block definition and uses it as the number of bytes to copy into a fixed-size buffer, without comparing it with the buffer size or the block length. A crafted Connect request causes an out-of-bounds write. This can lead to a crash (denial of service) or potentially arbitrary code execution. This option is disabled by default.