D-Link DGS-1500 Ax devices before 2.51B021 have a hardcoded password, which allows remote attackers to obtain shell access.
ftp://ftp2.dlink.com/PRODUCTS/DGS-1500-20/REVA/DGS-1500_REVA_FIRMWARE_PATCH_NOTES_2.51.021_EN.pdf
ftp://ftp2.dlink.com/PRODUCTS/DGS-1500-28/REVA/DGS-1500_REVA_FIRMWARE_PATCH_NOTES_2.51.021_EN.pdf
ftp://ftp2.dlink.com/PRODUCTS/DGS-1500-28P/REVA/DGS-1500_REVA_FIRMWARE_PATCH_NOTES_2.51.021_EN.pdf
ftp://ftp2.dlink.com/PRODUCTS/DGS-1500-52/REVA/DGS-1500_REVA_FIRMWARE_PATCH_NOTES_2.51.021_EN.pdf