Critical Flaw Found In Widely Used Netmask Open Source Module

The vulnerability was discovered while doing work to fix another vulnerability in a widely used NPM library known as Private IP. That module, which was also widely used by open source developers, enables applications to block request forgery attacks by filtering out attempts to access private IP4 addresses and other restricted IP4 address ranges, as defined by ARIN.

The IP4 address 0127.0.0.01 should be evaluated as the public IP address 87.0.0.1 as the octal string “0127” is the same as the integer “87.” However, netmask reads the address as 127.0.0.1, a trusted, localhost address. Treating an untrusted public IP address as a trusted private IP address opens the door to local- and remote file inclusion (LFI/RFI) attacks, in which a remote authenticated or unauthenticated attacker can bypass packages that rely on netmask to filter IP address blocks.