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  • April 18, 2021

Exclusive: IPv4 Parsing Flaw In NPM Netmask Could Affect 270,000 Apps

 Exclusive: IPv4 Parsing Flaw In NPM Netmask Could Affect 270,000 Apps

chicksdaddy shares a report from The Security Ledger: Independent security researchers analyzing the widely used open source component netmask have discovered security vulnerabilities that could leave more than a quarter million open source applications vulnerable to attack, according to a report released Monday, The Security Ledger reports. According to a report by the site Sick Codes, the flaws open applications that rely on netmask to a wide range of malicious attacks including Server Side Request Forgeries (SSRF) and Remote- and Local File Includes (RFI, LFI) that could enable attackers to ferry malicious code into a protected network, or siphon sensitive data out of one. Even worse, the flaws appear to stretch far beyond a single open source module, affecting a wide range of open source development languages, researchers say.

Netmask is a widely used package that allows developers to evaluate whether a IP address attempting to access an application was inside or outside of a given IPv4 range. Based on an IP address submitted to netmask, the module will return true or false about whether or not the submitted IP address is in the defined “block.” According to the researcher using the handle “Sick Codes,” the researchers discovered that netmask had a big blind spot. Specifically: it evaluates certain IP addresses incorrectly: improperly validating so-called “octal strings” rendering IPv4 addresses that contain certain octal strings as integers. For example, the IP4 address 0177.0.0.1 should be evaluated by netmask as the private IP address, as the octal string “0177” translates to the integer “127.” However, netmask evaluates it as a public IPv4 address:, simply stripping off the leading zero and reading the remaining parts of the octal string as an integer.

The implications for modules that are using the vulnerable version of netmask are serious. According to Sick Codes, remote attackers can use SSRF attacks to upload malicious files from the public Internet without setting off alarms, because applications relying on netmask would treat a properly configured external IP address as an internal address. Similarly, attackers could also disguise remote IP addresses local addresses, enabling remote file inclusion (RFI) attacks that could permit web shells or malicious programs to be placed on target networks. But researchers say much more is to come. The problems identified in netmask are not unique to that module. Researchers have noted previously that textual representation of IPv4 addresses were never standardized, leading to disparities in how different but equivalent versions of IPv4 addresses (for example: octal strings) are rendered and interpreted by different applications and platforms.

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