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Critical Flaw Found In Widely Used Netmask Open Source Module

Security researchers analyzing a widely used open source component have discovered security vulnerabilities that leave hundreds of thousands of software applications vulnerable to attack, according to a report released Monday.

The group of five researchers found the security vulnerabilities in netmask, an open source library used in a staggering 270,000 software projects. According to the report, the flaws open the door to a wide range of malicious attacks that could enable attackers to ferry malicious code into a protected network, or siphon sensitive data out of one.

Exploitable Flaw in NPM Private IP App Lurks Everywhere, Anywhere

Among the attacks enabled by the flaw are so-called server-side request forgeries (SSRF), as well as remote file inclusion, local file inclusion and more, the researcher called Sick Codes told The Security Ledger. Work to discover the extent of the flaws continues. The researchers have received a preliminary vulnerability ID for the flaw, CVE-2021-28918.

Even worse, the flaws appear the stretch far beyond a single open source module, affecting a wide range of open source development languages, researchers say.

Work on one vulnerable open source module uncovers another

In addition to Sick Codes (@sickcodes), the researchers conducting the investigation are Victor Viale (@koroeskohr), Kelly Kaoudis (@kaoudis), John Jackson (@johnjhacking) and Nick Sahler (@tensor_bodega).

According to Sick Codes, 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. In a report published in November, researchers revealed that the Private IP module didn’t work very well and was susceptible to being bypassed using SSRF attacks against top tier applications.

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SSRF attacks allow malicious actors to abuse functionality on a server: reading data from internal resources or modifying the code running on the server. Private-ip was created to help application developers spot and block such attacks. SSRF is one of the most common forms of attack on web applications according to OWASP.

The researchers working to fix the Private IP flaw turned to netmask, 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.”

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The module has a range of other useful features, as well, such as reporting back on how many IPs are inside a given block. And, with no other “dependencies,” netmask seemed like the perfect fit to fix Private IP’s problems.

There’s only one problem: netmask itself was flawed, as the researchers soon discovered. Specifically: the module 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 127.0.0.1, as the octal string “0177” translates to the integer “127.” However, netmask evaluates it as a public IPv4 address: 177.0.0.1, simply stripping off the leading zero and reading the remaining parts of the octal string as an integer.

Sample output from the netmask NPM module showing incorrect parsing of the IPv4 address. The flaw could enable a range of attacks on applications using the popular open source module.
Sample output from the netmask NPM module showing incorrect parsing of the IPv4 address. The flaw could enable a range of attacks on applications using the popular open source module. (Image courtesy of Sick Codes.)

And the flaw works both ways. 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.

A Popular Module with a Sterling Pedigree

Olivier Poitrey is netmask’s author and the Director of Engineering at Netflix.

Netmask is the creation of Olivier Poitrey, an accomplished developer who is the Director of Engineering at Netflix and a co-founder of the firm NextDNS. The module was first released nine years ago and gained prominence around 5 years ago, with version 1.06, which has been downloaded more than 3 million times.

The netmask module currently sees around 3.1 million downloads weekly, though development on netmask appears to have ceased during the last five years, up until the release of version 2.0.0 around 10 days ago, after discovery of the security holes.

Throwing open the doors to hackers

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.

Parsing IPv4 Addresses: Dark Magic

Updates to address the flaws in netmask began appearing on March 19 with the release of version 2.0.0. A subsequent update, 2.0.1, was released on Monday. To date, only 6,641 downloads of the updated netmask module have completed, meaning the vast majority of open source projects using it remain vulnerable. Contacted via chat, Poitrey advised netmask users to upgrade to the latest version of the module.

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 weren’t ever 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.

In the case of the netmask flaw, the true culprit ends up being “some unexpected behaviors of the parseInt function in Javascript combined with an oversight on those odd IP notations nobody (uses),” Poitrey told Security Ledger. “Parsing IPs correctly is black magic,” he said. “Those weird notations should just be deprecated, so nobody can exploit them like this.”

More to come

The impact of the flaw will become more clear in the weeks ahead, according to Sick Codes, as more examples of the IPv4 parsing problem are identified and patched in other popular open interpreted languages. “The implications are infinite,” he said.

6 Comments

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