New findings from cybersecurity firm JFrog show that malware targeting the npm ecosystem can evade security checks by taking advantage of an "unexpected behavior" in the npm command line interface (CLI) tool.

npm CLI's install and audit commands have built-in capabilities to check a package and all of its dependencies for known vulnerabilities, effectively acting as a warning mechanism for developers by highlighting the flaws.

But as JFrog established, the security advisories are not displayed when the packages follow certain version formats, creating a scenario where critical flaws could be introduced into their systems either directly or via the package's dependencies.

Specifically, the problem arises only when the installed package version contains a hyphen (e.g., 1.2.3-a), which is included to denote a pre-release version of an npm module.

While the project maintainers treat the discrepancy between regular npm package versions and pre-release versions as an intended functionality, this also makes it ripe for abuse by attackers looking to poison the open source ecosystem.

"Threat actors could exploit this behavior by intentionally planting vulnerable or malicious code in their innocent-looking packages which will be included by other developers due to valuable functionality or as a mistake due to infection techniques such as typosquatting or dependency confusion," Or Peles said.

In other words, an adversary could publish a seemingly benign package that's in the pre-release version format, which could then be potentially picked up by other developers and not be alerted to the fact that the package is malicious despite evidence to the contrary.

The development once again reiterates how the software supply chain is built as a chain of trust between various parties, and how a compromise of one link can affect all downstream applications that consume the rogue third-party dependency.

To counter such threats, it's recommended that developers avoid installing npm packages with a pre-release version, unless the source is known to be completely reliable.