Attackers are exploiting a major weakness that has allowed them access to the NPM code repository with more than 100 credential-stealing packages since August, mostly without detection.

The finding, laid out Wednesday by security firm Koi, brings attention to an NPM practice that allows installed packages to automatically pull down and run unvetted packages from untrusted domains. Koi said a campaign it tracks as PhantomRaven has exploited NPM’s use of “Remote Dynamic Dependences” to flood NPM with 126 malicious packages that have been downloaded more than 86,000 times. Some 80 of those packages remained available as of Wednesday morning, Koi said.

A blind spot

“PhantomRaven demonstrates how sophisticated attackers are getting [better] at exploiting blind spots in traditional security tooling,” Koi’s Oren Yomtov wrote. “Remote Dynamic Dependencies aren’t visible to static analysis.”

Remote Dynamic Dependencies provide greater flexibility in accessing dependencies—the code libraries that are mandatory for many other packages to work. Normally, dependencies are visible to the developer installing the package. They’re usually downloaded from NPM’s trusted infrastructure.

RDD works differently. It allows a package to download dependencies from untrusted websites, even those that connect over HTTP, which is unencrypted. The PhantomRaven attackers exploited this leniency by including code in the 126 packages uploaded to NPM. The code downloads malicious dependencies from URLs, including http://packages.storeartifact.com/npm/unused-imports. Koi said these dependencies are “invisible” to developers and many security scanners. Instead, they show the package contains “0 Dependencies.” An NPM feature causes these invisible downloads to be automatically installed.

Compounding the weakness, the dependencies are downloaded “fresh” from the attacker server each time a package is installed, rather than being cached, versioned, or otherwise static, as Koi explained: