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Malicious npm Package Bypasses Install-Script Defenses, Hides Malware Inside Runtime Code

The indexed-btree npm package evades npm v12's install-script blocking by hiding its malware loader inside a runtime method instead, Checkmarx reports.

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Overview

An ongoing npm malware campaign built around a package called indexed-btree shows how threat actors are adapting to npm’s newest defenses by moving their malicious code out of install scripts entirely, according to Checkmarx. The package, which impersonates the legitimate sorted-btree library, achieved nearly 2 million weekly downloads despite carrying no installation scripts at all, and instead triggers its payload only when application code actually calls one of its functions, according to BleepingComputer.

What We Know

  • Checkmarx researchers spotted indexed-btree impersonating sorted-btree, a legitimate library, and found it had amassed nearly 2 million weekly downloads, according to Checkmarx.
  • Rather than relying on preinstall or postinstall scripts, “the malicious code activates when the library is used in application code,” according to Checkmarx. The loader specifically hides inside the package’s BTree.prototype.set() method, which executes at runtime when an application calls it, according to BleepingComputer.
  • “The malware loader hides inside the library’s own BTree.prototype.set method, which is the main function that every user would call constantly. This triggers the sharedLoad.min.js, which contains the obfuscated first stage of the malware. This is a well-built way to sneak past standard taint-analysis tools and most static scanners,” Checkmarx explains, according to BleepingComputer.
  • Because it avoids installation scripts altogether, the package “triggers none of npm v12’s approval mechanisms,” which block preinstall, install, and postinstall scripts by default, according to BleepingComputer. GitHub’s own changelog describes that change plainly: “npm install will no longer execute preinstall, install, or postinstall scripts from dependencies unless they are explicitly allowed in your project,” according to GitHub’s official changelog. Machine Herald has previously reported on npm v12’s rollout of that install-script blocking.
  • Once triggered, the malware collects host details including architecture, hostname, CPU, memory, and uptime, then exfiltrates the data through hardcoded Slack and Telegram channels, according to BleepingComputer.
  • The malware also polls an Ethereum smart contract on the Sepolia test network for command-and-control information, using X25519 key exchange to derive an AES key and decrypt a second-stage payload stored in the contract, according to BleepingComputer. The malware fingerprints host systems and derives its encryption keys through elliptic-curve Diffie-Hellman key exchange, according to Checkmarx.
  • When operators choose to end the attack, the malware can delete its files and remove the malicious trigger from the package code to erase its traces, according to BleepingComputer.
  • The threat actors built a “legitimate-looking GitHub repository,” populated its commit history, and curated the developer account behind the package to make the project appear authentic, according to BleepingComputer.
  • Checkmarx discovered nine additional npm packages linked to the same operation and has had them removed from npm: ordered-kv-index (448,184 downloads), btree-leaderboard (493,685 downloads), priority-slot-queue (402,860 downloads), btree-range-store (468,092 downloads), btree-core (1,951,274 downloads), btree-time-index (425,312 downloads), btree-lru-cache (372,185 downloads), neighbor-key-map (366,019 downloads), and sliding-score-window (448,024 downloads), according to Checkmarx and BleepingComputer.
  • The attackers’ wallet held 109 ETH at the time of Checkmarx’s report, though Checkmarx’s findings do not establish that those funds came from cryptocurrency theft, according to BleepingComputer.
  • Developers are advised not to rely on install-time scanning alone and to also employ runtime behavioral analysis, according to BleepingComputer. Anyone who installed indexed-btree or the related packages should rotate all secrets and restore their development environment from a safe backup, according to BleepingComputer.

What We Don’t Know

Neither report identifies who is behind the campaign or how long it ran before Checkmarx discovered it. It also remains unconfirmed whether the funds in the attackers’ Ethereum wallet originated from this campaign or another source. It is not yet clear whether npm or GitHub plans a specific technical countermeasure targeting runtime-triggered payloads, as opposed to the install-time script blocking that indexed-btree was built to evade.

Analysis

The campaign underscores a pattern security researchers have flagged since npm began restricting install scripts by default: closing one execution path shifts attacker behavior rather than eliminating the underlying risk. npm v12’s script-blocking defaults, which Machine Herald previously covered as a response to a string of self-propagating npm worms, targeted the specific mechanism — automatic script execution during installation — that those worms relied on. indexed-btree’s runtime-triggered loader sidesteps that mechanism entirely by waiting for the package to be used rather than merely installed, a distinction that static, install-time scanners are not built to catch.