systeminformation has prototype pollution in `networkStats()` via the interface name

Summary

networkStats() keeps a per interface cache for computing rx_sec / tx_sec between calls. The cache is a plain JavaScript object and is indexed with the interface name the caller supplied. Passing the name __proto__ makes every cache write land on Object.prototype instead of the cache, polluting every object in the Node.js process.

An application is affected if it passes untrusted input as the ifaces argument of si.networkStats() and runs on FreeBSD, OpenBSD or NetBSD.

Details

The vulnerable code

lib/network.js declares the cache as a plain object:

const _network = {};

calcNetworkSpeed() writes into it, keyed by the interface name:

} else {
  if (!_network[iface]) {
    _network[iface] = {};
  }
  _network[iface].rx_bytes  = rx_bytes;
  _network[iface].tx_bytes  = tx_bytes;
  _network[iface].rx_sec    = null;
  _network[iface].tx_sec    = null;
  _network[iface].ms        = Date.now();
  _network[iface].last_ms   = 0;
  _network[iface].operstate = operstate;
}

With iface === '__proto__' this misbehaves twice:

1. _network['__proto__'] does not read a missing own property — it invokes the inherited __proto__ getter and returns Object.prototype, which is truthy. The if (!_network[iface]) guard therefore never creates the {} placeholder.
2. Each following assignment writes onto Object.prototype.

After one call, rx_bytes, tx_bytes, rx_sec, tx_sec, ms, last_ms and operstate are visible on every object in the process. A second call takes the other branch (_network['__proto__'].ms is now truthy) and keeps them updated.

Why the existing sanitizing does not help

The interface name does pass through util.sanitizeString(iface, true) before it is used as the cache key. That function is a shell metacharacter filter, built to stop command injection in the netstat / ifconfig invocations — a different threat. Property names made of letters and underscores are legitimate interface names and pass through untouched:

"a;id"        -> "aid"
"a$(id)"      -> "aid"
"a|id"        -> "aid"
"a`id`"       -> "aid"
"__proto__"   -> "__proto__"     <-- unchanged
"constructor" -> "constructor"   <-- unchanged

The sanitizer is doing its job; the gap is that no layer treats the value as an object key.

Reachability

The cache write is guarded incidentally on three of four platform branches. These guards exist for unrelated reasons and are not a deliberate defence:

The important part is not that three platforms happen to be safe, but that nothing in the design prevents the write. Any future platform branch without such a guard reopens the issue, which is why the fix is applied at the cache rather than at the call sites.

Age

The pattern predates the v5 line — git log -S "_network[iface] = {}" traces it back to a refactoring before v3.42.5. The v3 and v4 lines are end of life; the advisory covers v5.

Impact

The attacker controls which attributes appear on Object.prototype but not their values — those come from the interface statistics (rx_bytes, ms, operstate …). The attribute names are fixed by the library, so this is not an arbitrary-key pollution primitive.

The consequences are therefore application specific and arise from name collisions:

• Any application code that tests if (obj.operstate), if (obj.ms) or similar on an object that should not have those properties now takes the wrong branch.
• for…in loops over plain objects gain seven extra enumerable keys, which can corrupt serialisation, configuration merging or validation logic.
• Libraries that use {} as a lookup map and check membership with in or truthiness can return values for keys that were never inserted.

No path to remote code execution is known, and the values cannot be chosen by the attacker.

Proof of concept

The underlying primitive (any platform)

const { calcNetworkSpeed } = require('systeminformation/lib/network.js'); // internal
const cache = {};                       // the declaration used by the library
calcNetworkSpeed('__proto__', 1, 2, 'up', 0, 0, 0, 0);

console.log({}.rx_bytes);               // 1
console.log({}.operstate);              // 'up'
console.log(Object.prototype.hasOwnProperty('rx_bytes')); // true

End to end through the public API (FreeBSD / OpenBSD / NetBSD)

const si = require('systeminformation');

(async () => {
  console.log('before:', {}.operstate);   // undefined
  await si.networkStats('__proto__');     // an attacker supplied interface name
  console.log('after :', {}.operstate);   // 'up'  -> Object.prototype is polluted
})();

On Linux, macOS and Windows the same call returns an empty result set and leaves Object.prototype untouched, for the reasons in the Reachability table.

Patch

lib/network.js — give the cache a null prototype, so __proto__ becomes an ordinary own property instead of a setter into the prototype chain:

-const _network = {};
+const _network = Object.create(null);

This is the smallest change that fixes the class of problem rather than one reachable path:

• It is independent of the platform branches, so no guard has to be added or maintained.
• It keeps behaviour identical for every legitimate interface name — Object.keys(), member reads and writes all work unchanged on a null prototype object.
• It matches the pattern already used elsewhere in the codebase (lib/filesystem.js:114-115, lib/graphics.js:1060, lib/graphics.js:1087).
• Object.create is ES5.1, well below the declared engines.node >= 10.0.0.

After the patch the hostile name is stored as a normal cache entry:

const cache = Object.create(null);
// ... cache['__proto__'] holds the stats object, Object.prototype is untouched

Workarounds

For users who cannot upgrade immediately, either of these is sufficient:

1. Validate the interface name before calling — accept only names returned by si.networkInterfaces(), or reject the names __proto__, constructor and prototype.
2. Never pass untrusted input as the ifaces argument. Passing '*' or omitting the argument is unaffected, because those names come from the system rather than the caller.

Freezing Object.prototype (Object.freeze(Object.prototype)) also prevents the write, but it has wide-reaching side effects on other dependencies and is not recommended as a general mitigation.

Affected versions

All 5.x releases up to and including 5.33.12. Earlier major lines contain the same pattern but are end of life.

What was verified, and what was not

Verified on this machine (macOS 27, Node 24):

• the pollution primitive, by calling the cache write with iface = '__proto__';
• that util.sanitizeString('__proto__', true) returns the string unchanged, while shell metacharacters are stripped;
• that unpatched v5 on macOS does not pollute through the public API;
• that netstat -bdnI <unknown-interface> prints only its header and exits 0;
• that the patch stores the hostile key as an own property and leaves Object.prototype clean.

Not verified, because no such host was available:

• end to end exploitation on FreeBSD, OpenBSD or NetBSD. That conclusion is derived from reading the BSD branch of networkStatsSingle() — where the calcNetworkSpeed() call sits outside the row parsing loop and is reached whenever netstat exits successfully — combined with the observed netstat exit behaviour above. Worth confirming on a BSD host before publishing, since the affected-platform list depends on it.