Successful Zip Bomb attacks occur when an application expands untrusted archive files without controlling the size of the expanded data, which can
lead to denial of service. A Zip bomb is usually a malicious archive file of a few kilobytes of compressed data but turned into gigabytes of
uncompressed data. To achieve this extreme compression ratio, attackers will
compress irrelevant data (eg: a long string of repeated bytes).
Ask Yourself Whether
Archives to expand are untrusted and:
- There is no validation of the number of entries in the archive.
- There is no validation of the total size of the uncompressed data.
- There is no validation of the ratio between the compressed and uncompressed archive entry.
There is a risk if you answered yes to any of those questions.
Recommended Secure Coding Practices
- Define and control the ratio between compressed and uncompressed data, in general the data compression ratio for most of the legit archives is
1 to 3.
- Define and control the threshold for maximum total size of the uncompressed data.
- Count the number of file entries extracted from the archive and abort the extraction if their number is greater than a predefined threshold, in
particular it’s not recommended to recursively expand archives (an entry of an archive could be also an archive).
Sensitive Code Example
For tar module:
const tar = require('tar');
tar.x({ // Sensitive
file: 'foo.tar.gz'
});
For adm-zip module:
const AdmZip = require('adm-zip');
let zip = new AdmZip("./foo.zip");
zip.extractAllTo("."); // Sensitive
For jszip module:
const fs = require("fs");
const JSZip = require("jszip");
fs.readFile("foo.zip", function(err, data) {
if (err) throw err;
JSZip.loadAsync(data).then(function (zip) { // Sensitive
zip.forEach(function (relativePath, zipEntry) {
if (!zip.file(zipEntry.name)) {
fs.mkdirSync(zipEntry.name);
} else {
zip.file(zipEntry.name).async('nodebuffer').then(function (content) {
fs.writeFileSync(zipEntry.name, content);
});
}
});
});
});
For yauzl module
const yauzl = require('yauzl');
yauzl.open('foo.zip', function (err, zipfile) {
if (err) throw err;
zipfile.on("entry", function(entry) {
zipfile.openReadStream(entry, function(err, readStream) {
if (err) throw err;
// TODO: extract
});
});
});
For extract-zip module:
const extract = require('extract-zip')
async function main() {
let target = __dirname + '/test';
await extract('test.zip', { dir: target }); // Sensitive
}
main();
Compliant Solution
For tar module:
const tar = require('tar');
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
let fileCount = 0;
let totalSize = 0;
tar.x({
file: 'foo.tar.gz',
filter: (path, entry) => {
fileCount++;
if (fileCount > MAX_FILES) {
throw 'Reached max. number of files';
}
totalSize += entry.size;
if (totalSize > MAX_SIZE) {
throw 'Reached max. size';
}
return true;
}
});
For adm-zip module:
const AdmZip = require('adm-zip');
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
const THRESHOLD_RATIO = 10;
let fileCount = 0;
let totalSize = 0;
let zip = new AdmZip("./foo.zip");
let zipEntries = zip.getEntries();
zipEntries.forEach(function(zipEntry) {
fileCount++;
if (fileCount > MAX_FILES) {
throw 'Reached max. number of files';
}
let entrySize = zipEntry.getData().length;
totalSize += entrySize;
if (totalSize > MAX_SIZE) {
throw 'Reached max. size';
}
let compressionRatio = entrySize / zipEntry.header.compressedSize;
if (compressionRatio > THRESHOLD_RATIO) {
throw 'Reached max. compression ratio';
}
if (!zipEntry.isDirectory) {
zip.extractEntryTo(zipEntry.entryName, ".");
}
});
For jszip module:
const fs = require("fs");
const pathmodule = require("path");
const JSZip = require("jszip");
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
let fileCount = 0;
let totalSize = 0;
let targetDirectory = __dirname + '/archive_tmp';
fs.readFile("foo.zip", function(err, data) {
if (err) throw err;
JSZip.loadAsync(data).then(function (zip) {
zip.forEach(function (relativePath, zipEntry) {
fileCount++;
if (fileCount > MAX_FILES) {
throw 'Reached max. number of files';
}
// Prevent ZipSlip path traversal (S6096)
const resolvedPath = pathmodule.join(targetDirectory, zipEntry.name);
if (!resolvedPath.startsWith(targetDirectory)) {
throw 'Path traversal detected';
}
if (!zip.file(zipEntry.name)) {
fs.mkdirSync(resolvedPath);
} else {
zip.file(zipEntry.name).async('nodebuffer').then(function (content) {
totalSize += content.length;
if (totalSize > MAX_SIZE) {
throw 'Reached max. size';
}
fs.writeFileSync(resolvedPath, content);
});
}
});
});
});
Be aware that due to the similar structure of sensitive and compliant code the issue will be raised in both cases. It is up to the developer to
decide if the implementation is secure.
For yauzl module
const yauzl = require('yauzl');
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
const THRESHOLD_RATIO = 10;
yauzl.open('foo.zip', function (err, zipfile) {
if (err) throw err;
let fileCount = 0;
let totalSize = 0;
zipfile.on("entry", function(entry) {
fileCount++;
if (fileCount > MAX_FILES) {
throw 'Reached max. number of files';
}
// The uncompressedSize comes from the zip headers, so it might not be trustworthy.
// Alternatively, calculate the size from the readStream.
let entrySize = entry.uncompressedSize;
totalSize += entrySize;
if (totalSize > MAX_SIZE) {
throw 'Reached max. size';
}
if (entry.compressedSize > 0) {
let compressionRatio = entrySize / entry.compressedSize;
if (compressionRatio > THRESHOLD_RATIO) {
throw 'Reached max. compression ratio';
}
}
zipfile.openReadStream(entry, function(err, readStream) {
if (err) throw err;
// TODO: extract
});
});
});
Be aware that due to the similar structure of sensitive and compliant code the issue will be raised in both cases. It is up to the developer to
decide if the implementation is secure.
For extract-zip module:
const extract = require('extract-zip')
const MAX_FILES = 10000;
const MAX_SIZE = 1000000000; // 1 GB
const THRESHOLD_RATIO = 10;
async function main() {
let fileCount = 0;
let totalSize = 0;
let target = __dirname + '/foo';
await extract('foo.zip', {
dir: target,
onEntry: function(entry, zipfile) {
fileCount++;
if (fileCount > MAX_FILES) {
throw 'Reached max. number of files';
}
// The uncompressedSize comes from the zip headers, so it might not be trustworthy.
// Alternatively, calculate the size from the readStream.
let entrySize = entry.uncompressedSize;
totalSize += entrySize;
if (totalSize > MAX_SIZE) {
throw 'Reached max. size';
}
if (entry.compressedSize > 0) {
let compressionRatio = entrySize / entry.compressedSize;
if (compressionRatio > THRESHOLD_RATIO) {
throw 'Reached max. compression ratio';
}
}
}
});
}
main();
See
