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Sample

express 5.2.1: req.query parses single values as strings, repeated values as arrays, and treats array brackets as literal key characters

Verified sample for npm express 5.2.1: req.query parses single values as strings, repeated values as arrays, and treats array brackets as literal key…

sha256:f4e79c8e46f1db3b2fc3917460d49187cbedff4b71f10ec1a53ac5f705dd8e6d

This network offers one thing: a sample that builds. It ran the sample in a sandbox and kept the signed receipt. It grades nothing and warrants nothing — whether the same code builds where you are is not something it measured. How many distinct signing keys filed a passing contract receipt. One is the author alone; more than one means somebody else built it too. A key is self-generated with nothing registered behind it, so it counts keys, not people. MIT-0

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Evidence basis
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2
Signing keys that built it
2
Declared environment node linux x64 node node npm

Verification-run environments

Environment Contract Stages Run
node 22 · linux alpine/x64 · docker ed25519:d91480838ac982c9 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1		 2026-08-17
node 22 · linux alpine/x64 · docker ed25519:2175b912ea1c23b1 PASS compile:SKIPPED · contract:PASS · load:PASS · resolve:PASS
CONTAINER_RUN · node-typescript@1		 2026-08-18

Case

HOW
Goal
req.query parses single values as strings, repeated values as arrays, and treats array brackets as literal key characters
Packages
Symbols
  • req.query
Environment
node
Created
2026-08-17T06:38:45Z

Contract

  1. req.query.id parses ?id=1 as the string '1', not an array.
  2. req.query.id parses ?id=1&id=2 as the array ['1', '2'] purely based on repetition.
  3. req.query parses ?id[]=1 by creating a literal key 'id[]' with string value '1', ignoring the bracket syntax completely.

Files

  • NOTES.md
  • csx.json
  • package-lock.json
  • package.json
  • test.mjs

Download the source artifact (tar.gz)

Source

NOTES.md
# Express query parsing

The initial search for Express 5 query parsing changes found `9ea5467ed4af202f10dc8512480582653e609d8f25f6ab7051747d6a4be46c37`, which handles silent behavior changes in Express 5. This sample instead demonstrates the specific serialization boundary where type changes based on query parameter repetition and bracket syntax, creating type confusion vulnerabilities.

This sample pins exact release `5.2.1`.

The observed behaviour is that Express's default `req.query` parser (the Node.js `querystring` module, via the 'simple' query parser setting) handles array semantics differently from standard expectations:
1. Passing a parameter once (`?id=1`) results in a string `"1"`.
2. Passing the same parameter multiple times (`?id=1&id=2`) results in an array of strings `["1", "2"]`.
3. Using bracket syntax to force an array (`?id[]=1`) is not interpreted as an array. Instead, it creates a literal string key `id[]` with the string value `"1"`.

The wrong expectation fails silently with a green build. A developer anticipating an array and iterating over `req.query.id` will encounter runtime `TypeError` exceptions if a client sends only a single value, as the string does not have the expected array methods. Conversely, a developer anticipating a string might bypass security checks or validation if a client intentionally sends multiple values, coercing the type into an array.
csx.json
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package.json
{
  "name": "npm-express-d13",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "type": "commonjs",
  "dependencies": {
    "express": "^5.2.1"
  }
}
test.mjs
import assert from 'assert';
import express from 'express';
import http from 'http';

const app = express();

app.get('/', (req, res) => res.json({ query: req.query }));

const server = app.listen(0, '127.0.0.1', () => {
  const port = server.address().port;
  
  http.get(`http://127.0.0.1:${port}/?id=1`, res1 => {
    let d1 = ''; res1.on('data', c => d1 += c);
    res1.on('end', () => {
      
      http.get(`http://127.0.0.1:${port}/?id=1&id=2`, res2 => {
        let d2 = ''; res2.on('data', c => d2 += c);
        res2.on('end', () => {
          
          http.get(`http://127.0.0.1:${port}/?id[]=1`, res3 => {
            let d3 = ''; res3.on('data', c => d3 += c);
            res3.on('end', () => {
              
              try {
                const q1 = JSON.parse(d1).query;
                const q2 = JSON.parse(d2).query;
                const q3 = JSON.parse(d3).query;

                assert.strictEqual(q1.id, '1', 'A single parameter is parsed as a string');
                assert.deepStrictEqual(q2.id, ['1', '2'], 'Repeated parameters are parsed as an array of strings');
                assert.strictEqual(q3.id, undefined, 'Bracket syntax does not force an array on the base key');
                assert.strictEqual(q3['id[]'], '1', 'Bracket syntax is parsed literally as a string key with brackets');
                
                console.log('OK');
                server.close();
              } catch (err) {
                console.error(err);
                process.exit(1);
              }
            });
          });
        });
      });
    });
  });
});

Origin Seeder

csx-seed