Ldap Injection in Express with Hmac Signatures

LDAP Injection is an attack technique where untrusted input is concatenated into LDAP queries, allowing an attacker to alter query logic and bypass authentication or extract data. In Express applications that use HMAC signatures to bind incoming requests to a trusted identity (for example, to verify webhook origins or API message integrity), a common misconception is that HMAC validation alone protects backend calls such as LDAP queries. If the application uses the HMAC-verified data to dynamically build LDAP filters without proper sanitization, the trust placed in the signature does not prevent malicious payloads from being passed into the LDAP search.

Consider an Express route that expects an HMAC-signed JSON payload containing a username. The route verifies the HMAC, extracts the username, and uses it directly to construct an LDAP filter like (uid={username}). If the HMAC verification passes but the username is not escaped or parameterized, an attacker can supply a value such as admin)(uid=*). Depending on the LDAP library’s behavior, this can change the filter semantics to (uid=admin)(uid=*) or otherwise cause the query to return unintended entries. The HMAC signature does not mitigate this because the signature covers the attacker-controlled value; the vulnerability is in how the application uses that value in LDAP rather than in how it validates the request origin.

Another scenario involves query parameters or headers that are HMAC-signed and later used in constructing distinguished names (DNs) or search bases. An attacker may attempt to inject additional filter components via special LDAP filter characters such as *, (, ), or by leveraging encoded UTF-8 to bypass naive string checks. Because the HMAC ensures integrity but not safety, the application may treat the injected components as intended data, leading to over-privileged searches, information disclosure, or authentication bypass via an always-true condition. This is an authentication bypass vector at the application layer, not a flaw in the HMAC algorithm itself, but a failure to treat HMAC-verified input as untrusted for downstream protocols like LDAP.

These patterns are relevant to the 12 security checks run by middleBrick, including Authentication, BOLA/IDOR, and Input Validation. A scan can surface LDAP Injection findings when HMAC-sourced data reaches an LDAP query without canonicalization, escaping, or use of parameterized APIs. The reported risk reflects the ability of an attacker to manipulate directory queries through otherwise integrity-protected inputs, emphasizing the need to treat all data as hostile even after signature validation.

Remediation centers on strict input validation, canonicalization, and using LDAP-safe APIs that avoid string concatenation. Even when HMAC signatures confirm the origin and integrity of a value, you must treat that value as untrusted for protocol-specific operations. Below are concrete Express patterns that combine HMAC verification with secure LDAP usage.

Example 1: HMAC verification with parameterized LDAP queries

Use a library that supports parameterized filters or explicitly escape special characters. Do not directly interpolate user data into filter strings.

const crypto = require('crypto');
const ldap = require('ldapjs');

function verifyHmac(payload, providedHmac, secret) {
  const hmac = crypto.createHmac('sha256', secret);
  hmac.update(JSON.stringify(payload));
  const expected = hmac.digest('hex');
  return crypto.timingSafeEqual(Buffer.from(expected), Buffer.from(providedHmac));
}

app.post('/login', (req, res) => {
  const { username, providedHmac } = req.body;
  const secret = process.env.HMAC_SECRET;

  if (!verifyHmac({ username }, providedHmac, secret)) {
    return res.status(401).json({ error: 'invalid signature' });
  }

  // Safe: use parameterized approach via escapeFilterComponent
  const safeUsername = ldap.filter.encodeValue(username);
  const filter = `(uid=${safeUsername})`;

  const client = ldap.createClient({ url: 'ldap://dc.example.com' });
  client.bind('cn=bind,dc=example,dc=com', 'bindPassword', (bindErr) => {
    if (bindErr) { return res.status(500).json({ error: 'ldap bind failed' }); }
    client.search('dc=example,dc=com', {
      filter: filter,
      scope: 'sub',
    }, (searchErr, searchRes) => {
      if (searchErr) { return res.status(500).json({ error: 'search failed' }); }
      const entries = [];
      searchRes.on('searchEntry', (entry) => { entries.push(entry.object); });
      searchRes.on('error', () => { res.status(500).json({ error: 'search error' }); });
      searchRes.on('end', () => { res.json({ user: entries[0] }); });
    });
  });
});

Example 2: Canonicalizing and validating before use

Normalize input (trim, enforce permitted characters), and avoid building DNs or filters via concatenation when libraries support structured APIs.

const crypto = require('crypto');
const ldap = require('ldapjs');

function safeUsername(value) {
  // Allow only alphanumeric and a few safe characters; reject control chars and wildcards
  if (!/^[a-zA-Z0-9._-]{1,64}$/.test(value)) {
    throw new Error('invalid username');
  }
  return value;
}

app.post('/search', (req, res) => {
  const { username, providedHmac } = req.body;
  const secret = process.env.HMAC_SECRET;

  if (!verifyHmac({ username }, providedHmac, secret)) {
    return res.status(401).json({ error: 'invalid signature' });
  }

  const usernameSafe = safeUsername(username);
  // Use an administrative bind and a parameterized subtree search
  const client = ldap.createClient({ url: 'ldap://dc.example.com' });
  client.bind('cn=admin,dc=example,dc=com', process.env.LDAP_ADMIN_PASS, (bindErr) => {
    if (bindErr) { return res.status(500).json({ error: 'admin bind failed' }); }
    const filter = `(uid=${ldap.filter.escape(usernameSafe)})`;
    client.search('dc=example,dc=com', {
      filter: filter,
      scope: 'sub',
    }, (searchErr, searchRes) => {
      // handle results as before
    });
  });
});

Key remediation practices

• Always verify HMAC before processing, but do not assume verified data is safe for downstream protocols.
• Use library-provided encoding/escaping functions (e.g., ldap.filter.encodeValue) when constructing filters.
• Prefer parameterized APIs or strict allow-lists for characters instead of blacklists.
• Avoid building DNs or search bases via string concatenation; use library utilities when available.
• Combine HMAC integrity checks with robust input validation to reduce the impact of compromised secrets or implementation errors.

By applying these patterns, you maintain the integrity benefits of HMAC while preventing LDAP Injection through HMAC-verified inputs. This aligns with secure coding guidance and helps satisfy checks related to Authentication, Input Validation, and BOLA/IDOR that middleBrick evaluates.