Ldap Injection in Buffalo with Hmac Signatures

Ldap Injection occurs when an attacker can manipulate LDAP query construction through untrusted input. In the Buffalo web framework for Go, this typically arises when building LDAP filters or DNs using string concatenation or insufficiently parameterized bindings. When Hmac Signatures are introduced—commonly to validate webhook callbacks, API tokens, or session identifiers—the interaction can unintentionally expose or amplify injection risks if the signature or its derived payload is used to influence LDAP operations.

Consider a scenario where a Buffalo app receives an HTTP request containing an HMAC-signed query parameter (e.g., id) that is later used to construct an LDAP filter. If the application verifies the HMAC but then directly interpolates the decoded payload into an LDAP search filter without sanitization or parameterization, it may allow an attacker to terminate the filter string early and inject additional LDAP syntax. For example, a signature verification flow that extracts a user identifier from the payload and builds a filter like (&(uid=attacker-controlled-value)) becomes vulnerable if the value contains special LDAP characters such as *, (, ), or \. These characters can change the filter’s semantics, leading to authentication bypass or unauthorized data retrieval.

In Buffalo, Hmac Signatures are often handled via middleware or helper functions that validate a signature before processing request context. If the validated data is used to dynamically construct LDAP queries—such as binding to a directory or searching for entries—without strict input validation, the trust placed in the signature does not mitigate injection. The signature ensures integrity and authenticity of the data, but it does not sanitize or escape special characters for LDAP. This creates a mismatch: the application assumes that because the Hmac is valid, the content is safe for LDAP, which is not guaranteed. Attack patterns like filter prefix/postfix injection, escaping, and wildcard abuse become feasible when untrusted data derived from a trusted signature is passed to LDAP library calls such as ldap.Search.

Real-world implications align with common attack vectors cataloged in OWASP API Security Testing, where input validation and authorization are critical. An attacker could exploit this by sending a crafted Hmac-signed request where the payload includes LDAP metacharacters, potentially achieving unauthorized authentication as another user or extracting sensitive directory information. Because Buffalo does not inherently sanitize data used in LDAP calls, developers must treat all inputs—even those protected by Hmac Signatures—as untrusted when building directory queries.

To secure Buffalo applications that use Hmac Signatures in combination with LDAP operations, apply strict input validation and avoid direct string interpolation. Hmac verification should remain separate from data transformation for LDAP. Always treat payloads extracted from verified signatures as opaque and validate or encode them before use in directory queries.

Remediation pattern 1: Validate and sanitize before LDAP use

After verifying the Hmac, sanitize any extracted user-provided values. For LDAP filters, use proper escaping for special characters defined in RFC 4515. In Go, you can leverage libraries such as github.com/go-ldap/ldap/v3 which provide filter building utilities that reduce injection risk.

package controllers

import (
    "github.com/gobuffalo/buffalo"
    "github.com/gobuffalo/packr/v2"
    "github.com/go-ldap/ldap/v3"
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "errors"
)

// verifyHmac checks the Hmac signature of a payload.
func verifyHmac(payload, receivedSig, secret string) (bool, error) {
    key := []byte(secret)
    mac := hmac.New(sha256.New, key)
    mac.Write([]byte(payload))
    expected := hex.EncodeToString(mac.Sum(nil))
    if !hmac.Equal([]byte(expected), []byte(receivedSig)) {
        return false, errors.New("invalid signature")
    }
    return true, nil
}

// sanitizeLDAPFilter escapes special characters for safe LDAP filter use.
func sanitizeLDAPFilter(input string) string {
    // ldap.EscapeFilter escapes reserved characters as required by RFC 4515.
    return ldap.EscapeFilter(input)
}

// Example Buffalo action using Hmac-signed input safely with LDAP.
func UserSearch(c buffalo.Context) error {
    signedPayload := c.Param("data")      // e.g., "uid=alice"
    receivedSig := c.Param("sig")         // Hmac signature
    secret := "your-secure-key"

    ok, err := verifyHmac(signedPayload, receivedSig, secret)
    if err != nil || !ok {
        return c.Error(401, errors.New("unauthorized"))
    }

    // At this point, payload integrity is confirmed, but it is NOT safe to interpolate directly.
    safeFilter := sanitizeLDAPFilter(signedPayload)
    lfilter := ldap.NewFilter(safeFilter)

    // Use the filter with an ldap.Client (assumed initialized elsewhere).
    // ldapClient.Search(...) would use lfilter to avoid injection.
    _ = lfilter // placeholder to show usage

    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

Remediation pattern 2: Use parameterized construction or allowlists

Instead of building filters from raw input, prefer allowlists for known attributes and values. If the Hmac payload is expected to contain identifiers, validate them against a strict pattern (e.g., alphanumeric usernames) before using them in LDAP calls.

package controllers

import (
    "github.com/gobuffalo/buffalo"
    "regexp"
)

// isValidUsername ensures the identifier matches an expected safe pattern.
func isValidUsername(input string) bool {
    // Allow only letters, digits, hyphens, and underscores.
    pattern := regexp.MustCompile(`^[A-Za-z0-9_-]+$`)
    return pattern.MatchString(input)
}

func SecureBind(c buffalo.Context) error {
    payload := c.Param("data")
    sig := c.Param("sig")

    ok, _ := verifyHmac(payload, sig, "your-secure-key")
    if !ok {
        return c.Error(401, errors.New("unauthorized"))
    }

    // Enforce allowlist validation before any LDAP operation.
    if !isValidUsername(payload) {
        return c.Error(400, errors.New("invalid identifier"))
    }

    // Proceed with LDAP operations using the validated input.
    // ldapClient.Bind(...) would use the verified payload safely.
    _ = payload

    return c.Render(200, r.JSON(map[string]string{"status": "bound"}))
}

By combining Hmac verification with rigorous input validation and LDAP-aware escaping or allowlists, Buffalo applications can safely use signed data without exposing LDAP injection surfaces.