Ldap Injection in Gin with Hmac Signatures

LDAP Injection is an injection attack that occurs when an application passes untrusted input into LDAP query construction. In the Go ecosystem, the Gin web framework does not provide built-in LDAP helpers; developers typically construct LDAP filters or DNs using string concatenation or formatting. When HMAC signatures are used to bind or authorize requests—commonly to verify the origin of a request or to sign a payload that includes LDAP identifiers—the combination can inadvertently expose injection surfaces if the signed data includes user-controlled LDAP parameters that are later interpolated into LDAP queries without sanitization.

Consider a Gin handler that receives a user identifier and an HMAC signature. The signature is verified, and then the identifier is used to build an LDAP filter such as (uid={{.Username}}). Even though the HMAC ensures the request was generated by a trusted source, it does not guarantee the content of the signed fields are safe for LDAP. If an attacker can control the signed payload (for example, through a compromised client or a signature replay), they may inject LDAP metacharacters like *, (, ), or & into the signed field. Upon deserialization and use in an LDAP query, these characters can change the query logic, leading to unauthorized directory reads or privilege escalation. This is a BOLA-like pattern in the context of directory services: a trusted signature does not equate to trusted data.

Additionally, if the HMAC is computed over a serialized representation that includes LDAP distinguished names (DNs), special characters such as commas, plus signs, or backslashes can break DN parsing and lead to malformed searches or injection when the DN is reused in a search base. For instance, a signed DN like cn=admin,dc=example,dc=com is benign, but if an attacker can include a DN fragment like cn=admin,dc=example,dc=com + )+(objectClass=*) as part of a signed parameter that gets concatenated into a search base, the effective query scope can be widened unexpectedly. The middleware that validates HMACs in Gin should not be conflated with data validation; signature verification is an integrity check, not a sanitization step.

The risk is compounded when the same signed token is used both for authentication/authorization and to drive LDAP operations, as seen in some SSO or API gateway patterns. A scanner like middleBrick, which runs checks such as Input Validation and BOLA/IDOR in parallel, can surface these scenarios by correlating unsigned inputs, signed payload fields, and LDAP query construction points. Without explicit allowlists for LDAP-safe characters and strict separation between identity binding and query building, the HMAC layer becomes a false sense of security in the presence of injection-prone LDAP usage.

To remediate LDAP injection risks in Gin when HMAC signatures are used, treat signed data as untrusted for LDAP contexts. Validate and sanitize any field that will be used in LDAP queries, even if it was covered by the HMAC. Use parameterized LDAP queries or, where possible, avoid constructing filters via string interpolation. Below are concrete, working examples that demonstrate secure handling in Gin.

Example 1: HMAC verification with strict allowlist validation before LDAP use

In this example, a Gin handler verifies an HMAC and then validates the username against a strict character allowlist before using it in an LDAP filter. This ensures that LDAP metacharacters cannot reach the query layer.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "regexp"

    "github.com/gin-gonic/gin"
)

var sharedSecret = []byte("your-256-bit-secret")

func isValidUsername(username string) bool {
    // Allow only alphanumeric and underscore, length 1–64
    matched, _ := regexp.MatchString(`^[A-Za-z0-9_]{1,64}$`, username)
    return matched
}

func verifyHMAC(payload, receivedMAC string) bool {
    mac := hmac.New(sha256.New, sharedSecret)
    mac.Write([]byte(payload))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(receivedMAC))
}

func LoginHandler(c *gin.Context) {
    var req struct {
        Username string `json:"username"`
        MAC      string `json:"mac"`
    }
    if err := c.BindJSON(&req); err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request"})
        return
    }

    payload := req.Username // in practice, include nonce/timestamp
    if !verifyHMAC(payload, req.MAC) {
        c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
        return
    }

    if !isValidUsername(req.Username) {
        c.JSON(http.StatusBadRequest, gin.H{"error": "invalid username format"})
        return
    }

    // Safe to use req.Username in LDAP query, e.g., as a filter value
    ldapFilter := "(uid=" + req.Username + ")"
    // Proceed with LDAP search using ldapFilter
    c.JSON(http.StatusOK, gin.H{"status": "verified"})
}

Example 2: Avoiding DN concatenation; using a parameterized bind

Instead of building a DN from signed components, use a bind operation that accepts structured identifiers or map the identifier to a pre-approved account. If you must construct a DN, validate each component separately and avoid direct concatenation with user input.

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "strings"

    "github.com/gin-gonic/gin"
)

var sharedSecret = []byte("your-256-bit-secret")

// sanitizeDNComponent removes characters that can break DN parsing
func sanitizeDNComponent(s string) string {
    // Remove characters that have special meaning in RDNs
    s = strings.ReplaceAll(s, "+", "")
    s = strings.ReplaceAll(s, ",", "")
    s = strings.ReplaceAll(s, "=", "")
    s = strings.ReplaceAll(s, "#", "")
    s = strings.ReplaceAll(s, ";", "")
    s = strings.ReplaceAll(s, "<", "")
    s = strings.ReplaceAll(s, ">", "")
    s = strings.ReplaceAll(s, "\\", "")
    return s
}

func verifyHMAC(payload, receivedMAC string) bool {
    mac := hmac.New(sha256.New, sharedSecret)
    mac.Write([]byte(payload))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(receivedMAC))
}

func BindHandler(c *gin.Context) {
    var req struct {
        Username string `json:"username"`
        MAC      string `json:"mac"`
    }
    if err := c.BindJSON(&req); err != nil {
        c.JSON(http.StatusBadRequest, gin.H{"error": "invalid request"})
        return
    }

    payload := req.Username
    if !verifyHMAC(payload, req.MAC) {
        c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid signature"})
        return
    }

    safeUsername := sanitizeDNComponent(req.Username)
    // Use a pre-defined base DN; do not concatenate raw user input
    baseDN := "ou=people,dc=example,dc=com"
    searchFilter := "(uid=" + safeUsername + ")"
    // Perform LDAP search with baseDN and searchFilter
    c.JSON(http.StatusOK, gin.H{"status": "bind prepared safely"})
}

These examples illustrate that HMAC verification ensures request integrity but does not sanitize data for LDAP. Explicit validation and sanitization, combined with avoiding dynamic DN assembly, are necessary to prevent LDAP injection in Gin applications that use Hmac Signatures.