Ldap Injection in Chi with Mutual Tls

LDAP Injection is an injection attack where untrusted input is concatenated into LDAP query strings, allowing an attacker to alter query logic, bypass authentication, or extract directory data. Chi is a lightweight HTTP router for Go that is commonly used to build API endpoints. When a Chi-based service accepts user-supplied values (e.g., username, search filter, or tenant identifier) and uses them to construct LDAP queries without proper escaping, it can be vulnerable to LDAP Injection even when Mutual TLS (mTLS) is enforced for transport-layer authentication.

Mutual TLS provides strong client authentication and encrypts the transport, which prevents passive network eavesdropping and ensures that only clients with valid certificates can reach the endpoint. However, mTLS does not sanitize or validate the semantic content of requests. If your Chi handler parses the client certificate (e.g., to extract a username or group) and then directly uses that data in an LDAP filter, the trust boundary shifts from the network to the application logic. For example, an attacker who possesses a valid client certificate can supply crafted input in headers, query parameters, or JSON bodies that are later interpolated into LDAP filters. Because mTLS only authenticates the client and does not escape special LDAP characters such as (, ), *, \00, and control characters, the application may inadvertently construct filters like (&(uid=attacker)(objectClass=person)) or bypass intended scoping, leading to authentication bypass or data exfiltration.

Consider a Chi route that retrieves the authenticated identity from the TLS state and builds an LDAP filter without escaping:

// Dangerous: direct concatenation of identity into LDAP filter
func handler(w http.ResponseWriter, r *http.Request) {
    tlsState := r.TLS
    if len(tlsState.PeerCertificates) == 0 {
        http.Error(w, "missing client cert", http.StatusUnauthorized)
        return
    }
    subject := tlsState.PeerCertificates[0].Subject.CommonName // attacker-controlled once cert is obtained
    filter := fmt.Sprintf("(uid=%s)", subject) // LDAP Injection possible if subject contains special characters
    ldapQuery := ldap.NewSearchRequest(
        "dc=example,dc=com",
        ldap.ScopeWholeSubtree, ldap.NeverDerefAliases, 0, 0, false,
        filter,
        []string{"mail"},
        nil,
    )
    // ... execute ldapQuery
}

If an attacker obtains a valid client certificate with Common Name admin)(objectClass=person), the resulting filter becomes (&(uid=admin)(objectClass=person)), which may bypass authorization or return unintended entries. Even when mTLS enforces strict access, without input validation and escaping at the application layer, LDAP Injection remains a viable attack vector.

Additionally, mTLS does not protect against injection via other request dimensions such as JSON payloads or URL query parameters that the handler uses to build dynamic filters. For instance, a query parameter like ?search=* can be used to manipulate wildcard matching in LDAP filters. Therefore, treating mTLS as a comprehensive security control for input handling is a misconception; you must still validate, sanitize, and escape all data used in LDAP queries.

To mitigate LDAP Injection in a Chi application with mTLS, treat all identity data derived from the TLS handshake as untrusted and apply strict escaping and schema validation before using it in LDAP filters. Do not rely on mTLS to sanitize input.

1. Use parameterized LDAP filters or an LDAP library that supports escaping. For example, with the github.com/go-ldap/ldap/v3 package, prefer constructing filters with ldap.NewFilterEqualityAssertion instead of string concatenation:

import (
    "github.com/go-ldap/ldap/v3"
    "net/http"
)

func safeHandler(w http.ResponseWriter, r *http.Request) {
    tlsState := r.TLS
    if len(tlsState.PeerCertificates) == 0 {
        http.Error(w, "missing client cert", http.StatusUnauthorized)
        return
    }
    subject := tlsState.PeerCertificates[0].Subject.CommonName
    // Use library-provided escaping for safe filter construction
    filter, err := ldap.NewFilterEqualityAssertion("uid", subject)
    if err != nil {
        http.Error(w, "invalid input", http.StatusBadRequest)
        return
    }
    ldapQuery := ldap.NewSearchRequest(
        "dc=example,dc=com",
        ldap.ScopeWholeSubtree, ldap.NeverDerefAliases, 0, 0, false,
        filter,
        []string{"mail"},
        nil,
    )
    // ... execute ldapQuery
}

2. Validate and normalize the subject before using it. Enforce allowed characters and length, and normalize case if your directory is case-insensitive:

import "regexp"

var safeCN = regexp.MustCompile(`^[a-zA-Z0-9._-]{1,64}$`)

func validateCN(cn string) bool {
    return safeCN.MatchString(cn)
}

func validatedHandler(w http.ResponseWriter, r *http.Request) {
    tlsState := r.TLS
    if len(tlsState.PeerCertificates) == 0 {
        http.Error(w, "missing client cert", http.StatusUnauthorized)
        return
    }
    subject := tlsState.PeerCertificates[0].Subject.CommonName
    if !validateCN(subject) {
        http.Error(w, "invalid certificate subject", http.StatusBadRequest)
        return
    }
    filter, err := ldap.NewFilterEqualityAssertion("uid", subject)
    if err != nil {
        http.Error(w, "internal error", http.StatusInternalServerError)
        return
    }
    ldapQuery := ldap.NewSearchRequest(
        "dc=example,dc=com",
        ldap.ScopeWholeSubtree, ldap.NeverDerefAliases, 0, 0, false,
        filter,
        []string{"mail"},
        nil,
    )
    // ... execute ldapQuery
}

3. If you must build complex filters, use explicit allowlists for attribute values and avoid concatenating multiple user-influenced fields into a single filter expression. Also ensure that any data extracted from the mTLS certificate is treated as an identity claim and validated against your authorization model rather than being directly trusted for query construction.

4. Complement mTLS with application-level security: enable continuous scanning with tools like middleBrick to detect LDAP Injection and related issues in your Chi endpoints. The CLI tool can be run locally with middlebrick scan <url>, and the GitHub Action can add API security checks to your CI/CD pipeline to fail builds if risk scores drop below your chosen threshold.

By combining mTLS for transport and client authentication with strict input validation, escaping, and schema enforcement, you reduce the risk of LDAP Injection in Chi-based services without assuming mTLS alone suffices for input safety.