Ldap Injection in Aspnet with Dynamodb

LDAP Injection is an attack technique that manipulates LDAP query construction to bypass authentication or access unauthorized directory data. In an ASP.NET application that uses Amazon DynamoDB as a user store, this typically occurs when the app builds LDAP filter strings using unsanitized user input—for example, a username or search parameter taken directly from a form field or header.

Consider an ASP.NET authentication flow that first queries DynamoDB to retrieve a user’s LDAP filter template or additional attributes, then constructs an LDAP search filter in C# like this:

var username = httpContext.Request.Query["username"];
var userRecord = await dynamoClient.GetItemAsync<UserRecord>(new GetItemRequest {
    TableName = "Users",
    Key = new Dictionary<string, AttributeValue> {
        { "PK", new AttributeValue { S = $"USER#{username}" } }
    }
});
string ldapFilter = $"(uid={username})"; // Unsafe string interpolation

If the attacker supplies username as admin)(&objectClass=*), the resulting LDAP filter becomes (uid=admin)(&objectClass=*) or (&(uid=admin)(objectClass=*)) depending on concatenation, potentially bypassing UID checks and returning all directory entries. In a typical setup, the application may then pass this filter to an LDAP client library to perform group membership validation or additional attribute lookups. Because DynamoDB does not interpret LDAP syntax, the unsafe string handling resides entirely in the application code, and DynamoDB simply stores and retrieves the attacker-controlled strings used in the filter construction.

This combination exposes two distinct risk dimensions: DynamoDB may contain configuration templates or group mappings that an attacker can leverage via injection to alter LDAP behavior, and the application’s lack of input validation allows crafted payloads to change the semantics of the LDAP query. Attack outcomes can include authentication bypass, unauthorized group discovery, or information disclosure about directory entries. Because the scan is unauthenticated, middleBrick tests this surface by submitting suspicious strings in the username parameter and observing whether the LDAP filter is improperly formed or exposes sensitive attributes.

Additionally, if the application uses the retrieved DynamoDB data to dynamically construct LDAP comparisons—such as using a displayName attribute stored in DynamoDB directly in the filter—any stored malicious content or lack of canonicalization further amplifies the impact. The vulnerability is not in DynamoDB itself but in how the application builds and uses LDAP filter strings with data sourced from or influenced by the database.

Remediation centers on strict input validation, parameterized LDAP filter construction, and avoiding direct string interpolation with user-controlled values. Instead of embedding user input into LDAP filter strings, use a dictionary of allowed attributes and a whitelist-based approach to select safe filter components. When integrating with DynamoDB, treat retrieved data as untrusted and apply the same validation rigor.

Below is a secure pattern in ASP.NET that combines DynamoDB retrieval with safe LDAP filter building. It avoids concatenating raw input and uses a controlled mapping to construct filters:

var username = httpContext.Request.Query["username"];
// Whitelist validation: only allow alphanumeric and a few safe symbols
if (!System.Text.RegularExpressions.Regex.IsMatch(username, @"^[a-zA-Z0-9._-]{1,64}$")) {
    return Results.BadRequest("Invalid username format");
}

var userRecord = await dynamoClient.GetItemAsync<UserRecord>(new GetItemRequest {
    TableName = "Users",
    Key = new Dictionary<string, AttributeValue> {
        { "PK", new AttributeValue { S = $"USER#{username}" } }
    }
});

// Use a parameterized approach: treat user input as a value, not as filter syntax
var safeFilter = $"(uid={EscapeLdapFilterValue(username)})";

// Example helper to escape special LDAP filter characters
string EscapeLdapFilterValue(string input) {
    // Replace * (0x2A), ( (0x28), ) (0x29), \ (0x5C), NUL as per RFC 4515
    var sb = new System.Text.StringBuilder();
    foreach (char c in input) {
        switch (c) {
            case '\\': sb.Append("\\5c"); break;
            case '*': sb.Append("\\2a"); break;
            case '(' : sb.Append("\\28"); break;
            case ')' : sb.Append("\\29"); break;
            case '\0': sb.Append("\\00"); break;
            default: sb.Append(c); break;
        }
    }
    return sb.ToString();
}

In this pattern, DynamoDB is used only to fetch user-specific metadata, and the LDAP filter is built with a strict escape routine for special characters defined by RFC 4515. This neutralizes injection attempts that rely on control characters like parentheses or wildcards.

For applications that must construct complex multi-attribute filters, prefer a parameterized builder that assembles filter components from a controlled set rather than concatenating raw strings:

var filterBuilder = new System.Collections.Generic.List<string>();
filterBuilder.Add($"(uid={EscapeLdapFilterValue(username)})");
if (!string.IsNullOrEmpty(userRecord.Department)) {
    filterBuilder.Add($"(department={EscapeLdapFilterValue(userRecord.Department)})");
}
string complexFilter = $"(&{string.Join("", filterBuilder)})";

By combining DynamoDB data retrieval with strict validation and escaping, the application mitigates LDAP Injection while preserving the flexibility to use stored attributes safely. middleBrick’s unauthenticated scans can verify that no user-controlled input is directly interpolated into LDAP filters and that remediation reduces the attack surface to a safe subset of characters.