Ldap Injection in Axum with Mutual Tls

Ldap Injection occurs when an attacker can manipulate LDAP query construction by injecting malicious input. In Axum, a Rust web framework, this typically arises when user-controlled data is concatenated into LDAP filter strings without proper escaping. Even when Mutual TLS (mTLS) is enforced to authenticate clients at the transport layer, Ldap Injection remains possible because mTLS verifies identity but does not sanitize application-level input. An authenticated client with a valid certificate can still supply a username or search base parameter that alters the LDAP query logic.

For example, consider an endpoint in Axum that binds to an LDAP server after mTLS authentication. If the developer builds the filter using string formatting, a payload like (uid=*)(objectClass=*) could be injected into the filter, causing the server to return unintended entries or bypass authorization checks. The combination of mTLS and unchecked input creates a false sense of security: transport security ensures the client is known, but it does not prevent the application from building malicious LDAP queries. Attack patterns such as CVE-2021-28091 illustrate how improper filter construction leads to unauthorized data access.

To detect this in a black-box scan, middleBrick runs checks across the unauthenticated attack surface, including input validation and security misconfiguration. Even with mTLS in place, if the API endpoint accepts query parameters that influence LDAP filters and those parameters are not validated or escaped, the scan will flag the endpoint as vulnerable. The presence of mTLS does not mitigate injection; it only strengthens authentication, leaving the injection vector intact at the application layer.

Remediation focuses on treating all user input as untrusted, regardless of mTLS status. In Axum, you should validate and escape input before constructing LDAP filters. Use a dedicated LDAP escaping library to ensure that special characters such as asterisk, parentheses, and backslash are properly encoded. Never concatenate strings to form filters. Instead, use parameterized queries or an LDAP library that supports safe filter building.

Below are concrete code examples for Axum with mTLS. The first example shows a vulnerable route where user input is directly interpolated into an LDAP filter. The second example demonstrates a secure implementation using the ldap3 crate’s filter building utilities, which handle escaping automatically.

// Vulnerable Axum route with LDAP injection risk
use axum::{routing::get, Router};
use ldap3::{LdapConnAsync, Scope, SearchEntry};

async fn vulnerable_search(
    user_input: String,
) -> Result> {
    let (conn, mut ldap) = LdapConnAsync::new("ldap://ldap.example.com").await?;
    // mTLS is configured at the transport layer, but input is not escaped
    let filter = format!("(uid={})", user_input);
    let (rs, _res) = ldap.search(
        "dc=example,dc=com",
        Scope::Subtree,
        &filter,
        vec!["mail"],
    ).await?.success()?;
    for entry in rs {
        let e = SearchEntry::construct(entry);
        println!("{:?}", e.attrs);
    }
    Ok("done".to_string())
}

// Secure Axum route with proper escaping
use ldap3::filter::Filter;
use axum::{routing::get, Router};

async fn secure_search(
    user_input: String,
) -> Result> {
    let (conn, mut ldap) = LdapConnAsync::new("ldap://ldap.example.com").await?;
    // mTLS is still enforced at the transport layer
    // Build filter safely using ldap3's Filter API
    let base = "dc=example,dc=com";
    let scope = ldap3::Scope::Subtree;
    let filter = Filter::new(&user_input).build(); // escapes special characters
    let (rs, _res) = ldap.search(base, scope, &filter, vec!["mail"]).await?.success()?;
    for entry in rs {
        let e = SearchEntry::construct(entry);
        println!("{:?}", e.attrs);
    }
    Ok("done".to_string())
}

In the secure example, Filter::new ensures that characters like *, (, and ) are escaped, preventing injection. The mTLS configuration remains in place to authenticate clients, while the application logic correctly handles untrusted input. This approach aligns with the principle that transport security and input validation are complementary but independent controls.