Ldap Injection in Axum with Jwt Tokens

Ldap Injection occurs when untrusted input is concatenated into LDAP queries without validation or escaping, allowing an attacker to alter query logic. In an Axum application that uses JWT Tokens for initial authentication or identity claims, an LDAP backend may still be used for authorization, group membership, or supplementary attribute lookups. If the application takes a claim (for example, sub or a custom employee_id) from the validated JWT and directly embeds it into an LDAP filter, the boundary between data and control shifts. A malicious actor who can influence the JWT (via a weak issuer verification, algorithm confusion, or a compromised signing key) or who can supply a token with crafted payloads can inject LDAP metacharacters such as (, ), *, and & into the resulting search filter.

Consider an Axum handler that retrieves a user’s group membership from an LDAP server after a JWT is verified. If the identity value from the token is interpolated into the filter like (&(member={user_input})(objectClass=group)), special characters can break the filter syntax or expand the result set. For instance, a user_input of admin)(objectClass=group)(cn=* rewrites the logical structure, potentially returning all groups or bypassing intended restrictions. Even when the JWT itself is cryptographically sound, the application must treat its contents as untrusted for authorization purposes. The LDAP layer does not inherently understand JWT semantics; it only sees strings. Therefore, injection arises not from the JWT standard but from unsafe composition of LDAP queries using data originating from the token. This becomes a critical issue when the same token is used both for authentication and for building authorization queries, effectively chaining trust boundaries.

Additionally, JWT Tokens may carry attributes that the application uses to construct LDAP filters for role-based or attribute-based access control. If these attributes include user-supplied data that is not strictly validated and normalized, attackers can exploit reserved LDAP characters to perform privilege escalation or information disclosure. For example, a filter like (&(uid={username})(memberOf=cn={group_name},ou=groups,dc=example,dc=com)) becomes vulnerable when {group_name} is derived from a JWT claim that is not strictly constrained. Real-world attack patterns such as CVE-2023-21750 class vulnerabilities often stem from similar trust boundary confusion where identity tokens feed downstream directory queries without adequate sanitization.

Remediation centers on strict validation, canonicalization, and parameterization of any data derived from JWT Tokens before it reaches the LDAP layer. In Axum, this means applying typed extractors, schema validation, and encoding functions before constructing queries. Instead of string interpolation, use a parameterized approach or, when interfacing with LDAP clients that support it, prepared search templates that treat input as data only.

First, enforce strict claim validation. Validate issuer, audience, and expiration rigorously so that the token cannot be trivially manipulated. Then, normalize and constrain claims used in directory queries. For example, if a claim must map to an LDAP search filter, restrict it to safe characters and length limits.

use axum::{routing::get, Json, Router};
use jsonwebtoken::{decode, Algorithm, DecodingKey, Validation};
use serde::{Deserialize, Serialize};

#[derive(Debug, Serialize, Deserialize)]
struct Claims {
    sub: String,
    username: String,
}

async fn user_groups_handler(
    Json(payload): Json,
) -> Result {
    // Validate token with strict algorithm and claims
    let validation = Validation::new(Algorithm::HS256);
    let token_data = decode::(
        &extract_token_from_header()?, // implement header extraction separately
        &DecodingKey::from_secret("secret".as_ref()),
        &validation,
    )
    .map_err(|_| (axum::http::StatusCode::UNAUTHORIZED, "Invalid token".to_string()))?;

    // Canonicalize and constrain values used in LDAP queries
    let safe_username = sanitize_ldap_input(&token_data.claims.username);
    let filter = format!("(&(uid={})(objectClass=person))", safe_username);

    // Use parameterized LDAP client call if available, or safely encode
    let result = ldap_search_with_params(filter).await?;
    Ok(result)
}

fn sanitize_ldap_input(value: &str) -> String {
    // Remove or escape LDAP metacharacters
    value.replace('*', "\\2a").replace('(', "\\28").replace(')', "\\29")
}

async fn ldap_search_with_params(filter: String) -> Result {
    // Implementation depends on LDAP client library; ensure filter is passed as a parameter,
    // not concatenated into a raw search string.
    Ok("filtered_result".to_string())
}

Second, avoid using JWT-derived values directly in multi-valued or nested filters. If group membership is needed, prefer mapping claims to a pre-approved set of roles or identifiers rather than injecting raw strings into complex LDAP expressions. When constructing filters, explicitly encode special characters or use an LDAP library that provides safe filter building utilities. Never trust the contents of JWT Tokens for query construction without canonicalization and allowlisting.

Finally, audit the trust boundary between authentication and authorization. If your architecture requires LDAP queries based on JWT Tokens, ensure that the token’s scope and claims are strictly limited and that the LDAP client uses secure connections (e.g., LDAPS). This reduces the impact of potential token compromise and prevents injection paths that exploit format strings or wildcard injections in directory services.