Email Injection in Axum with Jwt Tokens

Email injection in Axum combined with JWT token handling can expose authentication and authorization logic when user-controlled data influences email-related operations. Axum is a web application framework for Rust, and while it does not directly handle email sending, integrations often introduce risks when constructing email headers or bodies using unchecked input that later participates in JWT-based workflows.

Consider a scenario where an endpoint accepts an email address to send a verification or reset link, and the application embeds that email value into a JWT claim (for example, an email claim) without strict validation or canonicalization. If the input is not validated, an attacker can supply additional email headers (e.g., injecting %0d%0a or newline sequences) that alter the semantics of the downstream email process. Even though Axum does not send email itself, the payload may be forwarded to an email service or logged in a way that enables header injection. This becomes a security boundary issue when the JWT is later used to identify the user in a trust boundary that assumes the email claim is immutable and tied to a verified identity.

Additionally, when APIs expose JWT-signed URLs or tokens that include email-based identifiers, unchecked email input can lead to token confusion or account takeover. For example, if an attacker injects a second email header such as Cc: [email protected], and the application logs or echoes this value into a JWT without normalization, the forged email may be treated as the authenticated principal. This can bypass expected access controls where authorization checks rely on the email claim derived from user input rather than the authenticated subject.

The risk is compounded when Axum endpoints directly parse JSON payloads into structures that are then signed with JWTs. Without schema validation and type-safe deserialization, an attacker can supply nested or malformed email values that distort the resulting token contents. Although Axum does not perform validation automatically, frameworks often rely on Serde to deserialize request bodies; if the deserialization logic does not enforce strict string patterns, the resulting JWT may carry an unexpected email value that weakens identity linkage.

Operational visibility also matters: because middleBrick scans the unauthenticated attack surface and tests input validation across API endpoints, it can surface cases where email parameters are reflected in JWT-related responses or error messages. This aligns with checks such as Input Validation and Property Authorization, which help detect injection-like behaviors even in backend frameworks that do not directly manage email sending.

To secure Axum endpoints that interact with JWT tokens and email-related data, apply strict validation and canonicalization before any token creation or signing. Use well-maintained crates such as jsonwebtoken for signing and verification, and validate email formats with a robust parser rather than simple string checks.

First, define a request structure that constrains the email field to a validated type. Use a dedicated validator that enforces RFC 5322-like rules and rejects newline or control characters that could enable injection:

use axum::extract::Json;
use serde::{Deserialize, Serialize};
use validator::ValidateEmail;

#[derive(Debug, Deserialize, Serialize)]
struct EmailRequest {
    email: String,
}

// Custom validation layer
impl ValidateEmail for EmailRequest {
    fn is_valid_email(&self) -> bool {
        // Reject newlines, carriage returns, and other control characters
        if self.email.chars().any(|c| c.is_control()) {
            return false;
        }
        // Use a crate or regex that follows RFC 6530 rules for safety
        email_validator::validate_email(&self.email)
    }
}

async fn request_handler(
    Json(payload): Json<EmailRequest>,
) -> Result<impl IntoResponse, (StatusCode, String)> {
    if !payload.is_valid_email() {
        return Err((StatusCode::BAD_REQUEST, "Invalid email".to_string()));
    }
    // Continue with JWT creation using a canonical email
    let canonical_email = payload.email.trim().to_lowercase();
    // Token creation omitted for brevity
    Ok(Json("OK"))
}

Second, when creating JWTs, ensure the email claim is derived from the validated canonical form and avoid including raw user input directly in the payload. Use explicit claims structures and a secure signing key:

use jsonwebtoken::{encode, Algorithm, EncodingKey, Header, Validation};
use serde::{Deserialize, Serialize};

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

fn create_token(email: &str, secret: &[u8]) -> Result<String, jsonwebtoken::errors::Error> {
    let claims = Claims {
        sub: email.to_string(), // Prefer a user ID; email as subject only if canonicalized
        email: email.to_string(),
        exp: (chrono::Utc::now() + chrono::Duration::hours(1)).timestamp() as usize,
   ;
    encode(
        &Header::new(Algorithm::HS256),
        &claims,
        &EncodingKey::from_secret(secret),
    )
}

Third, apply consistent validation on the consuming side. When verifying tokens, do not re-trust the email claim for routing or authorization decisions unless it matches the authenticated subject stored server-side:

fn verify_and_use(token: &str, secret: &[u8]) -> Result<Claims, jsonwebtoken::errors::Error> {
    let mut validation = Validation::new(Algorithm::HS256);
    validation.validate_exp = true;
    let token_data = decode<Claims>(token, &DecodingKey::from_secret(secret), &validation)?;
    // Do not use token_data.claims.email as a sole identity proof; cross-check with a trusted directory
    Ok(token_data.claims)
}

These steps align with the framework-agnostic nature of Axum while ensuring that email values do not escape their intended boundary. By combining strict input validation, canonicalization, and disciplined JWT usage, you reduce the attack surface associated with email injection and token confusion.