Email Injection in Actix with Jwt Tokens

Email injection in Actix applications that also use JWT tokens can occur when user-controlled data, such as an email address or username, is reflected into authentication flows or error messages without proper validation. If an API endpoint accepts an email parameter to construct a redirect, send a confirmation link, or select a user record, and that input is not strictly validated, an attacker can inject newline characters or additional headers to alter the behavior of the email layer or session handling.

When JWT tokens are involved, the risk pattern shifts to how the token is issued, validated, and associated with an email claim. An attacker may attempt to inject crafted email values into a parameter that influences which subject or recipient is used when generating a JWT, potentially leading to token issuance for an unintended identity or to a session fixation scenario. Even in black-box scanning, middleBrick flags this as an authentication/authorization concern because the injection can undermine identity binding that the JWT is meant to protect.

During a scan, middleBrick tests unauthenticated endpoints that accept email-like inputs and inspect how tokens are issued or validated. If the API reflects user input into email headers, redirects, or JWT custom claims without canonicalization and strict allow-listing, the scan produces findings under Authentication and BOLA/IDOR. The scanner does not assume trust in the JWT structure; it checks whether the email used during token introspection or verification can be manipulated via newline or header injection patterns that violate RFC 5322 and RFC 7519 expectations.

Example exploit flow: an API accepts email as a query parameter to build a password reset link and also embeds the email into a JWT claim without normalization. An attacker submits email=attacker%40example.com%0D%0AX-Mailer:%20Malicious. If the server concatenates headers or uses the email directly in a redirect, the injected header may alter downstream mail handling or session binding. middleBrick detects missing input validation and improper claim handling as high-severity findings, referencing the common risks around email header injection and JWT claim misuse.

These findings map to OWASP API Top 10 controls, particularly Improper Neutralization of Special Elements and Security Misconfiguration. Because the API exposes identity-related parameters used in token issuance, the scan highlights the need for strict validation and canonicalization before any JWT operation. middleBrick does not fix the implementation but provides prioritized remediation guidance to help developers address the injection surface and preserve the integrity of token-bound identities.

To remediate email injection risks in Actix while using JWT tokens, enforce strict input validation and canonicalization before using any user-supplied email in authentication logic or token generation. Use allow-listed patterns for local-part and domain, and avoid directly concatenating user input into headers, redirects, or JWT claims.

Example secure email validation and JWT creation in Actix using the jsonwebtoken crate:

use actix_web::{web, HttpResponse, Responder};
use serde::{Deserialize, Serialize};
use jsonwebtoken::{encode, Header, EncodingKey};
use regex::Regex;

#[derive(Deserialize)]
pub struct LoginPayload {
    pub email: String,
}

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

fn is_valid_email(email: &str) -> bool {
    // Basic RFC 5322-ish allow-list; tighten per policy
    let re = Regex::new(r"^[a-zA-Z0-9.!#$%&'*+/=?^_`{|}~-]+@[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?(?:\.[a-zA-Z0-9](?:[a-zA-Z0-9-]{0,61}[a-zA-Z0-9])?)*$").unwrap();
    re.is_match(email)
}

pub async fn login_handler(payload: web::Json<LoginPayload>) -> impl Responder {
    let email = payload.email.trim().to_lowercase();
    if !is_valid_email(&email) {
        return HttpResponse::BadRequest().json(serde_json::json!({ "error": "invalid_email" }));
    }
    let claims = Claims {
        sub: email.clone(),
        email,
        exp: (chrono::Utc::now() + chrono::Duration::hours(1)).timestamp() as usize,
    };
    let token = encode(
        &Header::default(),
        &claims,
        &EncodingKey::from_secret("your-secret-key".as_ref()),
    ).unwrap_or_else(|_| "error".to_string());
    HttpResponse::Ok().json(serde_json::json!({ "token": token }))
}

Key practices:

• Normalize and lower-case the email before validation and claim population.
• Use a strict regex or a dedicated email parsing library to reject newlines, carriage returns, and control characters that enable injection.
• Do not directly interpolate user input into redirect URLs or mail headers; use typed structures and safe builders.
• When issuing JWTs, ensure the sub and any custom email claims are derived from the validated, canonical email, not the raw input.
• Apply consistent validation on both client hints and server-side checks; treat the JWT as an assertion of a verified identity, not a trust boundary for raw input.

In a production Actix service, combine this with secure cookie/session policies and HTTPS. middleBrick’s scans will validate that such canonicalization is present by testing email inputs across authentication-related endpoints and inspecting token issuance patterns, ensuring that injection attempts cannot alter headers or claims.