Format String in Actix with Hmac Signatures

A format string vulnerability occurs when user-controlled input is passed directly into a formatting function without proper sanitization, allowing an attacker to read from or write to memory. In Actix web applications, this risk can intersect with HMAC signature handling when developers inadvertently incorporate untrusted data into the process of generating or validating signatures. For example, using user input in a logging or tracing statement that includes a format specifier like %s or %x can expose parts of the stack, including the HMAC secret or intermediate signature values.

Consider an Actix handler that logs incoming requests together with a signature derived from a secret and the request payload. If the developer uses a format string that incorporates unchecked user data—such as a query parameter or header value—into the logging call, an attacker can supply format specifiers to leak the HMAC secret or the computed signature. This becomes a chain where the signature integrity, which should protect message authenticity, is undermined by a basic formatting mistake.

More critically, if the HMAC verification logic uses formatted output that includes attacker-controlled strings (for example, building an error message that embeds the expected versus received signature using something like format!("Expected: {}, Got: {}", expected, user_sig) without sanitization), an attacker can exploit the format string to infer memory contents or trigger crashes that aid further exploitation. In the context of API security checks run by middleBrick, such patterns are surfaced as data exposure and input validation findings, because the HMAC mechanism is meant to ensure authenticity and should not depend on uncontrolled string interpolation.

Because Actix applications often handle high-throughput requests, a format string bug tied to HMAC handling can lead to information leakage across requests or enable denial-of-service conditions that indirectly weaken authentication guarantees. The scanner’s input validation and data exposure checks look for places where user data enters formatted strings, particularly around cryptographic operations like HMAC, to highlight these classes of issues before they can be weaponized.

To remediate format string risks in Actix when working with HMAC signatures, ensure that all logging, tracing, and error reporting uses strict format strings with no user-controlled format specifiers. Prefer using structured logging libraries or explicit argument substitution so that user data is never interpreted as a format directive.

Below are examples of vulnerable and safe patterns in Actix with HMAC handling.

Vulnerable pattern

use actix_web::{web, HttpResponse, Responder};
use hmac::{Hmac, Mac};
use sha2::Sha256;

type HmacSha256 = Hmac<Sha256>;

async fn vulnerable_login(
    payload: web::Json<LoginPayload>,
    query: web::Query<HashMap<String, String>>
) -> impl Responder {
    let secret = b"super-secret-key";
    let mut mac = HmacSha256::new_from_slice(secret).expect("HMAC can take key of any size");
    mac.update(payload.body().as_bytes());
    let result = mac.finalize();
    let signature = result.into_bytes();

    // Vulnerable: user-controlled value from query is used directly in format string
    let tag = query.get("tag").unwrap_or(&"none".to_string());
    println!("Processing request with tag: %s and signature: %x", tag, signature);

    HttpResponse::Ok().body("OK")
}

In the example above, the format string contains %s and %x placeholders directly interpolated with tag and signature. If tag contains format specifiers, it can lead to memory disclosure or crashes.

Safe remediation

use actix_web::{web, HttpResponse, Responder};
use hmac::{Hmac, Mac};
use sha2::Sha256;

type HmacSha256 = Hmac<Sha256>;

async fn secure_login(
    payload: web::Json<LoginPayload>,
    query: web::Query<HashMap<String, String>>
) -> impl Responder {
    let secret = b"super-secret-key";
    let mut mac = HmacSha256::new_from_slice(secret).expect("HMAC can take key of any size");
    mac.update(payload.body().as_bytes());
    let result = mac.finalize();
    let signature = result.into_bytes();

    // Safe: use structured logging or explicit formatting without user-controlled specifiers
    let tag = query.get("tag").unwrap_or(&"none".to_string());
    // Using format! to build a single message ensures no interpretation of format specifiers in user data
    let log_message = format!("Processing request with tag: {} and signature: {:?}", tag, signature);
    println!("{}", log_message);

    HttpResponse::Ok().body("OK")
}

In the secure version, the format string is a literal with no placeholders derived from user input, and the values are passed as regular arguments. This prevents any injected format specifiers from being interpreted by println!. For production-grade Actix services, consider using a logging facade such as log with a structured logger backend to further reduce risk around sensitive data exposure around HMAC operations.