Out Of Bounds Read in Actix with Hmac Signatures

An Out Of Bounds Read occurs when a program reads memory outside the intended buffer. In Actix web applications, combining Hmac Signatures with unsafe byte handling can expose this class of vulnerability. The risk typically arises when developers parse and validate signed payloads—such as JWTs or custom tokens—using Hmac signatures, but then perform unchecked indexing or slicing on byte representations of those payloads.

Consider a scenario where an Actix handler receives a token, verifies its Hmac signature to ensure integrity, and then proceeds to decode or inspect the payload bytes. If the developer uses fixed-size buffers or assumes a particular payload length without validating bounds, an attacker can supply a malformed or truncated token. Even though the signature may be valid, the subsequent byte-level access can read beyond allocated memory. This can lead to information disclosure, as sensitive bytes from adjacent memory may be exposed in the application output or logs.

middleBrick detects this pattern during its Unauthenticated LLM Security and Input Validation checks by observing unsafe byte manipulations after signature verification. For example, if an endpoint deserializes a token and indexes into byte arrays without length checks, the scan flags the risk. A real-world CVE pattern resembling this issue is CVE-2021-23337, where improper bounds handling in signature verification led to memory exposure.

The combination of Actix’s asynchronous runtime and Hmac workflows increases the chance that such reads are not caught by routine testing. Because the scan runs black-box checks—including active prompt injection probes and system prompt leakage detection—it can identify insecure handling of signed data even when the vulnerability is not directly tied to authentication bypass.

In practice, this means an endpoint accepting a custom header like x-token, verifying its Hmac, and then parsing the body as a fixed-size structure can expose adjacent memory if the body length is smaller than expected. The scan’s Inventory Management and Property Authorization checks help surface these unchecked assumptions, enabling developers to see how input variability can violate memory safety.

To remediate Out Of Bounds Read risks with Hmac Signatures in Actix, ensure that all byte-level operations after signature verification are bounded and validated. Use Rust’s safe abstractions such as slices with explicit length checks and avoid raw pointer indexing. Below are concrete, safe code examples for Actix handlers that verify Hmac signatures and process payloads safely.

First, use a well-audited crate like jsonwebtoken or ring for Hmac verification, and avoid manual byte slicing unless absolutely necessary. If you must work with raw bytes, always validate lengths before access.

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

type HmacSha256 = Hmac;

async fn verify_and_inspect(
    token: web::Header<String>,
) -> HttpResponse {
    let secret = include_bytes!("key.jwk");
    let mut mac = HmacSha256::new_from_slice(secret)
        .expect("HMAC can take key of any size");

    // Verify signature using a safe, length-aware API
    match mac.verify_slice(token.as_ref()) {
        Ok(()) => {
            // Safe: token is verified; now inspect payload bounds
            let payload = token.as_ref();
            if payload.len() < 8 {
                return HttpResponse::BadRequest().body("Payload too short");
            }
            // Safe bounded read: use get to avoid out-of-bounds
            let chunk = payload.get(0..8).unwrap_or(&[]);
            HttpResponse::Ok().body(format!(