Bleichenbacher Attack in Actix with Hmac Signatures

A Bleichenbacher attack exploits adaptive chosen-ciphertext in RSA-based signature schemes, including when signatures are used alongside HMAC in Actix-based services. The classic Bleichenbacher attack targets PKCS#1 v1.5 padding in RSA decryption, but the same adaptive pattern can manifest when an Actix endpoint validates HMAC-based signatures in a way that leaks validity through timing or error behavior.

Consider an Actix web service that uses HMAC-SHA256 to sign a JSON payload and expects the client to provide both the payload and the signature. If the server first checks a hash prefix or length before performing a full constant-time comparison, an attacker can send many modified signatures and observe differing response times or error messages. This oracle behavior lets an attacker iteratively guess the signature or forge a valid one without knowing the secret key. In a scenario where RSA is also used for key exchange or token signing (e.g., JWT with RS256) and Actix routes mix signature verification logic with HMAC checks, a Bleichenbacher-style adaptive attack becomes feasible when validation is not strictly constant-time and does not enforce strict MAC-before-verify discipline.

For example, an attacker might intercept a signed message from an Actix endpoint, then modify the ciphertext or signature and resend it. If the server returns distinct errors for padding failures versus HMAC mismatches, the attacker gains an oracle. Repeating this with many adaptively chosen ciphertexts, the attacker can recover the effective signature or produce a valid one for a manipulated payload. This violates integrity guarantees and can lead to privilege escalation or unauthorized actions when the forged message elevates permissions or changes critical parameters.

Real-world context: While CVE-2017-13097 describes a Bleichenbacher-style padding oracle in RSAES-PKCS1-v1_5, the conceptual pattern applies to any signature or MAC validation that is not strictly constant-time and deterministic. In Actix, if HMAC verification is implemented with simple string equality or short-circuits on length or initial bytes, the service becomes an oracle. This is especially risky when the same endpoint handles both business logic and security-sensitive validation, as an attacker can exploit timing differences or error messages to gradually learn about the signature or secret.

To map this to the middleware scanning capabilities of middleBrick: the tool checks for timing-invariant validation patterns and flags inconsistent error handling across authentication and signature checks. In an OpenAPI spec describing an Actix-based API, middleBrick can detect endpoints where signature verification is performed without constant-time comparison and where error responses vary by failure type. These findings align with the Unauthenticated LLM Detection and Property Authorization checks, highlighting risks when public endpoints expose validation logic that can be probed.

Example of vulnerable code in Actix (Rust) that should be avoided:

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

type HmacSha256 = Hmac<Sha256>;

#[post("/verify")]
async fn verify_data(form: web::Form<VerifyForm>) -> HttpResponse {
    let secret = b"super-secret-key";
    let mut mac = HmacSha256::new_from_slice(secret).expect("HMAC can take key of any size");
    mac.update(form.payload.as_bytes());
    let expected = mac.finalize();
    let result = expected.into_bytes();

    // Vulnerable: non-constant-time comparison
    if form.signature[..] == result[..] {
        HttpResponse::Ok().body("OK")
    } else {
        HttpResponse::Unauthorized().body("Invalid signature")
    }
}

The above comparison is vulnerable because it may short-circuit on the first mismatching byte, creating a timing side channel. An attacker can send many modified signatures and measure response latency to infer correctness byte-by-byte, effectively conducting a Bleichenbacher-style adaptive attack against the HMAC-based integrity check in Actix.

Remediation centers on using constant-time comparison and strict validation order to remove any adaptive oracle. In Actix, this means replacing byte-wise equality checks with a cryptographic constant-time comparison that always processes all bytes and returns in fixed time. Additionally, ensure that signature verification does not expose distinct error paths for padding, length, or MAC failures; return a single generic error for any validation failure.

Use the subtle crate in Rust to perform constant-time equality checks. This eliminates timing differences an attacker could exploit in a Bleichenbacher-style probe. Below is a secure version of the previous Actix handler that mitigates the adaptive oracle risk.

Secure Actix code example with constant-time HMAC verification:

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

type HmacSha256 = Hmac<Sha256>;

#[post("/verify")]
async fn verify_data(form: web::Form<VerifyForm>) -> HttpResponse {
    let secret = b"super-secret-key";
    let mut mac = HmacSha256::new_from_slice(secret).expect("HMAC can take key of any size");
    mac.update(form.payload.as_bytes());
    let expected = mac.finalize();
    let result = expected.into_bytes();

    // Constant-time comparison to prevent timing side channels
    if form.signature.len() == result.len() && form.signature.ct_eq(&result).into() {
        HttpResponse::Ok().body("OK")
    } else {
        // Generic error to avoid oracle behavior
        HttpResponse::Unauthorized().body("Invalid request")
    }
}

Key remediation steps:

• Use subtle::ConstantTimeEq for comparing the provided signature with the computed HMAC, ensuring the comparison always takes the same amount of time regardless of where the first mismatch occurs.
• Check length equality before constant-time compare to avoid panics, but ensure the length check does not short-circuit the error path in a way that leaks information.
• Return a uniform error response for any validation failure, preventing an attacker from distinguishing between a bad signature, a bad payload format, or other internal issues.
• If you use JWTs or tokens with RSA signatures in Actix, apply the same constant-time principles to the final verification step and avoid exposing distinct error messages for padding versus signature mismatches.
• Rotate signing keys and use key derivation with salt where feasible to limit the impact of any potential leakage.

These changes close the oracle that a Bleichenbacher attack would exploit by ensuring that an attacker cannot learn incremental information about the signature through timing or error messages. middleBrick can validate that your Actix API enforces constant-time checks and does not expose variable error responses through its runtime checks and spec cross-referencing.