Heap Overflow in Actix with Basic Auth

A heap overflow in an Actix web service that uses Basic Authentication can occur when untrusted input from the Authorization header is copied into fixed-size buffers on the heap without proper length checks. In C++ Actix applications, this typically manifests when a developer parses the base64-encoded credentials and stores the decoded username or password into a std::string or character array that has a constrained capacity. If the attacker sends an exceptionally long credential value, the allocator may satisfy the request with a heap block, but the subsequent copy routine does not enforce the destination buffer’s bounds, writing past the allocated region. This corrupts adjacent heap metadata, can redirect execution flow, and may lead to arbitrary code execution under the same process privileges.

When Basic Auth is handled in Actix via middleware or extractor logic, the framework decodes the header and passes the credentials to application code. If the developer assumes a maximum length and uses functions like memcpy or std::copy without verifying the source size, the mismatch between expected and actual length becomes exploitable. For example, an attacker can send an Authorization header with thousands of characters, causing the service to allocate a small buffer based on a flawed length estimate and then overflow it during copy. The overflow may overwrite saved return addresses or function pointers on the heap, altering control flow without triggering immediate crashes.

Because this vulnerability is exercised during unauthenticated scanning, middleBrick’s black-box checks can detect unsafe handling of Authorization input by probing endpoints that accept Basic Auth and observing whether abnormally long credentials cause service instability or unexpected behavior. The 12 parallel security checks, including Input Validation and Unsafe Consumption, increase the likelihood of identifying such heap corruption patterns. Note that middleBrick detects and reports these risks and provides remediation guidance but does not fix, patch, or block the endpoint.

To mitigate heap overflow risks in Actix with Basic Authentication, always treat credentials as untrusted input and enforce strict length and format checks before any heap allocation. Prefer using Actix’s built-in extractors with size-constrained strings and avoid low-level buffer manipulation. Validate the decoded username and password lengths against realistic upper bounds and reject requests that exceed those limits before any copying occurs.

Below are two concrete Actix examples. The first demonstrates a vulnerable approach where a raw base64 string is decoded into a fixed-size buffer. The second shows a safe implementation using std::string with explicit length checks and Actix’s extractor pattern.

Vulnerable Actix Basic Auth handling (do not use)

#include <actix-web>
#include <vector>
#include <cstring>

// WARNING: unsafe fixed-size buffer on the heap via std::vector<char> with unchecked copy
ACTIX_WEB_ROUTE(login, web::post()) {
    auto auth = req.headers().find("authorization");
    if (auth != req.headers().end()) {
        std::string header = auth->to_str().value_or("");
        if (header.substr(0, 6) == "Basic ") {
            std::string token = header.substr(6);
            std::vector<char> buffer(256); // fixed heap buffer
            std::vector<unsigned char> decoded = base64_decode(token); // hypothetical
            // VULNERABLE: no check that decoded size fits buffer
            std::memcpy(buffer.data(), decoded.data(), decoded.size());
            // process credentials
        }
    }
    return HttpResponse::Ok();
}

Secure Actix Basic Auth handling with length validation

#include <actix-web>
#include <string>n>

// Safe extractor that enforces maximum lengths and uses std::string
struct BasicCredentials {
    std::string username;
    std::string password;
};

bool validate_basic_auth(const std::string& header, BasicCredentials& out) {
    if (header.substr(0, 6) != "Basic ") return false;
    std::string token = header.substr(6);
    std::vector<unsigned char> decoded = base64_decode(token); // hypothetical
    std::string decoded_str(decoded.begin(), decoded.end());
    size_t sep = decoded_str.find(':');
    if (sep == std::string::npos) return false;
    std::string user = decoded_str.substr(0, sep);
    std::string pass = decoded_str.substr(sep + 1);
    const size_t MAX_LEN = 256;
    if (user.size() > MAX_LEN || pass.size() > MAX_LEN) return false;
    out.username = std::move(user);
    out.password = std::move(pass);
    return true;
}

ACTIX_WEB_ROUTE(login_safe, web::post()) {
    auto auth = req.headers().find("authorization");
    if (auth == req.headers().end()) {
        return HttpResponse::Unauthorized().finish();
    }
    BasicCredentials creds;
    if (!validate_basic_auth(auth->to_str().value_or(""), creds)) {
        return HttpResponse::BadRequest().finish();
    }
    // Use creds.username and creds.password safely; no fixed buffers
    return HttpResponse::Ok().body("Authenticated as " + creds.username);
}

In the secure example, base64 decoding produces a std::string, and we explicitly verify that both username and password lengths remain under a defined maximum before moving them into the output structure. This avoids any fixed heap buffer and ensures that internal copies remain bounded. Combine this with transport-layer encryption and consider using higher-level authentication mechanisms where feasible to reduce the attack surface associated with handling raw credentials.