Out Of Bounds Read in Chi with Basic Auth

An Out Of Bounds Read occurs when a service reads memory beyond the intended allocation, often due to missing or incorrect boundary checks. In the Chi web framework for Go, combining routing with Basic Auth can expose this vulnerability when authorization logic does not properly validate input lengths or slices before accessing them.

Chi uses middleware to handle authentication, including Basic Auth. If the handler or middleware reads request data—such as headers, query parameters, or body content—without validating the size or range of the data, it may read beyond the underlying buffer or slice. For example, a handler that copies a header value into a fixed-size byte array without length checking can read adjacent memory, potentially leaking sensitive information or causing a crash.

Consider a route defined with Chi that enforces Basic Auth via middleware. The middleware decodes the Authorization header, which contains a base64-encoded username:password string. If the decoded credentials are stored in a fixed-size structure and the code does not verify the length of the username or password, an attacker can supply an overly long credential string. This can lead to an Out Of Bounds Read as the code accesses memory beyond the allocated storage for the credential fields.

When using the BasicAuth middleware provided by Chi, the framework does not inherently limit the length of the credentials it processes. If downstream code assumes a maximum length and copies data into a fixed buffer based on that assumption without validation, the boundary check is bypassed. This is especially risky when the handler directly references indices or slices derived from user-controlled input, such as splitting the Authorization header value by the colon character and storing the parts in pre-allocated arrays.

In an API security scan performed by middleBrick, an endpoint using Basic Auth in Chi may reveal an Out Of Bounds Read finding when the unauthenticated scan sends long or malformed Authorization headers. The scanner tests how the service handles inputs that exceed expected sizes and observes whether memory is accessed outside intended bounds. Since the scan tests the unauthenticated attack surface, it can trigger this behavior without requiring credentials, highlighting a flaw in input validation rather than authentication itself.

Real-world attack patterns related to this issue align with OWASP API Top 10 categories and can map to findings around improper input validation. Unlike some frameworks that provide built-in protections, Chi requires developers to explicitly validate the length and format of data used in routing and middleware logic. middleBrick’s checks include input validation and authentication testing, which can surface these misconfigurations when Basic Auth is combined with unchecked data handling.

Remediation focuses on validating the length and format of credentials before they are used in memory operations. In Chi, you should enforce strict size limits on the username and password extracted from the Basic Auth header and avoid fixed-size buffers. Use slices with dynamic allocation or explicit length checks instead of assuming bounded input.

Below is an example of insecure code that decodes Basic Auth and copies values into fixed-size arrays, which can lead to an Out Of Bounds Read:

// Insecure example: fixed-size buffers without validation
var user [32]byte
var pass [32]byte

basicAuth := req.Header.Get("Authorization")
if basicAuth != "" {
    parts := strings.Split(strings.TrimPrefix(basicAuth, "Basic "), ":")
    if len(parts) == 2 {
        copy(user[:], parts[0]) // No length check
        copy(pass[:], parts[1]) // No length check
    }
}

An attacker can provide a username or password longer than 32 bytes, causing the copy to write beyond the array bounds. This may result in an Out Of Bounds Read, depending on how the memory is accessed later.

A secure approach uses length validation and dynamically sized slices:

// Secure example: validate length and use slices
const maxCredentialLength = 256

basicAuth := req.Header.Get("Authorization")
var username, password string

if basicAuth != "" {
    parts := strings.Split(strings.TrimPrefix(basicAuth, "Basic "), ":")
    if len(parts) == 2 {
        if len(parts[0]) > maxCredentialLength || len(parts[1]) > maxCredentialLength {
            http.Error(w, "invalid credentials", http.StatusBadRequest)
            return
        }
        username = parts[0]
        password = parts[1]
    }
}

This remediation ensures that each credential is bounded by a reasonable maximum length before being used. By avoiding fixed-size arrays and performing explicit checks, you prevent memory reads beyond intended allocations. The same principle applies when storing credentials in structs or passing them to other functions: always validate input lengths and use types that grow as needed.

middleBrick can help identify these issues during a scan by testing endpoints with oversized Basic Auth headers and analyzing how the service handles them. With the Pro plan, you can enable continuous monitoring so that future changes to authentication handling are flagged before they reach production. The GitHub Action can also fail builds if an endpoint returns signs of improper input validation when Basic Auth is in use.