Integer Overflow in Chi with Basic Auth

Integer overflow in Chi, when paired with HTTP Basic Authentication, can lead to authentication bypass or privilege escalation when values used in authorization logic wrap around. Chi is a lightweight HTTP router for Go that relies on route patterns and middleware to control access. Basic Auth is commonly applied via middleware that decodes the Authorization header, parses the credentials, and validates them before allowing the request to proceed.

Consider a scenario where a Chi application uses an integer-based user ID or counter to enforce per-user limits or to construct authorization decisions. If that integer is derived from user-controlled input, such as a request header, query parameter, or a value parsed from the Basic Auth credentials (for example, a numeric tenant or account identifier), an attacker may supply a value like 4294967295 (max uint32) and cause an overflow when the value is incremented, cast, or used in arithmetic. This can result in a smaller-than-expected integer, which may incorrectly satisfy a bounds check or comparison, effectively granting access to another user’s resources (BOLA/IDOR) or escalating privileges.

During a middleBrick scan, the Authentication and BOLA/IDOR checks will flag cases where Basic Auth is accepted without strict type and range validation. The scanner also inspects OpenAPI specs for type definitions and cross-references them with runtime behavior to detect mismatches. For instance, if the spec defines an account_id as integer but the implementation uses uint32 in a calculation that increments it, the overflow risk is high. Input validation checks will highlight missing upper-bound enforcement, and the findings will include remediation guidance to use int64 or big integers and to validate before arithmetic is performed.

An example vulnerable pattern in Chi might decode Basic Auth and extract a numeric realm or tenant ID from the user part or a custom header, then add it to a counter without checking for overflow:

// Vulnerable: potential integer overflow when parsing and adding parsed value
userPart := strings.Split(basicAuth, "@")[0]
n, err := strconv.ParseUint(userPart, 10, 32)
if err != nil {
    http.Error(w, "invalid credentials", http.StatusUnauthorized)
    return
}
// If n is near 2^32-1, increment can overflow in further logic
counter := uint32(0)
newVal := counter + uint32(n) // overflow possible at runtime
if newVal < counter {
    // logic may incorrectly allow access
}

middleBrick will highlight that the unchecked addition and the cast from parsed input to fixed-width integer can produce unexpected values. The scanner’s LLM/AI Security checks do not apply here, but the Inventory Management and Unsafe Consumption checks may note insecure handling of parsed credentials. The output will include prioritized findings with severity and remediation steps, such as validating input ranges, using larger integer types, and avoiding arithmetic on user-controlled values without sanitization.

To mitigate integer overflow risks in Chi when using Basic Auth, ensure that credential-derived numeric values are validated, bounded, and handled with types that avoid overflow. Always parse user input defensively and perform checks before using values in arithmetic or comparisons. Below are concrete code examples showing a vulnerable approach and a remediated version that uses safer parsing and validation.

First, a vulnerable Chi route with Basic Auth and unsafe integer use:

//go:generate go run github.com/go-chi/chi/v5@latest
package main

import (
    "net/http"
    "strconv"
    "strings"
)

func basicAuthMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, pass, ok := r.BasicAuth()
        if !ok || user == "" || pass == "" {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        // Vulnerable: parsing part of user as integer without validation
        parts := strings.Split(user, "-")
        if len(parts) != 2 {
            http.Error(w, "Bad request", http.StatusBadRequest)
            return
        }
        id, err := strconv.ParseUint(parts[1], 10, 32)
        if err != nil {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        // Dangerous use of id in further logic, risk of overflow
        _ = uint32(id) + 1000
        next.ServeHTTP(w, r)
    })
}

func mainHandler(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("OK"))
}

func main() {
    r := chi.NewRouter()
    r.Use(basicAuthMiddleware)
    r.Get("/", mainHandler)
    http.ListenAndServe(":8080", r)
}

Remediated version with proper validation and safe arithmetic:

package main

import (
    "errors"
    "math"
    "net/http"
    "strconv"
    "strings"
)

func safeParseUint64(s string, max uint64) (uint64, error) {
    v, err := strconv.ParseUint(s, 10, 64)
    if err != nil {
        return 0, err
    }
    if v > max {
        return 0, errors.New("value exceeds maximum")
    }
    return v, nil
}

func basicAuthMiddlewareSafe(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, pass, ok := r.BasicAuth()
        if !ok || user == "" || pass == "" {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        parts := strings.Split(user, "-")
        if len(parts) != 2 {
            http.Error(w, "Bad request", http.StatusBadRequest)
            return
        }
        // Use a bounded parse and safe arithmetic
        id, err := safeParseUint64(parts[1], math.MaxUint32)
        if err != nil {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        // Safe: id is guaranteed <= math.MaxUint32, and addition is checked
        const limit = 1000
        if id > math.MaxUint64 - uint64(limit) {
            http.Error(w, "Forbidden", http.StatusForbidden)
            return
        }
        _ = id + uint64(limit)
        next.ServeHTTP(w, r)
    })
}

func mainHandler(w http.ResponseWriter, r *http.Request) {
    w.Write([]byte("OK"))
}

func main() {
    r := chi.NewRouter()
    r.Use(basicAuthMiddlewareSafe)
    r.Get("/", mainHandler)
    http.ListenAndServe(":8080", r)
}

These examples show how to avoid overflow by bounding input, using larger integer types, and checking before arithmetic. middleBrick’s findings will map these issues to OWASP API Top 10 and compliance frameworks, and the remediation guidance will help you implement safer Basic Auth handling in Chi.