Out Of Bounds Write in Chi with Mutual Tls

An Out Of Bounds Write occurs when a program writes data outside the intended memory region, which can corrupt stack or heap structures and lead to arbitrary code execution. In Chi, this often arises from unchecked slice or buffer manipulation when handling request payloads. When Mutual TLS (mTLS) is enabled, the server performs client certificate verification before routing requests to application handlers. If the certificate-parsing logic or the per-request context constructed after mTLS validation does not properly bound writes to fixed-size buffers or slices, an attacker can supply a carefully crafted request (e.g., with large headers or maliciously encoded certificate extensions) that triggers an out-of-bounds write during downstream request processing.

Specifically in Chi, the combination of mTLS and Out Of Bounds Write is exposed when application code uses unchecked indices while copying data from the request context or from mTLS-verified connection state into fixed-length arrays or slices. For example, if a handler iterates over a list of values extracted from the client certificate or from headers and writes each value into a pre-allocated buffer without validating length, an oversized value can write beyond the buffer boundary. This corrupts adjacent memory, which in a compiled language like Go may overwrite function pointers or return addresses. Because mTLS ensures the request is authenticated at the transport layer, developers might mistakenly assume that downstream code is safe from injection, leading to insufficient bounds checks on data extracted from the verified context.

Chi does not provide automatic runtime bounds checking; it relies on the developer to validate lengths. An out-of-bounds write can be triggered via path parameters, headers, or body fields that are processed after mTLS authentication. Consider a handler that reads a numeric ID from the URL, uses it to index into a slice of configuration objects obtained after mTLS verification, and writes a computed value back into a response buffer. If the ID is not validated against the slice length, an attacker can supply an index that writes beyond the slice, modifying adjacent memory. The mTLS layer confirms the client’s identity but does not protect against logic errors in bounds management, making it essential to apply strict length checks on all data derived from the request context.

Real-world patterns include using copy without verifying source length or using raw indices in loops. The OWASP API Security Top 10 categorizes this as a vulnerability leading to crashes or potential code execution. Tools like middleBrick can detect such issues by analyzing OpenAPI specs and runtime behavior, flagging missing length validations on data extracted after mTLS authentication. Developers should treat mTLS as a transport assurance and still apply strict input validation and bounds checks on all request data to prevent Out Of Bounds Write in Chi.

Remediation focuses on ensuring that all data extracted from the request context after mTLS authentication is handled with strict bounds. In Chi, use explicit length checks before indexing into slices or writing into buffers. Prefer using built-in copy functions with verified lengths and avoid manual index manipulation where possible.

Example 1: Safe indexing with bounds check

import (
    "net/http"
    "github.com/go-chi/chi/v5"
)

func safeHandler(configs []Config) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // ID extracted from URL parameter after mTLS authentication
        id := chi.URLParam(r, "id")
        var index int
        if _, err := fmt.Sscanf(id, "%d", &index); err != nil {
            http.Error(w, "invalid id", http.StatusBadRequest)
            return
        }
        // Bounds check before using index
        if index < 0 || index >= len(configs) {
            http.Error(w, "out of range", http.StatusBadRequest)
            return
        }
        cfg := configs[index]
        // Use cfg safely
        w.Write([]byte(cfg.Name))
    }
}

Example 2: Using copy with length validation

import (
    "io"
    "net/http"
    "github.com/go-chi/chi/v5"
)

func copyHandler(buf []byte) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        // Assume src is derived from request context after mTLS verification
        src := []byte(r.Header.Get("X-Custom-Data"))
        // Ensure we do not write beyond the destination buffer
        n := copy(buf, src)
        if n < len(src) {
            // src was larger than buf; handle truncation safely
            http.Error(w, "data too large", http.StatusRequestEntityTooLarge)
            return
        }
        w.Write(buf[:n])
    }
}

Example 3: Struct field size limits

import (
    "fmt"
    "net/http"
    "github.com/go-chi/chi/v5"
)

type UserMeta struct {
    Key   [32]byte // Fixed-size buffer to prevent overflow
    Value [128]byte
}

func metaHandler(r *http.Request) UserMeta {
    var meta UserMeta
    key := r.Header.Get("X-Key")
    // Ensure key fits into fixed-size array
    if len(key) > 31 {
        key = key[:31]
    }
    copy(meta.Key[:], key) // copy respects length of destination
    val := r.Header.Get("X-Value")
    if len(val) > 127 {
        val = val[:127]
    }
    copy(meta.Value[:], val)
    return meta
}

General practices

• Always validate lengths of slices and buffers before indexing or copying.
• Use fixed-size arrays for small, predictable data instead of slices when feasible.
• Leverage Go’s built-in copy which prevents writing beyond the destination length, but ensure the destination is sized appropriately.
• Treat mTLS as transport security; continue to validate and bound all data used in application logic.

By combining these patterns, developers mitigate Out Of Bounds Write risks in Chi even when mTLS is enforced, ensuring that authenticated requests do not bypass critical memory safety checks.