Out Of Bounds Read in Chi with Mutual Tls

An Out Of Bounds Read in Chi with Mutual Tls occurs when a service reads beyond the allocated buffer while validating or processing mTLS handshakes and client certificates. Because mutual TLS requires both the server and the client to present and verify certificates, the code path that parses, verifies, and indexes certificate fields (e.g., subject fields, SANs, serial numbers) can read past the end of a byte slice or C string if lengths are not rigorously checked.

In Chi, this often surfaces when developers use C-style string operations or unchecked slice bounds while iterating over certificate bytes. For example, reading a certificate field by assuming a fixed offset may succeed in benign cases but trigger an out-of-bounds read when a certificate is malformed or unusually large. The vulnerability is exposed during the TLS handshake, where Chi must parse raw certificates; if the parser trusts input length rather than validating it, an attacker providing a crafted certificate can cause the program to read sensitive memory or crash the service.

Consider a handler that inspects a client certificate’s Common Name without validating length:

import (
    "crypto/tls"
    "fmt"
    "net/http"
)

func insecureCommonName(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tlsState := r.TLS
        if tlsState == nil || len(tlsState.PeerCertificates) == 0 {
            http.Error(w, "missing client cert", http.StatusUnauthorized)
            return
        }
        cert := tlsState.PeerCertificates[0]
        raw := cert.RawSubjectCommonName // hypothetical field used for illustration
        // Risk: raw may be empty or shorter than expected; indexing without bounds check
        fmt.Fprintf(w, "CN byte at offset 128: %d\n", raw[128])
        next.ServeHTTP(w, r)
    })
}

If raw is shorter than 128 bytes, this triggers an Out Of Bounds Read. In a mutual TLS flow, the client certificate is supplied by the peer, so an attacker can send a certificate that causes the read to go out of bounds. Because Chi is often used to build high-performance APIs, such reads can leak stack or heap contents, leading to information disclosure or instability.

The scanner checks such patterns by correlating TLS configuration and certificate-handling code paths. Findings include insecure indexing of certificate fields, missing length checks on parsed headers, and assumptions about buffer sizes. These map to broader issues in input validation and memory safety within the context of authenticated protocols.

Remediation focuses on validating lengths before indexing, using safe abstractions, and ensuring certificate parsing does not rely on fixed offsets. Below are concrete, safe examples for Chi services using mutual TLS.

1. Validate slice bounds before reading certificate fields

Always check the length of raw certificate data before accessing specific offsets:

import (
    "crypto/x509"
    "net/http"
)

func safeCommonName(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tlsState := r.TLS
        if tlsState == nil || len(tlsState.PeerCertificates) == 0 {
            http.Error(w, "missing client cert", http.StatusUnauthorized)
            return
        }
        cert := tlsState.PeerCertificates[0]
        raw := cert.RawSubjectCommonName
        if len(raw) < 129 {
            http.Error(w, "invalid certificate field length", http.StatusBadRequest)
            return
        }
        // Safe: bounds checked
        fmt.Fprintf(w, "CN byte at offset 128: %d\n", raw[128])
        next.ServeHTTP(w, r)
    })
}

2. Use x509 certificate structures instead of raw offsets

Parsing structured fields via x509 avoids manual offset arithmetic entirely:

import (
    "crypto/tls"
    "fmt"
    "net/http"
)

func handleWithParsedFields(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        tlsState := r.TLS
        if tlsState == nil || len(tlsState.PeerCertificates) == 0 {
            http.Error(w, "missing client cert", http.StatusUnauthorized)
            return
        }
        cert := tlsState.PeerCertificates[0]
        for _, email := range cert.EmailAddresses {
            fmt.Fprintf(w, "Email: %s\n", email)
        }
        for _, uri := range cert.URIs {
            fmt.Fprintf(w, "URI: %s\n", uri)
        }
        next.ServeHTTP(w, r)
    })
}

3. Enforce certificate constraints in TLS config

Configure Chi’s TLS settings to require certificates and set verification options that reduce the surface for malformed inputs:

import (
    "crypto/tls"
    "net/http"
)

func newTLSConfig() *tls.Config {
    return &tls.Config{
        ClientAuth:   tls.RequireAndVerifyClientCert,
        MinVersion:   tls.VersionTLS12,
        PreferServerCipherSuites: true,
        // Provide a custom verification function to enforce policies
        VerifyPeerCertificate: func(rawCerts [][]byte, verifiedChains [][]*x509.Certificate) error {
            for _, raw := range rawCerts {
                if len(raw) == 0 {
                    return fmt.Errorf("empty certificate")
                }
                // Additional policy checks can go here
            }
            return nil
        },
    }
}

func Setup() http.Handler {
    r := chi.NewRouter()
    r.Use(middleware.RealIP)
    r.Get("/secure", func(w http.ResponseWriter, r *http.Request) {
        w.Write([]byte("mTLS enforced and safe"))
    })
    return r
}

These practices reduce the likelihood of Out Of Bounds Reads by ensuring that certificate data is accessed within valid ranges and by relying on structured parsing. The scanner can detect remaining risks by correlating TLS settings with index operations on certificate bytes.