diff --git a/app/auth/plugins/twilio_signature/incoming.go b/app/auth/plugins/twilio_signature/incoming.go
index 0a8132ee..4855c19e 100644
--- a/app/auth/plugins/twilio_signature/incoming.go
+++ b/app/auth/plugins/twilio_signature/incoming.go
@@ -6,8 +6,11 @@ import (
 	"crypto/sha1"
 	"encoding/base64"
 	"fmt"
+	"mime"
 	"net/http"
+	"net/url"
 	"sort"
+	"strings"
 
 	"github.com/winhowes/AuthTranslator/app/auth"
 	"github.com/winhowes/AuthTranslator/app/secrets"
@@ -41,24 +44,58 @@ func (t *TwilioSignatureAuth) ParseParams(m map[string]interface{}) (interface{}
 	return p, nil
 }
 
-func canonicalString(r *http.Request) string {
+func canonicalString(r *http.Request, body []byte) string {
 	// Use the full URL including query string as seen by the proxy
 	base := r.URL.String()
-	// Include POST form parameters sorted by key
-	if err := r.ParseForm(); err == nil {
-		keys := make([]string, 0, len(r.PostForm))
-		for k := range r.PostForm {
-			keys = append(keys, k)
+
+	form, ok := parseCanonicalForm(r, body)
+	if !ok || len(form) == 0 {
+		return base
+	}
+
+	keys := make([]string, 0, len(form))
+	totalLen := len(base)
+	for k, vals := range form {
+		keys = append(keys, k)
+		for _, v := range vals {
+			totalLen += len(k) + len(v)
 		}
-		sort.Strings(keys)
-		for _, k := range keys {
-			vals := r.PostForm[k]
-			for _, v := range vals {
-				base += k + v
-			}
+	}
+	sort.Strings(keys)
+
+	var builder strings.Builder
+	builder.Grow(totalLen)
+	builder.WriteString(base)
+	for _, k := range keys {
+		vals := form[k]
+		for _, v := range vals {
+			builder.WriteString(k)
+			builder.WriteString(v)
 		}
 	}
-	return base
+	return builder.String()
+}
+
+func parseCanonicalForm(r *http.Request, body []byte) (url.Values, bool) {
+	if r.PostForm != nil {
+		return r.PostForm, true
+	}
+
+	switch r.Method {
+	case http.MethodPost, http.MethodPut, http.MethodPatch:
+	default:
+		return nil, false
+	}
+
+	mediaType, _, err := mime.ParseMediaType(r.Header.Get("Content-Type"))
+	if err != nil || mediaType != "application/x-www-form-urlencoded" {
+		return nil, false
+	}
+	form, err := url.ParseQuery(string(body))
+	if err != nil {
+		return nil, false
+	}
+	return form, true
 }
 
 func (t *TwilioSignatureAuth) Authenticate(ctx context.Context, r *http.Request, params interface{}) bool {
@@ -70,11 +107,13 @@ func (t *TwilioSignatureAuth) Authenticate(ctx context.Context, r *http.Request,
 	if sig == "" {
 		return false
 	}
-	// Ensure body is read and restored for ParseForm
-	if _, err := authplugins.GetBody(r); err != nil {
+	// Use the shared body cache so signature validation does not consume the
+	// request body before the proxy forwards it upstream.
+	body, err := authplugins.GetBody(r)
+	if err != nil {
 		return false
 	}
-	base := canonicalString(r)
+	base := canonicalString(r, body)
 	for _, ref := range cfg.Secrets {
 		secret, err := secrets.LoadSecret(ctx, ref)
 		if err != nil {
diff --git a/app/auth/plugins/twilio_signature/twilio_signature_test.go b/app/auth/plugins/twilio_signature/twilio_signature_test.go
index 82e6d86f..8f0d9997 100644
--- a/app/auth/plugins/twilio_signature/twilio_signature_test.go
+++ b/app/auth/plugins/twilio_signature/twilio_signature_test.go
@@ -5,14 +5,15 @@ import (
 	"crypto/hmac"
 	"crypto/sha1"
 	"encoding/base64"
-	authplugins "github.com/winhowes/AuthTranslator/app/auth"
 	"io"
 	"net/http"
 	"net/url"
 	"sort"
+	"strconv"
 	"strings"
 	"testing"
 
+	authplugins "github.com/winhowes/AuthTranslator/app/auth"
 	_ "github.com/winhowes/AuthTranslator/app/secrets/plugins"
 )
 
@@ -58,6 +59,102 @@ func TestTwilioSignatureAuth(t *testing.T) {
 	}
 }
 
+func TestCanonicalStringUsesCachedBody(t *testing.T) {
+	body := []byte("B=two&A=one&A=uno")
+	r := &http.Request{
+		Method: http.MethodPost,
+		URL:    &url.URL{Path: "/callback"},
+		Header: http.Header{"Content-Type": []string{"application/x-www-form-urlencoded; charset=utf-8"}},
+		Body:   io.NopCloser(strings.NewReader("unread")),
+	}
+
+	got := canonicalString(r, body)
+	if got != "/callbackAoneAunoBtwo" {
+		t.Fatalf("unexpected canonical string %q", got)
+	}
+
+	remaining, err := io.ReadAll(r.Body)
+	if err != nil {
+		t.Fatalf("unexpected body read error: %v", err)
+	}
+	if string(remaining) != "unread" {
+		t.Fatalf("canonicalString consumed request body: %q", string(remaining))
+	}
+}
+
+func TestCanonicalStringNoFormReturnsURL(t *testing.T) {
+	r := &http.Request{
+		Method: http.MethodPost,
+		URL:    &url.URL{Path: "/callback", RawQuery: "q=1"},
+		Header: http.Header{"Content-Type": []string{"application/x-www-form-urlencoded"}},
+	}
+
+	if got := canonicalString(r, nil); got != "/callback?q=1" {
+		t.Fatalf("unexpected canonical string %q", got)
+	}
+}
+
+func TestParseCanonicalFormBranches(t *testing.T) {
+	preParsed := &http.Request{PostForm: url.Values{"A": []string{"one"}}}
+	form, ok := parseCanonicalForm(preParsed, nil)
+	if !ok || form.Get("A") != "one" {
+		t.Fatalf("expected pre-parsed form, got %v %t", form, ok)
+	}
+
+	unsupportedMethod := &http.Request{
+		Method: http.MethodGet,
+		Header: http.Header{"Content-Type": []string{"application/x-www-form-urlencoded"}},
+	}
+	if form, ok := parseCanonicalForm(unsupportedMethod, []byte("A=one")); ok || form != nil {
+		t.Fatalf("expected unsupported method to skip form, got %v %t", form, ok)
+	}
+
+	unsupportedMedia := &http.Request{
+		Method: http.MethodPost,
+		Header: http.Header{"Content-Type": []string{"text/plain"}},
+	}
+	if form, ok := parseCanonicalForm(unsupportedMedia, []byte("A=one")); ok || form != nil {
+		t.Fatalf("expected unsupported media type to skip form, got %v %t", form, ok)
+	}
+
+	malformedForm := &http.Request{
+		Method: http.MethodPost,
+		Header: http.Header{"Content-Type": []string{"application/x-www-form-urlencoded"}},
+	}
+	if form, ok := parseCanonicalForm(malformedForm, []byte("A=%zz")); ok || form != nil {
+		t.Fatalf("expected malformed form to fail, got %v %t", form, ok)
+	}
+}
+
+func TestTwilioSignatureAuthenticatePreservesBody(t *testing.T) {
+	form := url.Values{"Body": []string{"hello"}, "From": []string{"+15551234567"}}
+	urlStr := "/path"
+	sig := sign(urlStr, form, "tok")
+	body := form.Encode()
+	r := &http.Request{Method: "POST", URL: &url.URL{Path: urlStr}, Header: http.Header{
+		"X-Twilio-Signature": []string{sig},
+		"Content-Type":       []string{"application/x-www-form-urlencoded"},
+	}, Body: io.NopCloser(strings.NewReader(body))}
+
+	p := TwilioSignatureAuth{}
+	t.Setenv("TOK", "tok")
+	cfg, err := p.ParseParams(map[string]interface{}{"secrets": []string{"env:TOK"}})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !p.Authenticate(context.Background(), r, cfg) {
+		t.Fatal("expected authentication to succeed")
+	}
+
+	remaining, err := io.ReadAll(r.Body)
+	if err != nil {
+		t.Fatalf("unexpected body read error: %v", err)
+	}
+	if string(remaining) != body {
+		t.Fatalf("expected body to remain %q, got %q", body, string(remaining))
+	}
+}
+
 func TestTwilioSignatureAuthFail(t *testing.T) {
 	form := url.Values{"Foo": []string{"bar"}}
 	urlStr := "/cb"
@@ -216,3 +313,22 @@ func TestTwilioSignatureSecretLoadError(t *testing.T) {
 		t.Fatal("expected auth failure when secret load fails")
 	}
 }
+
+func BenchmarkCanonicalStringLargeForm(b *testing.B) {
+	form := make(url.Values, 2000)
+	for i := 0; i < 2000; i++ {
+		form.Set("Field"+strconv.Itoa(i), strings.Repeat("x", 24))
+	}
+	body := []byte(form.Encode())
+	r := &http.Request{
+		Method: http.MethodPost,
+		URL:    &url.URL{Path: "/callback", RawQuery: "q=1"},
+		Header: http.Header{"Content-Type": []string{"application/x-www-form-urlencoded"}},
+	}
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		_ = canonicalString(r, body)
+	}
+}