jws: improve fix for CVE-2025-22868

The fix for CVE-2025-22868 relies on strings.Count, which isn't ideal
because it precludes failing fast when the token contains an unexpected
number of periods. Moreover, Verify still allocates more than necessary.

Eschew strings.Count in favor of strings.Cut. Some benchmark results:

goos: darwin
goarch: amd64
pkg: golang.org/x/oauth2/jws
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
                              │      old       │                 new                 │
                              │     sec/op     │   sec/op     vs base                │
Verify/full_of_periods-8        24862.50n ± 1%   57.87n ± 0%  -99.77% (p=0.000 n=20)
Verify/two_trailing_periods-8      3.485m ± 1%   3.445m ± 1%   -1.13% (p=0.003 n=20)
geomean                            294.3µ        14.12µ       -95.20%

                              │     old      │                  new                   │
                              │     B/op     │     B/op      vs base                  │
Verify/full_of_periods-8          16.00 ± 0%     16.00 ± 0%        ~ (p=1.000 n=20) ¹
Verify/two_trailing_periods-8   2.001Mi ± 0%   1.001Mi ± 0%  -49.98% (p=0.000 n=20)
geomean                         5.658Ki        4.002Ki       -29.27%
¹ all samples are equal

                              │     old     │                 new                  │
                              │  allocs/op  │ allocs/op   vs base                  │
Verify/full_of_periods-8         1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=20) ¹
Verify/two_trailing_periods-8   12.000 ± 0%   9.000 ± 0%  -25.00% (p=0.000 n=20)
geomean                          3.464        3.000       -13.40%
¹ all samples are equal

Also, remove all remaining calls to strings.Split.

Updates golang/go#71490

Change-Id: Icac3c7a81562161ab6533d892ba19247d6d5b943
GitHub-Last-Rev: 3a82900f747798f5f36065126385880277c0fce7
GitHub-Pull-Request: golang/oauth2#774
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/655455
Commit-Queue: Neal Patel <nealpatel@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Neal Patel <nealpatel@google.com>

jws/jws.go

@@ -116,12 +116,12 @@ func (h *Header) encode() (string, error) {
 // Decode decodes a claim set from a JWS payload.
 func Decode(payload string) (*ClaimSet, error) {
 	// decode returned id token to get expiry
-	s := strings.Split(payload, ".")
+	_, claims, _, ok := parseToken(payload)
-	if len(s) < 2 {
+	if !ok {
 		// TODO(jbd): Provide more context about the error.
 		return nil, errors.New("jws: invalid token received")
 	}
-	decoded, err := base64.RawURLEncoding.DecodeString(s[1])
+	decoded, err := base64.RawURLEncoding.DecodeString(claims)
 	if err != nil {
 		return nil, err
 	}
@@ -165,18 +165,34 @@ func Encode(header *Header, c *ClaimSet, key *rsa.PrivateKey) (string, error) {
 // Verify tests whether the provided JWT token's signature was produced by the private key
 // associated with the supplied public key.
 func Verify(token string, key *rsa.PublicKey) error {
-	if strings.Count(token, ".") != 2 {
+	header, claims, sig, ok := parseToken(token)
+	if !ok {
 		return errors.New("jws: invalid token received, token must have 3 parts")
 	}
-
+	signatureString, err := base64.RawURLEncoding.DecodeString(sig)
-	parts := strings.SplitN(token, ".", 3)
-	signedContent := parts[0] + "." + parts[1]
-	signatureString, err := base64.RawURLEncoding.DecodeString(parts[2])
 	if err != nil {
 		return err
 	}
 
 	h := sha256.New()
-	h.Write([]byte(signedContent))
+	h.Write([]byte(header + tokenDelim + claims))
 	return rsa.VerifyPKCS1v15(key, crypto.SHA256, h.Sum(nil), signatureString)
 }
+
+func parseToken(s string) (header, claims, sig string, ok bool) {
+	header, s, ok = strings.Cut(s, tokenDelim)
+	if !ok { // no period found
+		return "", "", "", false
+	}
+	claims, s, ok = strings.Cut(s, tokenDelim)
+	if !ok { // only one period found
+		return "", "", "", false
+	}
+	sig, _, ok = strings.Cut(s, tokenDelim)
+	if ok { // three periods found
+		return "", "", "", false
+	}
+	return header, claims, sig, true
+}
+
+const tokenDelim = "."

jws/jws_test.go

@@ -7,6 +7,8 @@ package jws
 import (
 	"crypto/rand"
 	"crypto/rsa"
+	"net/http"
+	"strings"
 	"testing"
 )
 
@@ -39,8 +41,57 @@ func TestSignAndVerify(t *testing.T) {
 }
 
 func TestVerifyFailsOnMalformedClaim(t *testing.T) {
-	err := Verify("abc.def", nil)
+	cases := []struct {
-	if err == nil {
+		desc  string
-		t.Error("got no errors; want improperly formed JWT not to be verified")
+		token string
+	}{
+		{
+			desc:  "no periods",
+			token: "aa",
+		}, {
+			desc:  "only one period",
+			token: "a.a",
+		}, {
+			desc:  "more than two periods",
+			token: "a.a.a.a",
+		},
+	}
+	for _, tc := range cases {
+		f := func(t *testing.T) {
+			err := Verify(tc.token, nil)
+			if err == nil {
+				t.Error("got no errors; want improperly formed JWT not to be verified")
+			}
+		}
+		t.Run(tc.desc, f)
+	}
+}
+
+func BenchmarkVerify(b *testing.B) {
+	cases := []struct {
+		desc  string
+		token string
+	}{
+		{
+			desc:  "full of periods",
+			token: strings.Repeat(".", http.DefaultMaxHeaderBytes),
+		}, {
+			desc:  "two trailing periods",
+			token: strings.Repeat("a", http.DefaultMaxHeaderBytes-2) + "..",
+		},
+	}
+	privateKey, err := rsa.GenerateKey(rand.Reader, 2048)
+	if err != nil {
+		b.Fatal(err)
+	}
+	for _, bc := range cases {
+		f := func(b *testing.B) {
+			b.ReportAllocs()
+			b.ResetTimer()
+			for range b.N {
+				Verify(bc.token, &privateKey.PublicKey)
+			}
+		}
+		b.Run(bc.desc, f)
 	}
 }