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jws: improve fix for CVE-2025-22868 #774

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34 changes: 25 additions & 9 deletions jws/jws.go
Original file line number Diff line number Diff line change
Expand Up @@ -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, ".")
if len(s) < 2 {
_, claims, _, ok := parseToken(payload)
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
}
Expand Down Expand Up @@ -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")
}

parts := strings.SplitN(token, ".", 3)
signedContent := parts[0] + "." + parts[1]
signatureString, err := base64.RawURLEncoding.DecodeString(parts[2])
signatureString, err := base64.RawURLEncoding.DecodeString(sig)
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 = "."
57 changes: 54 additions & 3 deletions jws/jws_test.go
Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,8 @@ package jws
import (
"crypto/rand"
"crypto/rsa"
"net/http"
"strings"
"testing"
)

Expand Down Expand Up @@ -39,8 +41,57 @@ func TestSignAndVerify(t *testing.T) {
}

func TestVerifyFailsOnMalformedClaim(t *testing.T) {
err := Verify("abc.def", nil)
if err == nil {
t.Error("got no errors; want improperly formed JWT not to be verified")
cases := []struct {
desc string
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)
}
}