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Merged
ericchiang merged 1 commit into from
Aug 2, 2025
+22 −36
Merged

oidc: verify signature before parsing token #465

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2 changes: 1 addition & 1 deletion .github/workflows/test.yaml
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ jobs:
runs-on: ubuntu-latest
strategy:
matrix:
go: ['1.22', '1.23']
go: ['1.23', '1.24']
name: Linux Go ${{ matrix.go }}
steps:
- uses: actions/checkout@v4
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56 changes: 21 additions & 35 deletions verify.go
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Original file line number Diff line number Diff line change
@@ -1,11 +1,9 @@
package oidc

import (
"bytes"
"context"
"encoding/base64"
"encoding/json"
"errors"
"fmt"
"io/ioutil"
"net/http"
Expand Down Expand Up @@ -211,12 +209,29 @@ func (v *IDTokenVerifier) Verify(ctx context.Context, rawIDToken string) (*IDTok
return nil, fmt.Errorf("oidc: malformed jwt: %v", err)
}

// Throw out tokens with invalid claims before trying to verify the token. This lets
// us do cheap checks before possibly re-syncing keys.
payload, err := parseJWT(rawIDToken)
switch len(jws.Signatures) {
case 0:
return nil, fmt.Errorf("oidc: id token not signed")
case 1:
default:
return nil, fmt.Errorf("oidc: multiple signatures on id token not supported")
}

sig := jws.Signatures[0]
supportedSigAlgs := v.config.SupportedSigningAlgs
if len(supportedSigAlgs) == 0 {
supportedSigAlgs = []string{RS256}
}

if !contains(supportedSigAlgs, sig.Header.Algorithm) {
return nil, fmt.Errorf("oidc: id token signed with unsupported algorithm, expected %q got %q", supportedSigAlgs, sig.Header.Algorithm)
}

payload, err := v.keySet.VerifySignature(ctx, rawIDToken)
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@liggitt liggitt Jan 13, 2026 •
edited
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how did moving this first help the DOS? this still calls:

VerifySignature → jose.ParseSigned → parseSignedCompact

which still calls parts := strings.Split(input, ".")

If this was trying to catch spurious tokens with lots of . characters, something like the count check in https://go.dev/cl/652155 would seem more effective

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@ericchiang ericchiang Jan 13, 2026

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Hey! The v3 version of this package depends on a go-jose version that doesn't do that.

go-oidc/go.mod

Line 8 in 35b8e03

github.com/go-jose/go-jose/v4 v4.1.3

https://github.com/go-jose/go-jose/blob/v4.1.3/jws.go#L367

For v2, maybe we should do our own check before this to throw out invalid ID tokens early

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@liggitt liggitt Jan 13, 2026

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For v2, maybe we should do our own check before this to throw out invalid ID tokens early

Yeah, that's exactly what I'd recommend

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@liggitt liggitt Jan 13, 2026

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(also, sorry for the random 6-month-later drive-by ... I just got here while reviewing kubernetes/kubernetes#136162 (comment))

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@ericchiang ericchiang Jan 13, 2026

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Lol, no worries. Appreciate you actually checking my work.

Sent the change and tagged a release:

if err != nil {
return nil, fmt.Errorf("oidc: malformed jwt: %v", err)
return nil, fmt.Errorf("failed to verify signature: %v", err)
}

var token idToken
if err := json.Unmarshal(payload, &token); err != nil {
return nil, fmt.Errorf("oidc: failed to unmarshal claims: %v", err)
Expand Down Expand Up @@ -296,36 +311,7 @@ func (v *IDTokenVerifier) Verify(ctx context.Context, rawIDToken string) (*IDTok
}
}

switch len(jws.Signatures) {
case 0:
return nil, fmt.Errorf("oidc: id token not signed")
case 1:
default:
return nil, fmt.Errorf("oidc: multiple signatures on id token not supported")
}

sig := jws.Signatures[0]
supportedSigAlgs := v.config.SupportedSigningAlgs
if len(supportedSigAlgs) == 0 {
supportedSigAlgs = []string{RS256}
}

if !contains(supportedSigAlgs, sig.Header.Algorithm) {
return nil, fmt.Errorf("oidc: id token signed with unsupported algorithm, expected %q got %q", supportedSigAlgs, sig.Header.Algorithm)
}

t.sigAlgorithm = sig.Header.Algorithm

gotPayload, err := v.keySet.VerifySignature(ctx, rawIDToken)
if err != nil {
return nil, fmt.Errorf("failed to verify signature: %v", err)
}

// Ensure that the payload returned by the square actually matches the payload parsed earlier.
if !bytes.Equal(gotPayload, payload) {
return nil, errors.New("oidc: internal error, payload parsed did not match previous payload")
}

return t, nil
}

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