From 233d30b55f9b1a168219638bec63a3ab4b79f5dc Mon Sep 17 00:00:00 2001
From: Andrew Barnes <bortstheboat@gmail.com>
Date: Sun, 12 Jul 2026 23:40:21 -0400
Subject: [PATCH] fix: apply signing algorithms to extra JWT issuers

Signed-off-by: Andrew Barnes <bortstheboat@gmail.com>
---
 CHANGELOG.md                   |  2 ++
 pkg/validation/options.go      | 20 +++++++++++++-------
 pkg/validation/options_test.go | 18 ++++++++++++++++++
 3 files changed, 33 insertions(+), 7 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 788e82c2..7eb748d4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -8,6 +8,8 @@
 
 ## Changes since v7.15.3
 
+- [#3472](https://github.com/oauth2-proxy/oauth2-proxy/pull/3472) fix: apply configured signing algorithms to extra JWT issuers (#3471) (@Bortlesboat)
+
 # V7.15.3
 
 ## Release Highlights
diff --git a/pkg/validation/options.go b/pkg/validation/options.go
index 13ce2e0b..624a8c8d 100644
--- a/pkg/validation/options.go
+++ b/pkg/validation/options.go
@@ -61,6 +61,7 @@ func Validate(o *options.Options) error {
 				verifier, err := newVerifierFromJwtIssuer(
 					o.Providers[0].OIDCConfig.AudienceClaims,
 					o.Providers[0].OIDCConfig.ExtraAudiences,
+					o.Providers[0].OIDCConfig.EnabledSigningAlgs,
 					jwtIssuer,
 				)
 				if err != nil {
@@ -143,13 +144,8 @@ func parseJwtIssuers(issuers []string, msgs []string) ([]jwtIssuer, []string) {
 
 // newVerifierFromJwtIssuer takes in issuer information in jwtIssuer info and returns
 // a verifier for that issuer.
-func newVerifierFromJwtIssuer(audienceClaims []string, extraAudiences []string, jwtIssuer jwtIssuer) (internaloidc.IDTokenVerifier, error) {
-	pvOpts := internaloidc.ProviderVerifierOptions{
-		AudienceClaims: audienceClaims,
-		ClientID:       jwtIssuer.audience,
-		ExtraAudiences: extraAudiences,
-		IssuerURL:      jwtIssuer.issuerURI,
-	}
+func newVerifierFromJwtIssuer(audienceClaims []string, extraAudiences []string, supportedSigningAlgs []string, jwtIssuer jwtIssuer) (internaloidc.IDTokenVerifier, error) {
+	pvOpts := providerVerifierOptionsFromJwtIssuer(audienceClaims, extraAudiences, supportedSigningAlgs, jwtIssuer)
 
 	pv, err := internaloidc.NewProviderVerifier(context.TODO(), pvOpts)
 	if err != nil {
@@ -166,6 +162,16 @@ func newVerifierFromJwtIssuer(audienceClaims []string, extraAudiences []string,
 	return pv.Verifier(), nil
 }
 
+func providerVerifierOptionsFromJwtIssuer(audienceClaims []string, extraAudiences []string, supportedSigningAlgs []string, jwtIssuer jwtIssuer) internaloidc.ProviderVerifierOptions {
+	return internaloidc.ProviderVerifierOptions{
+		AudienceClaims:       audienceClaims,
+		ClientID:             jwtIssuer.audience,
+		ExtraAudiences:       extraAudiences,
+		IssuerURL:            jwtIssuer.issuerURI,
+		SupportedSigningAlgs: supportedSigningAlgs,
+	}
+}
+
 // jwtIssuer hold parsed JWT issuer info that's used to construct a verifier.
 type jwtIssuer struct {
 	issuerURI string
diff --git a/pkg/validation/options_test.go b/pkg/validation/options_test.go
index 5ea748c1..957c4b91 100644
--- a/pkg/validation/options_test.go
+++ b/pkg/validation/options_test.go
@@ -107,6 +107,24 @@ func TestInitializedOptions(t *testing.T) {
 	assert.Equal(t, nil, Validate(o))
 }
 
+func TestProviderVerifierOptionsFromJwtIssuer(t *testing.T) {
+	audienceClaims := []string{"aud", "client_id"}
+	extraAudiences := []string{"extra-audience"}
+	supportedSigningAlgs := []string{"RS256", "ES256"}
+	issuer := jwtIssuer{
+		issuerURI: "https://issuer.example.com",
+		audience:  "client-id",
+	}
+
+	got := providerVerifierOptionsFromJwtIssuer(audienceClaims, extraAudiences, supportedSigningAlgs, issuer)
+
+	assert.Equal(t, audienceClaims, got.AudienceClaims)
+	assert.Equal(t, issuer.audience, got.ClientID)
+	assert.Equal(t, extraAudiences, got.ExtraAudiences)
+	assert.Equal(t, issuer.issuerURI, got.IssuerURL)
+	assert.Equal(t, supportedSigningAlgs, got.SupportedSigningAlgs)
+}
+
 // Note that it's not worth testing nonparseable URLs, since url.Parse()
 // seems to parse damn near anything.
 func TestRedirectURL(t *testing.T) {