From 5993067505cac4c8e80192787ccd1f4cba05d994 Mon Sep 17 00:00:00 2001
From: Jan Larwig <jan@larwig.com>
Date: Sat, 8 Nov 2025 12:42:45 +0100
Subject: [PATCH] Merge commit from fork

Signed-off-by: Jan Larwig <jan@larwig.com>
---
 CHANGELOG.md                   |  1 +
 pkg/middleware/headers.go      | 37 ++++++++++++++++++++++++++++++----
 pkg/middleware/headers_test.go | 21 +++++++++++++++++++
 3 files changed, 55 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index dbb0974c..20a8f67f 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@
 - [#3244](https://github.com/oauth2-proxy/oauth2-proxy/pull/3244) chore(deps): upgrade to latest go1.25.3 (@tuunit)
 - [#3238](https://github.com/oauth2-proxy/oauth2-proxy/pull/3238) chore: Replace pkg/clock with narrowly targeted stub clocks (@dsymonds)
 - [#3237](https://github.com/oauth2-proxy/oauth2-proxy/pull/3237) - feat: add option to use organization id for preferred username in Google Provider (@pixeldrew)
+- [GHSA-vjrc-mh2v-45x6](https://github.com/oauth2-proxy/oauth2-proxy/security/advisories/GHSA-vjrc-mh2v-45x6) fix: request header smuggling by stripping all normalized header variants (@tuunit)
 
 # V7.12.0
 
diff --git a/pkg/middleware/headers.go b/pkg/middleware/headers.go
index 8d2f8e3e..f833848f 100644
--- a/pkg/middleware/headers.go
+++ b/pkg/middleware/headers.go
@@ -25,10 +25,10 @@ func NewRequestHeaderInjector(headers []options.Header) (alice.Constructor, erro
 }
 
 func newStripHeaders(headers []options.Header) alice.Constructor {
-	headersToStrip := []string{}
+	headersToStrip := []options.Header{}
 	for _, header := range headers {
 		if !header.PreserveRequestValue {
-			headersToStrip = append(headersToStrip, header.Name)
+			headersToStrip = append(headersToStrip, header)
 		}
 	}
 
@@ -50,10 +50,10 @@ func flattenHeaders(headers http.Header) {
 	}
 }
 
-func stripHeaders(headers []string, next http.Handler) http.Handler {
+func stripHeaders(headers []options.Header, next http.Handler) http.Handler {
 	return http.HandlerFunc(func(rw http.ResponseWriter, req *http.Request) {
 		for _, header := range headers {
-			req.Header.Del(header)
+			stripNormalizedHeader(req, header)
 		}
 		next.ServeHTTP(rw, req)
 	})
@@ -113,3 +113,32 @@ func injectResponseHeaders(injector header.Injector, next http.Handler) http.Han
 		next.ServeHTTP(rw, req)
 	})
 }
+
+// normalizeHeaderName normalizes the header name by lowercasing it
+// and replacing underscores with hyphens.
+func normalizeHeaderName(headerName string) string {
+	headerName = strings.ToLower(headerName)
+	headerName = strings.ReplaceAll(headerName, "_", "-")
+	return headerName
+}
+
+// stripNormalizedHeader removes any headers from the request that match
+// the normalized version of the provided header's name.
+func stripNormalizedHeader(req *http.Request, header options.Header) {
+	normalizedName := normalizeHeaderName(header.Name)
+
+	toBeDeleted := []string{}
+	for h := range req.Header {
+		if normalizeHeaderName(h) == normalizedName {
+			// necessary to avoid modifying the map while iterating
+			toBeDeleted = append(toBeDeleted, h)
+		}
+	}
+
+	for _, h := range toBeDeleted {
+		// necessary because req.Header.Del accesses the map via
+		// the header's canonicalized name. We need to delete by
+		// the original name.
+		delete(req.Header, h)
+	}
+}
diff --git a/pkg/middleware/headers_test.go b/pkg/middleware/headers_test.go
index 06440eea..14937955 100644
--- a/pkg/middleware/headers_test.go
+++ b/pkg/middleware/headers_test.go
@@ -205,6 +205,27 @@ var _ = Describe("Headers Suite", func() {
 			expectedHeaders: nil,
 			expectedErr:     "error building request header injector: error building request injector: error building injector for header \"X-Auth-Request-Authorization\": error loading basicAuthPassword: secret source is invalid: exactly one entry required, specify either value, fromEnv or fromFile",
 		}),
+		Entry("strips normalized variants before injecting (no preservation)", headersTableInput{
+			headers: []options.Header{
+				{
+					Name: "X-Auth-Request-User",
+					Values: []options.HeaderValue{
+						{
+							ClaimSource: &options.ClaimSource{Claim: "user"},
+						},
+					},
+				},
+			},
+			initialHeaders: http.Header{
+				"X-Auth-Request-User": []string{"old"},
+				"X-Auth_Request_User": []string{"evil"},
+			},
+			session: &sessionsapi.SessionState{User: "user-123"},
+			expectedHeaders: http.Header{
+				"X-Auth-Request-User": []string{"user-123"},
+			},
+			expectedErr: "",
+		}),
 	)
 
 	DescribeTable("the response header injector",