Allow marking custom headers as sensitive on redirects

src/client.toit

@@ -108,6 +108,7 @@ class Client:
   root-certificates_/List ::= []
   connection_/Connection? := null
   security-store_/SecurityStore
+  sensitive-headers_/List
 
   /**
   Constructs a new client instance over the given $network.
@@ -145,16 +146,27 @@ class Client:
     expose request headers and bodies over plaintext. Explicit HTTP or WS
     requests and requests that do not follow redirects are unaffected.
 
+  The $sensitive-headers list names additional headers to remove when a
+    redirect changes origin (scheme, host, or port). Header names are matched
+    without regard to ASCII case. `Authorization`, `Proxy-Authorization`, and
+    `Cookie` are always removed on such redirects. Other headers are forwarded;
+    callers must list custom credentials such as `X-Api-Key` here, or disable
+    redirect following. This applies to HTTP and WebSocket redirects.
+    The list is copied, and caller-owned request headers are not modified.
+    Removed headers are not restored if a redirect returns to the original origin.
+
   See the `certificate_roots` package for common roots:
     https://pkg.toit.io/package/github.com%2Ftoitware%2Ftoit-cert-roots
   */
   constructor network/tcp.Interface
       --root-certificates/List=[]
       --security-store/SecurityStore=SecurityStoreInMemory
-      --allow-insecure-redirects/bool=false:
+      --allow-insecure-redirects/bool=false
+      --sensitive-headers/List=[]:
     interface_ = network
     security-store_ = security-store
     allow-insecure-redirects_ = allow-insecure-redirects
+    sensitive-headers_ = sensitive-headers.copy
     root-certificates_ = root-certificates
     add-finalizer this:: this.finalize_
 
@@ -171,15 +183,19 @@ class Client:
     different server name from the one used to establish the connection.
 
   The $allow-insecure-redirects option has the same meaning as in $constructor.
+
+  The $sensitive-headers list has the same meaning as in $constructor.
   */
   constructor.tls .interface_
       --root-certificates/List=[]
       --server-name/string?=null
       --certificate/tls.Certificate?=null
       --security-store/SecurityStore=SecurityStoreInMemory
-      --allow-insecure-redirects/bool=false:
+      --allow-insecure-redirects/bool=false
+      --sensitive-headers/List=[]:
     security-store_ = security-store
     allow-insecure-redirects_ = allow-insecure-redirects
+    sensitive-headers_ = sensitive-headers.copy
     use-tls-by-default_ = true
     root-certificates_ = root-certificates
     server-name_ = server-name
@@ -467,6 +483,7 @@ class Client:
     result.remove "Authorization"
     result.remove "Proxy-Authorization"
     result.remove "Cookie"
+    sensitive-headers_.do: | name/string | result.remove name
     return result
 
   /**

tests/client-sensitive-headers-test.toit

@@ -0,0 +1,110 @@
+// Copyright (C) 2026 Toit contributors.
+// Use of this source code is governed by a Zero-Clause BSD license that can
+// be found in the tests/TESTS_LICENSE file.
+
+import expect show *
+import http
+
+import .memory-socket
+
+OK ::= "HTTP/1.1 200 OK\r\nContent-Length: 0\r\n\r\n"
+
+main:
+  [false, true].do: | tls-client/bool |
+    ["GET", "POST", "WebSocket"].do: | method/string |
+      [303, 307].do: | status/int |
+        ["/next", "//other.example/next", "//original.example:8080/next"].do: | location/string |
+          test-redirect method status location --tls-client=tls-client
+      test-return-to-origin method --tls-client=tls-client
+  test-scheme-change
+
+new-client network/ScriptedNetwork --tls-client/bool -> http.Client:
+  sensitive := ["x-aPi-kEy", "X-Session", "Authorization"]
+  client := tls-client
+      ? (http.Client.tls network --sensitive-headers=sensitive)
+      : (http.Client network --sensitive-headers=sensitive)
+  // Mutating the caller's configuration must not remove the protection.
+  sensitive.clear
+  return client
+
+new-headers -> http.Headers:
+  headers := http.Headers.from-map {
+    "X-Api-Key": ["synthetic-first", "synthetic-second"],
+    "x-session": "synthetic-session",
+    "Authorization": "Bearer synthetic-token",
+    "Cookie": "synthetic-cookie",
+    "Proxy-Authorization": "synthetic-proxy",
+    "X-Request": "keep",
+  }
+  return headers
+
+send client/http.Client method/string headers/http.Headers:
+  if method == "WebSocket":
+    // A final 200 intentionally stops the handshake after headers are sent.
+    expect-throw "WebSocket upgrade failed with 200 OK":
+      client.web-socket --uri="ws://original.example/" --headers=headers
+  else if method == "GET":
+    client.get --uri="http://original.example/" --headers=headers
+  else:
+    client.post "body".to-byte-array --uri="http://original.example/" --headers=headers
+
+redirect status/int location/string -> string:
+  return "HTTP/1.1 $status Redirect\r\nLocation: $location\r\nContent-Length: 0\r\n\r\n"
+
+check-headers sent/string --sensitive/bool:
+  expect (sent.contains "X-Request: keep\r\n")
+  ["X-Api-Key", "X-Session", "Authorization", "Cookie", "Proxy-Authorization"].do: | name/string |
+    expect-equals sensitive (sent.contains "$name: ")
+  if sensitive:
+    expect (sent.contains "X-Api-Key: synthetic-first\r\n")
+    expect (sent.contains "X-Api-Key: synthetic-second\r\n")
+
+test-redirect method/string status/int location/string --tls-client/bool:
+  same-origin := location == "/next"
+  first := MemorySocket [((redirect status location) + (same-origin ? OK : "")).to-byte-array]
+  second := MemorySocket [OK.to-byte-array]
+  client := new-client (ScriptedNetwork [first, second]) --tls-client=tls-client
+  headers := new-headers
+  original := headers.stringify
+  try:
+    send client method headers
+    sent := (same-origin ? first : second).out.buffer.to-string
+    start := sent.index-of --last " /next HTTP/1.1"
+    expect start >= 0
+    check-headers sent[start..] --sensitive=same-origin
+    check-headers first.out.buffer.to-string --sensitive
+    expect-equals original headers.stringify
+  finally:
+    client.close
+
+test-return-to-origin method/string --tls-client/bool:
+  // Keep credentials on a relative hop, strip them on departure, and never
+  // restore them when a later hop returns to the initial origin.
+  first := MemorySocket [((redirect 307 "/next") + (redirect 307 "//other.example/away")).to-byte-array]
+  second := MemorySocket [(redirect 307 "//original.example/back").to-byte-array]
+  third := MemorySocket [OK.to-byte-array]
+  client := new-client (ScriptedNetwork [first, second, third]) --tls-client=tls-client
+  headers := new-headers
+  original := headers.stringify
+  try:
+    send client method headers
+    first-sent := first.out.buffer.to-string
+    check-headers first-sent[(first-sent.index-of --last " /next HTTP/1.1")..] --sensitive
+    check-headers second.out.buffer.to-string --no-sensitive
+    check-headers third.out.buffer.to-string --no-sensitive
+    expect-equals original headers.stringify
+  finally:
+    client.close
+
+test-scheme-change:
+  client := new-client (ScriptedNetwork []) --tls-client
+  try:
+    previous := http.ParsedUri_.parse_ "http://original.example:8080/"
+    next := http.ParsedUri_.parse_ "https://original.example:8080/"
+    headers := new-headers
+    filtered := client.redirect-headers_ headers previous next
+    check-headers filtered.stringify --no-sensitive
+    check-headers headers.stringify --sensitive
+    expect-null (client.redirect-headers_ null previous next)
+  finally:
+    client.close