Use Form encoded body instead of JSON for OAuth requests (#3919)

* Parse JSON encoded Authorization Request Parameters

* Use `application/x-www-form-urlencoded` content instead of JSON for OAuth requests

Fixes: #3723

* Pre-process number too

* improved type checking

* Update packages/oauth/oauth-client/src/oauth-server-agent.ts

Co-authored-by: devin ivy <devinivy@gmail.com>

---------

Co-authored-by: devin ivy <devinivy@gmail.com>

.changeset/seven-pans-press.md

@@ -0,0 +1,5 @@
+---
+"@atproto/oauth-types": patch
+---
+
+Parse JSON encoded Authorization Request Parameters

.changeset/ten-tools-exercise.md

@@ -0,0 +1,5 @@
+---
+"@atproto/oauth-client": patch
+---
+
+Use `application/x-www-form-urlencoded` content instead of JSON for OAuth requests

packages/oauth/oauth-client/src/oauth-server-agent.ts

@@ -207,10 +207,17 @@ export class OAuthServerAgent {
 
     const auth = await this.buildClientAuth(endpoint)
 
+    // https://datatracker.ietf.org/doc/html/draft-ietf-oauth-v2-1-13#section-3.2.2
+    // https://datatracker.ietf.org/doc/html/rfc7009#section-2.1
+    // https://datatracker.ietf.org/doc/html/rfc7662#section-2.1
+    // https://datatracker.ietf.org/doc/html/rfc9126#section-2
     const { response, json } = await this.dpopFetch(url, {
       method: 'POST',
-      headers: { ...auth.headers, 'Content-Type': 'application/json' },
+      headers: {
-      body: JSON.stringify({ ...payload, ...auth.payload }),
+        ...auth.headers,
+        'Content-Type': 'application/x-www-form-urlencoded',
+      },
+      body: wwwFormUrlEncode({ ...payload, ...auth.payload }),
     }).then(fetchJsonProcessor())
 
     if (response.ok) {
@@ -294,3 +301,37 @@ export class OAuthServerAgent {
     throw new Error(`Unsupported ${endpoint} authentication method`)
   }
 }
+
+function wwwFormUrlEncode(payload: Record<string, undefined | unknown>) {
+  return new URLSearchParams(
+    Object.entries(payload)
+      .filter(entryHasDefinedValue)
+      .map(stringifyEntryValue),
+  ).toString()
+}
+
+function entryHasDefinedValue(
+  entry: [string, unknown],
+): entry is [string, null | NonNullable<unknown>] {
+  return entry[1] !== undefined
+}
+
+function stringifyEntryValue(entry: [string, unknown]): [string, string] {
+  const name = entry[0]
+  const value = entry[1]
+
+  switch (typeof value) {
+    case 'string':
+      return [name, value]
+    case 'number':
+    case 'boolean':
+      return [name, String(value)]
+    default: {
+      const enc = JSON.stringify(value)
+      if (enc === undefined) {
+        throw new Error(`Unsupported value type for ${name}: ${String(value)}`)
+      }
+      return [name, enc]
+    }
+  }
+}

packages/oauth/oauth-types/src/oauth-authorization-request-parameters.ts

@@ -10,8 +10,12 @@ import { oauthScopeSchema } from './oauth-scope.js'
 import { oidcClaimsParameterSchema } from './oidc-claims-parameter.js'
 import { oidcClaimsPropertiesSchema } from './oidc-claims-properties.js'
 import { oidcEntityTypeSchema } from './oidc-entity-type.js'
+import { jsonObjectPreprocess, numberPreprocess } from './util.js'
 
 /**
+ * @note non string parameters will be converted from their string
+ * representation since oauth request parameters are typically sent as URL
+ * encoded form data or URL encoded query string.
  * @see {@link https://openid.net/specs/openid-connect-core-1_0.html#AuthRequest | OIDC}
  */
 export const oauthAuthorizationRequestParametersSchema = z.object({
@@ -47,14 +51,17 @@ export const oauthAuthorizationRequestParametersSchema = z.object({
   // PAPE [OpenID.PAPE] max_auth_age request parameter.) When max_age is used,
   // the ID Token returned MUST include an auth_time Claim Value. Note that
   // max_age=0 is equivalent to prompt=login.
-  max_age: z.number().int().min(0).optional(),
+  max_age: z.preprocess(numberPreprocess, z.number().int().min(0)).optional(),
 
   claims: z
-    .record(
+    .preprocess(
-      oidcEntityTypeSchema,
+      jsonObjectPreprocess,
       z.record(
-        oidcClaimsParameterSchema,
+        oidcEntityTypeSchema,
-        z.union([z.literal(null), oidcClaimsPropertiesSchema]),
+        z.record(
+          oidcClaimsParameterSchema,
+          z.union([z.literal(null), oidcClaimsPropertiesSchema]),
+        ),
       ),
     )
     .optional(),
@@ -85,7 +92,9 @@ export const oauthAuthorizationRequestParametersSchema = z.object({
   prompt: z.enum(['none', 'login', 'consent', 'select_account']).optional(),
 
   // https://datatracker.ietf.org/doc/html/rfc9396
-  authorization_details: oauthAuthorizationDetailsSchema.optional(),
+  authorization_details: z
+    .preprocess(jsonObjectPreprocess, oauthAuthorizationDetailsSchema)
+    .optional(),
 })
 
 /**

packages/oauth/oauth-types/src/util.ts

@@ -66,3 +66,23 @@ export function extractUrlPath(url) {
 
   return url.substring(pathStart, pathEnd)
 }
+
+export const jsonObjectPreprocess = (val: unknown) => {
+  if (typeof val === 'string' && val.startsWith('{') && val.endsWith('}')) {
+    try {
+      return JSON.parse(val)
+    } catch {
+      return val
+    }
+  }
+
+  return val
+}
+
+export const numberPreprocess = (val: unknown): unknown => {
+  if (typeof val === 'string') {
+    const number = Number(val)
+    if (!Number.isNaN(number)) return number
+  }
+  return val
+}