diff --git a/DEPRECATIONS.md b/DEPRECATIONS.md
index cd001e7162..b1593c224a 100644
--- a/DEPRECATIONS.md
+++ b/DEPRECATIONS.md
@@ -23,6 +23,7 @@ The following is a list of deprecations, according to the [Deprecation Policy](h
 | DEPPS17 | Remove config option `playgroundPath`                                                        | [#10110](https://github.com/parse-community/parse-server/issues/10110) | 9.5.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
 | DEPPS18 | Config option `requestComplexity` limits enabled by default                                  | [#10207](https://github.com/parse-community/parse-server/pull/10207)   | 9.6.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
 | DEPPS19 | Remove config option `enableProductPurchaseLegacyApi`                                        | [#10228](https://github.com/parse-community/parse-server/pull/10228)   | 9.6.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
+| DEPPS20 | Remove config option `allowExpiredAuthDataToken`                                             |                                                                        | 9.6.0 (2026)                    | 10.0.0 (2027)                   | deprecated            | -     |
 
 [i_deprecation]: ## "The version and date of the deprecation."
 [i_change]: ## "The version and date of the planned change."
diff --git a/spec/vulnerabilities.spec.js b/spec/vulnerabilities.spec.js
index 5b82e2bb9c..f2bd2c3bdc 100644
--- a/spec/vulnerabilities.spec.js
+++ b/spec/vulnerabilities.spec.js
@@ -3221,4 +3221,131 @@ describe('(GHSA-5hmj-jcgp-6hff) Protected fields leak via LiveQuery afterEvent t
       obj.save({ publicField: 'changed' }, { useMasterKey: true }),
     ]);
   });
+
+  describe('(GHSA-pfj7-wv7c-22pr) AuthData subset validation bypass with allowExpiredAuthDataToken', () => {
+    let validatorSpy;
+
+    const testAdapter = {
+      validateAppId: () => Promise.resolve(),
+      validateAuthData: () => Promise.resolve(),
+    };
+
+    beforeEach(async () => {
+      validatorSpy = spyOn(testAdapter, 'validateAuthData').and.resolveTo({});
+      await reconfigureServer({
+        auth: { testAdapter },
+        allowExpiredAuthDataToken: true,
+      });
+    });
+
+    it('validates authData on login when incoming data is a strict subset of stored data', async () => {
+      // Sign up a user with full authData (id + access_token)
+      const user = new Parse.User();
+      await user.save({
+        authData: { testAdapter: { id: 'user123', access_token: 'valid_token' } },
+      });
+      validatorSpy.calls.reset();
+
+      // Attempt to log in with only the id field (subset of stored data)
+      const res = await request({
+        method: 'POST',
+        url: 'http://localhost:8378/1/users',
+        headers: {
+          'Content-Type': 'application/json',
+          'X-Parse-Application-Id': 'test',
+          'X-Parse-REST-API-Key': 'rest',
+        },
+        body: JSON.stringify({
+          authData: { testAdapter: { id: 'user123' } },
+        }),
+      });
+      expect(res.data.objectId).toBe(user.id);
+      // The adapter MUST be called to validate the login attempt
+      expect(validatorSpy).toHaveBeenCalled();
+    });
+
+    it('prevents account takeover via partial authData when allowExpiredAuthDataToken is enabled', async () => {
+      // Sign up a user with full authData
+      const user = new Parse.User();
+      await user.save({
+        authData: { testAdapter: { id: 'victim123', access_token: 'secret_token' } },
+      });
+      validatorSpy.calls.reset();
+
+      // Simulate an attacker sending only the provider ID (no access_token)
+      // The adapter should reject this because the token is missing
+      validatorSpy.and.rejectWith(
+        new Parse.Error(Parse.Error.SCRIPT_FAILED, 'Invalid credentials')
+      );
+
+      const res = await request({
+        method: 'POST',
+        url: 'http://localhost:8378/1/users',
+        headers: {
+          'Content-Type': 'application/json',
+          'X-Parse-Application-Id': 'test',
+          'X-Parse-REST-API-Key': 'rest',
+        },
+        body: JSON.stringify({
+          authData: { testAdapter: { id: 'victim123' } },
+        }),
+      }).catch(e => e);
+
+      // Login must be rejected — adapter validation must not be skipped
+      expect(res.status).toBe(400);
+      expect(validatorSpy).toHaveBeenCalled();
+    });
+
+    it('validates authData on login even when authData is identical', async () => {
+      // Sign up with full authData
+      const user = new Parse.User();
+      await user.save({
+        authData: { testAdapter: { id: 'user456', access_token: 'expired_token' } },
+      });
+      validatorSpy.calls.reset();
+
+      // Log in with the exact same authData (all keys present, same values)
+      const res = await request({
+        method: 'POST',
+        url: 'http://localhost:8378/1/users',
+        headers: {
+          'Content-Type': 'application/json',
+          'X-Parse-Application-Id': 'test',
+          'X-Parse-REST-API-Key': 'rest',
+        },
+        body: JSON.stringify({
+          authData: { testAdapter: { id: 'user456', access_token: 'expired_token' } },
+        }),
+      });
+      expect(res.data.objectId).toBe(user.id);
+      // Auth providers are always validated on login regardless of allowExpiredAuthDataToken
+      expect(validatorSpy).toHaveBeenCalled();
+    });
+
+    it('skips validation on update when authData is a subset of stored data', async () => {
+      // Sign up with full authData
+      const user = new Parse.User();
+      await user.save({
+        authData: { testAdapter: { id: 'user789', access_token: 'valid_token' } },
+      });
+      validatorSpy.calls.reset();
+
+      // Update the user with a subset of authData (simulates afterFind stripping fields)
+      await request({
+        method: 'PUT',
+        url: `http://localhost:8378/1/users/${user.id}`,
+        headers: {
+          'Content-Type': 'application/json',
+          'X-Parse-Application-Id': 'test',
+          'X-Parse-REST-API-Key': 'rest',
+          'X-Parse-Session-Token': user.getSessionToken(),
+        },
+        body: JSON.stringify({
+          authData: { testAdapter: { id: 'user789' } },
+        }),
+      });
+      // On update with allowExpiredAuthDataToken: true, subset data skips validation
+      expect(validatorSpy).not.toHaveBeenCalled();
+    });
+  });
 });
diff --git a/src/Deprecator/Deprecations.js b/src/Deprecator/Deprecations.js
index 18baa00927..dff88c78bb 100644
--- a/src/Deprecator/Deprecations.js
+++ b/src/Deprecator/Deprecations.js
@@ -76,4 +76,9 @@ module.exports = [
     changeNewKey: '',
     solution: "The product purchase API is an undocumented, unmaintained legacy feature that may not function as expected and will be removed in a future major version. We strongly advise against using it. Set 'enableProductPurchaseLegacyApi' to 'false' to disable it, or remove the option to accept the future removal.",
   },
+  {
+    optionKey: 'allowExpiredAuthDataToken',
+    changeNewKey: '',
+    solution: "Auth providers are always validated on login regardless of this setting. Set 'allowExpiredAuthDataToken' to 'false' or remove the option to accept the future removal.",
+  },
 ];
diff --git a/src/Options/Definitions.js b/src/Options/Definitions.js
index af9279d712..26c50a5d3f 100644
--- a/src/Options/Definitions.js
+++ b/src/Options/Definitions.js
@@ -72,7 +72,7 @@ module.exports.ParseServerOptions = {
   },
   allowExpiredAuthDataToken: {
     env: 'PARSE_SERVER_ALLOW_EXPIRED_AUTH_DATA_TOKEN',
-    help: 'Allow a user to log in even if the 3rd party authentication token that was used to sign in to their account has expired. If this is set to `false`, then the token will be validated every time the user signs in to their account. This refers to the token that is stored in the `_User.authData` field. Defaults to `false`.',
+    help: 'Deprecated. This option will be removed in a future version. Auth providers are always validated on login. On update, if this is set to `true`, auth providers are only re-validated when the auth data has changed. If this is set to `false`, auth providers are re-validated on every update. Defaults to `false`.',
     action: parsers.booleanParser,
     default: false,
   },
diff --git a/src/Options/docs.js b/src/Options/docs.js
index 20d74fb770..082f9cba38 100644
--- a/src/Options/docs.js
+++ b/src/Options/docs.js
@@ -15,7 +15,7 @@
  * @property {AccountLockoutOptions} accountLockout The account lockout policy for failed login attempts.<br><br>Note: Setting a user's ACL to an empty object `{}` via master key is a separate mechanism that only prevents new logins; it does not invalidate existing session tokens. To immediately revoke a user's access, destroy their sessions via master key in addition to setting the ACL.
  * @property {Boolean} allowClientClassCreation Enable (or disable) client class creation, defaults to false
  * @property {Boolean} allowCustomObjectId Enable (or disable) custom objectId
- * @property {Boolean} allowExpiredAuthDataToken Allow a user to log in even if the 3rd party authentication token that was used to sign in to their account has expired. If this is set to `false`, then the token will be validated every time the user signs in to their account. This refers to the token that is stored in the `_User.authData` field. Defaults to `false`.
+ * @property {Boolean} allowExpiredAuthDataToken Deprecated. This option will be removed in a future version. Auth providers are always validated on login. On update, if this is set to `true`, auth providers are only re-validated when the auth data has changed. If this is set to `false`, auth providers are re-validated on every update. Defaults to `false`.
  * @property {String[]} allowHeaders Add headers to Access-Control-Allow-Headers
  * @property {String|String[]} allowOrigin Sets origins for Access-Control-Allow-Origin. This can be a string for a single origin or an array of strings for multiple origins.
  * @property {Adapter<AnalyticsAdapter>} analyticsAdapter Adapter module for the analytics
diff --git a/src/Options/index.js b/src/Options/index.js
index 459494cdab..8707d4c87a 100644
--- a/src/Options/index.js
+++ b/src/Options/index.js
@@ -379,7 +379,7 @@ export interface ParseServerOptions {
   /* Set to true if new users should be created without public read and write access.
   :DEFAULT: true */
   enforcePrivateUsers: ?boolean;
-  /* Allow a user to log in even if the 3rd party authentication token that was used to sign in to their account has expired. If this is set to `false`, then the token will be validated every time the user signs in to their account. This refers to the token that is stored in the `_User.authData` field. Defaults to `false`.
+  /* Deprecated. This option will be removed in a future version. Auth providers are always validated on login. On update, if this is set to `true`, auth providers are only re-validated when the auth data has changed. If this is set to `false`, auth providers are re-validated on every update. Defaults to `false`.
   :DEFAULT: false */
   allowExpiredAuthDataToken: ?boolean;
   /* An array of keys and values that are prohibited in database read and write requests to prevent potential security vulnerabilities. It is possible to specify only a key (`{"key":"..."}`), only a value (`{"value":"..."}`) or a key-value pair (`{"key":"...","value":"..."}`). The specification can use the following types: `boolean`, `numeric` or `string`, where `string` will be interpreted as a regex notation. Request data is deep-scanned for matching definitions to detect also any nested occurrences. Defaults are patterns that are likely to be used in malicious requests. Setting this option will override the default patterns.
diff --git a/src/RestWrite.js b/src/RestWrite.js
index e986a35c67..80b233ec59 100644
--- a/src/RestWrite.js
+++ b/src/RestWrite.js
@@ -639,9 +639,10 @@ RestWrite.prototype.handleAuthData = async function (authData) {
         return;
       }
 
-      // Force to validate all provided authData on login
-      // on update only validate mutated ones
-      if (hasMutatedAuthData || !this.config.allowExpiredAuthDataToken) {
+      // Always validate all provided authData on login to prevent authentication
+      // bypass via partial authData (e.g. sending only the provider ID without
+      // an access token); on update only validate mutated ones
+      if (isLogin || hasMutatedAuthData || !this.config.allowExpiredAuthDataToken) {
         const res = await Auth.handleAuthDataValidation(
           isLogin ? authData : mutatedAuthData,
           this,