diff --git a/pods/authProviders/src/__tests__/cookieDomain.test.ts b/pods/authProviders/src/__tests__/cookieDomain.test.ts
new file mode 100644
index 00000000000..4ab3567def6
--- /dev/null
+++ b/pods/authProviders/src/__tests__/cookieDomain.test.ts
@@ -0,0 +1,55 @@
+//
+// Copyright © 2024 Hardcore Engineering, Inc.
+//
+// Licensed under the Eclipse Public License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License. You may
+// obtain a copy of the License at https://www.eclipse.org/legal/epl-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+import { isValidCookieDomain } from '../cookieDomain'
+
+describe('isValidCookieDomain', () => {
+  it('accepts a leading-dot parent domain', () => {
+    expect(isValidCookieDomain('.example.com')).toBe(true)
+  })
+
+  it('accepts a bare parent domain', () => {
+    expect(isValidCookieDomain('example.com')).toBe(true)
+  })
+
+  it('accepts nested subdomains', () => {
+    expect(isValidCookieDomain('.app.example.com')).toBe(true)
+    expect(isValidCookieDomain('dev.example.co.uk')).toBe(true)
+  })
+
+  it('accepts hyphenated labels', () => {
+    expect(isValidCookieDomain('.my-app.example.com')).toBe(true)
+  })
+
+  it('rejects a single-label TLD like "com"', () => {
+    expect(isValidCookieDomain('com')).toBe(false)
+    expect(isValidCookieDomain('.com')).toBe(false)
+  })
+
+  it('rejects values with whitespace', () => {
+    expect(isValidCookieDomain(' example.com')).toBe(false)
+    expect(isValidCookieDomain('example.com ')).toBe(false)
+    expect(isValidCookieDomain('exa mple.com')).toBe(false)
+  })
+
+  it('rejects protocol or port', () => {
+    expect(isValidCookieDomain('https://example.com')).toBe(false)
+    expect(isValidCookieDomain('example.com:8080')).toBe(false)
+  })
+
+  it('rejects empty and trailing-dot values', () => {
+    expect(isValidCookieDomain('')).toBe(false)
+    expect(isValidCookieDomain('example.')).toBe(false)
+  })
+})
diff --git a/pods/authProviders/src/cookieDomain.ts b/pods/authProviders/src/cookieDomain.ts
new file mode 100644
index 00000000000..2a2ad203667
--- /dev/null
+++ b/pods/authProviders/src/cookieDomain.ts
@@ -0,0 +1,28 @@
+//
+// Copyright © 2024 Hardcore Engineering, Inc.
+//
+// Licensed under the Eclipse Public License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License. You may
+// obtain a copy of the License at https://www.eclipse.org/legal/epl-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+
+/**
+ * Validates a `SESSION_COOKIE_DOMAIN` value before it is used to widen the
+ * session cookie scope across subdomains.
+ *
+ * Accepts only plausible parent-domain values: an optional leading dot
+ * followed by at least two dot-separated labels ending in a 2+ character TLD
+ * (e.g. `.example.com`, `example.com`). Rejects whitespace, protocol, port
+ * and single-label values like `com` — the latter would span the cookie
+ * across a whole TLD and is a common misconfiguration footgun.
+ */
+export function isValidCookieDomain (domain: string): boolean {
+  return /^\.?([a-z0-9-]+\.)+[a-z]{2,}$/i.test(domain)
+}
diff --git a/pods/authProviders/src/index.ts b/pods/authProviders/src/index.ts
index 40d7a83c7ba..b4ecf17dca8 100644
--- a/pods/authProviders/src/index.ts
+++ b/pods/authProviders/src/index.ts
@@ -2,6 +2,7 @@ import Koa from 'koa'
 import passport from 'koa-passport'
 import Router from 'koa-router'
 import session from 'koa-session'
+import { isValidCookieDomain } from './cookieDomain'
 import { registerGithub } from './github'
 import { registerGoogle } from './google'
 import { registerOpenid } from './openid'
@@ -45,7 +46,41 @@ export function registerProviders (
   }
 
   app.keys = [serverSecret]
-  app.use(session({}, app))
+  // koa-session defaults the cookie domain to the request host, which
+  // breaks OIDC flows that start on one subdomain (e.g. dev.example.com)
+  // and get the callback on another (e.g. app.example.com) — the session
+  // cookie set on dev isn't sent back to app, so the Passport state lookup
+  // fails with "did not find expected authorization request details in
+  // session". Setting SESSION_COOKIE_DOMAIN=.example.com makes the cookie
+  // span both hosts so the flow completes. Empty / unset preserves prior
+  // behaviour.
+  //
+  // Only set SESSION_COOKIE_DOMAIN when (a) all subdomains are equally
+  // trusted and (b) HTTPS is terminated in front of this service: widening
+  // the cookie scope forces secure=true + sameSite=lax below, so the
+  // signed httpOnly session cookie (carrying OIDC state/nonce/PKCE verifier)
+  // is never sent over plain HTTP nor cross-site. An invalid value (e.g.
+  // "com", or anything with whitespace/protocol/port) is ignored — the
+  // option falls back to prior behaviour (cookie scoped to the request host).
+  const sessionCookieDomain = process.env.SESSION_COOKIE_DOMAIN?.trim()
+  const sessionOpts: Partial<session.opts> = {}
+  if (sessionCookieDomain !== undefined && sessionCookieDomain.length > 0) {
+    // Accept only plausible parent-domain values: must contain a dot and be
+    // free of whitespace/protocol/port. Prevents misconfiguration like "com"
+    // (which would span the cookie across a whole TLD).
+    if (!isValidCookieDomain(sessionCookieDomain)) {
+      ctx.warn('SESSION_COOKIE_DOMAIN ignored: not a valid domain', { value: sessionCookieDomain })
+    } else {
+      sessionOpts.domain = sessionCookieDomain
+      // A cross-subdomain cookie MUST be hardened once its scope reaches
+      // beyond the request host (koa-session defaults: secure=false,
+      // sameSite unset). sameSite='lax' is enough for the top-level GET
+      // OIDC callback.
+      sessionOpts.secure = true
+      sessionOpts.sameSite = 'lax'
+    }
+  }
+  app.use(session(sessionOpts, app))
   app.use(passport.initialize())
   app.use(passport.session())