diff --git a/cypress/e2e/direct.spec.js b/cypress/e2e/direct.spec.js
index b182045951..61f9e48cdb 100644
--- a/cypress/e2e/direct.spec.js
+++ b/cypress/e2e/direct.spec.js
@@ -103,6 +103,48 @@ describe('Direct editing (legacy)', function() {
 		})
 	})
 
+	describe('PostMessage origin security', function() {
+		it('rejects messages from an unexpected origin', function() {
+			createDirectEditingLink(randUser, fileId)
+				.then((token) => {
+					cy.nextcloudTestingAppConfigSet('richdocuments', 'uiDefaults-UIMode', 'classic')
+					cy.logout()
+					cy.visit(token, {
+						onBeforeLoad(win) {
+							cy.spy(win, 'postMessage').as('postMessage')
+						},
+					})
+					cy.waitForCollabora(false)
+					cy.waitForPostMessage('App_LoadingStatus', { Status: 'Document_Loaded' })
+
+					cy.window().then(win => {
+						cy.spy(win.console, 'warn').as('consoleWarn')
+					})
+					cy.dispatchMessageFromOrigin('https://evil.example.com', { MessageId: 'Action_Save', Values: {} })
+					cy.get('@consoleWarn').should('have.been.calledWith',
+						'PostMessageService: rejected message from unexpected origin',
+						'https://evil.example.com'
+					)
+				})
+		})
+
+		it('sends messages with the Collabora targetOrigin', function() {
+			createDirectEditingLink(randUser, fileId)
+				.then((token) => {
+					cy.nextcloudTestingAppConfigSet('richdocuments', 'uiDefaults-UIMode', 'classic')
+					cy.logout()
+					cy.visit(token)
+					cy.waitForCollabora(false)
+					cy.get('[data-cy="coolframe"]').then($iframe => {
+						const collaboraOrigin = $iframe[0].contentWindow.location.origin
+						cy.spy($iframe[0].contentWindow, 'postMessage').as('postMessage')
+						cy.dispatchMessageFromOrigin(collaboraOrigin, { MessageId: 'App_LoadingStatus', Values: { Status: 'Document_Loaded' } })
+						cy.waitForPostMessage('Host_PostmessageReady', undefined, { targetOrigin: collaboraOrigin })
+					})
+				})
+		})
+	})
+
 	it('Open a remotely shared file', () => {
 		cy.createRandomUser().then(shareRecipient => {
 			cy.login(randUser)
diff --git a/cypress/e2e/open.spec.js b/cypress/e2e/open.spec.js
index 5226d2bb3d..18f8289222 100644
--- a/cypress/e2e/open.spec.js
+++ b/cypress/e2e/open.spec.js
@@ -152,3 +152,56 @@ describe('Open PDF with richdocuments', () => {
 		cy.closeDocument()
 	})
 })
+
+describe('PostMessage origin security', function() {
+	let randUser
+
+	before(function() {
+		cy.createRandomUser().then(user => {
+			randUser = user
+			cy.login(user)
+			cy.uploadFile(user, 'document.odt', 'application/vnd.oasis.opendocument.text', '/document.odt')
+		})
+	})
+
+	beforeEach(function() {
+		cy.login(randUser)
+	})
+
+	it('rejects messages from an unexpected origin', function() {
+		cy.visit('/apps/files', {
+			onBeforeLoad(win) {
+				cy.spy(win, 'postMessage').as('postMessage')
+			},
+		})
+		cy.openFile('document.odt')
+		cy.waitForViewer()
+		cy.waitForCollabora()
+		cy.waitForPostMessage('App_LoadingStatus', { Status: 'Document_Loaded' })
+
+		cy.window().then(win => {
+			cy.spy(win.console, 'warn').as('consoleWarn')
+		})
+		cy.dispatchMessageFromOrigin('https://evil.example.com', { MessageId: 'Action_Save', Values: {} })
+		cy.get('@consoleWarn').should('have.been.calledWith',
+			'PostMessageService: rejected message from unexpected origin',
+			'https://evil.example.com'
+		)
+		cy.closeDocument()
+	})
+
+	it('sends messages with the Collabora targetOrigin', function() {
+		cy.visit('/apps/files')
+		cy.openFile('document.odt')
+		cy.waitForViewer()
+		cy.waitForCollabora()
+		cy.get('[data-cy="coolframe"]').then($iframe => {
+			const collaboraOrigin = $iframe[0].contentWindow.location.origin
+			cy.spy($iframe[0].contentWindow, 'postMessage').as('postMessage')
+			cy.dispatchMessageFromOrigin(collaboraOrigin, { MessageId: 'App_LoadingStatus', Values: { Status: 'Document_Loaded' } })
+			cy.waitForPostMessage('Host_PostmessageReady', undefined, { targetOrigin: collaboraOrigin })
+		})
+
+		cy.closeDocument()
+	})
+})
diff --git a/cypress/support/commands.js b/cypress/support/commands.js
index 12a1030e9e..ebe4e178b1 100644
--- a/cypress/support/commands.js
+++ b/cypress/support/commands.js
@@ -276,18 +276,51 @@ Cypress.Commands.add('waitForCollabora', (wrapped = false, federated = false) =>
 	return cy.get('@loleafletframe')
 })
 
-Cypress.Commands.add('waitForPostMessage', (messageId, values = undefined) => {
+Cypress.Commands.add('waitForPostMessage', (messageId, expectedValues = undefined, options = {}) => {
+	const { targetOrigin } = options
+	const checkExpectedValues = (message, values) => {
+		for (const [key, value] of Object.entries(values)) {
+			if (!message.Values[key] || message.Values[key] !== value) {
+				return false
+			}
+		}
+
+		return true
+	}
+
 	cy.get('@postMessage', { timeout: 20000 }).should(spy => {
 		const calls = spy.getCalls()
-		const findMatchingCall = calls.find(call => call.args[0].indexOf('"MessageId":"' + messageId + '"') !== -1)
-		if (!findMatchingCall) {
-			return expect(findMatchingCall).to.not.be.undefined
+		const messagesMatchingId = []
+
+		// Find all messages matching the given ID
+		// We do it this way to avoid the shallow copy of Array.filter()
+		for (const call of calls) {
+			if (call.args[0].includes(`"MessageId":"${messageId}"`)) {
+				messagesMatchingId.push({ message: JSON.parse(call.args[0]), call })
+			}
 		}
-		if (!values) {
-			const object = JSON.parse(findMatchingCall.args[0])
-			values.forEach(value => {
-				expect(object.Values).to.have.property(value, values[value])
-			})
+
+		expect(messagesMatchingId.length).to.be.greaterThan(0)
+
+		if (expectedValues) {
+			const messagesMatchingValues = []
+
+			for (const { message } of messagesMatchingId) {
+				if (checkExpectedValues(message, expectedValues)) {
+					messagesMatchingValues.push(message)
+				}
+			}
+
+			expect(messagesMatchingValues.length).to.be.greaterThan(0)
+		}
+
+		if (targetOrigin) {
+			for (const { call } of messagesMatchingId) {
+				expect(call.args[1]).to.equal(
+					targetOrigin,
+					`Expected targetOrigin for ${messageId} to be ${targetOrigin}`,
+				)
+			}
 		}
 	})
 })
@@ -323,3 +356,12 @@ Cypress.Commands.add('uploadSystemTemplate', () => {
 	}, { force: true })
 	cy.get('#richdocuments-templates li').contains('systemtemplate.otp')
 })
+
+Cypress.Commands.add('dispatchMessageFromOrigin', (origin, message) => {
+	cy.window().then(win => {
+		win.dispatchEvent(new win.MessageEvent('message', {
+			origin,
+			data: JSON.stringify(message),
+		}))
+	})
+})
diff --git a/src/document.js b/src/document.js
index 6aa7682bbc..fe4e1120c0 100644
--- a/src/document.js
+++ b/src/document.js
@@ -554,7 +554,20 @@ const documentsMain = {
 	initSession() {
 		PostMessages.sendPostMessage('parent', 'loading')
 
-		documentsMain.urlsrc = Config.get('urlsrc')
+		const urlsrc = Config.get('urlsrc')
+		if (urlsrc) {
+			try {
+				PostMessages.setAllowedOrigins([
+					new URL(urlsrc).origin,
+					window.location.origin,
+				])
+				PostMessages.setTargetOrigins({
+					loolframe: new URL(urlsrc).origin,
+					parent: window.location.origin,
+				})
+			} catch (e) {}
+		}
+		documentsMain.urlsrc = urlsrc
 		documentsMain.fullPath = Config.get('path')
 		documentsMain.token = Config.get('token')
 		documentsMain.tokenTtl = Config.get('token_ttl') * 1000
@@ -600,7 +613,7 @@ const documentsMain = {
 		documentsMain.UI.hideEditor()
 		documentsMain.openLocally()
 
-		PostMessages.sendPostMessage('parent', 'close', '*')
+		PostMessages.sendPostMessage('parent', 'close')
 	},
 
 	onCloseViewer() {
diff --git a/src/services/postMessage.tsx b/src/services/postMessage.tsx
index 2d6b67a829..d8a67f0f76 100644
--- a/src/services/postMessage.tsx
+++ b/src/services/postMessage.tsx
@@ -38,22 +38,37 @@ interface WindowCallbackHandler { (): Window}
 export default class PostMessageService {
 	private readonly targets: {[name: string]: (Window|WindowCallbackHandler)};
 	private postMessageHandlers: Function[] = [];
+	private allowedOrigins: string[] = [];
+	private targetOrigins: {[name: string]: string} = {};
 
 	constructor(targets: {[name: string]: (Window|WindowCallbackHandler)}) {
 		this.targets = targets
 		window.addEventListener('message', (event: {source: MessageEventSource, data: any, origin: string}) => {
+			if (this.allowedOrigins.length > 0 && !this.allowedOrigins.includes(event.origin)) {
+				console.warn('PostMessageService: rejected message from unexpected origin', event.origin)
+				return
+			}
 			this.handlePostMessage(event.data)
 		}, false)
 	}
 
-	sendPostMessage(target: string, message: any, targetOrigin: string = '*') {
+	setAllowedOrigins(origins: string[]): void {
+		this.allowedOrigins = origins
+	}
+
+	setTargetOrigins(origins: {[name: string]: string}): void {
+		this.targetOrigins = origins
+	}
+
+	sendPostMessage(target: string, message: any, targetOrigin?: string) {
 		let targetElement: Window;
 		if (typeof this.targets[target] === 'function') {
 			targetElement = (this.targets[target] as WindowCallbackHandler)()
 		} else {
 			targetElement = this.targets[target] as Window
 		}
-		targetElement.postMessage(message, targetOrigin)
+		const origin = targetOrigin ?? this.targetOrigins[target] ?? '*'
+		targetElement.postMessage(message, origin)
 		console.debug('PostMessageService.sendPostMessage', target, message)
 	}
 
diff --git a/src/view/Office.vue b/src/view/Office.vue
index 48347311a3..418bf094a8 100644
--- a/src/view/Office.vue
+++ b/src/view/Office.vue
@@ -323,6 +323,12 @@ export default {
 			})
 
 			if (data.federatedUrl) {
+				try {
+					this.postMessage.setAllowedOrigins([new URL(data.federatedUrl).origin, window.location.origin])
+					this.postMessage.setTargetOrigins({ FRAME_DOCUMENT: new URL(data.federatedUrl).origin })
+				} catch (e) {
+					console.warn('[richdocuments] Could not derive origin from federatedUrl', e)
+				}
 				this.$set(this.formData, 'action', data.federatedUrl)
 				this.$nextTick(() => this.$refs.form.submit())
 				this.loading = LOADING_STATE.DOCUMENT_READY
@@ -330,6 +336,12 @@ export default {
 			}
 
 			Config.update('urlsrc', data.urlSrc)
+			try {
+				this.postMessage.setAllowedOrigins([new URL(data.urlSrc).origin, window.location.origin])
+				this.postMessage.setTargetOrigins({ FRAME_DOCUMENT: new URL(data.urlSrc).origin })
+			} catch (e) {
+				console.warn('[richdocuments] Could not derive Collabora origin from urlsrc', e)
+			}
 			Config.update('wopi_callback_url', loadState('richdocuments', 'wopi_callback_url', ''))
 
 			const forceReadOnly = this.isEmbedded && !this.hasWidgetEditingEnabled