diff --git a/apps/web/__tests__/unit/share-cap-authorization.test.ts b/apps/web/__tests__/unit/share-cap-authorization.test.ts
new file mode 100644
index 00000000000..e37fa9ac99e
--- /dev/null
+++ b/apps/web/__tests__/unit/share-cap-authorization.test.ts
@@ -0,0 +1,235 @@
+import {
+	sharedVideos,
+	spaces,
+	spaceVideos,
+	videos,
+} from "@cap/database/schema";
+import { Space, Video } from "@cap/web-domain";
+import { getTableColumns } from "drizzle-orm";
+import type { MySqlTable } from "drizzle-orm/mysql-core";
+import { beforeEach, describe, expect, it, vi } from "vitest";
+
+const mocks = vi.hoisted(() => ({ query: vi.fn(), user: vi.fn() }));
+
+vi.mock("@cap/database", async () => {
+	const { drizzle } = await import("drizzle-orm/mysql-proxy");
+	const client = drizzle(mocks.query);
+	return { db: () => client };
+});
+vi.mock("@cap/database/auth/session", () => ({
+	getCurrentUser: mocks.user,
+}));
+vi.mock("next/cache", () => ({ revalidatePath: vi.fn() }));
+
+import { shareCap } from "@/actions/caps/share";
+
+const capId = Video.VideoId.make("video-1");
+const spaceId = Space.SpaceId.make("space-1");
+let privacy: "Private" | "Public";
+let creatorId: string;
+let organizationRole: string | null;
+let spaceRole: string | null;
+let videoOwnerId: string;
+let accessExists: boolean;
+let accessError: boolean;
+let alreadyShared: boolean;
+let organizationShareExists: boolean;
+
+function row(table: MySqlTable, values: Record<string, unknown>) {
+	return Object.values(getTableColumns(table)).map(
+		(column) => values[column.name] ?? null,
+	);
+}
+
+beforeEach(() => {
+	privacy = "Private";
+	creatorId = "space-owner";
+	organizationRole = "member";
+	spaceRole = null;
+	videoOwnerId = "caller";
+	accessExists = true;
+	accessError = false;
+	alreadyShared = false;
+	organizationShareExists = false;
+	mocks.user.mockResolvedValue({ id: "caller" });
+	mocks.query.mockImplementation(async (query: string) => {
+		if (query.startsWith("delete from `shared_videos`")) {
+			organizationShareExists = false;
+		}
+		if (/^(insert|update|delete) /.test(query)) {
+			return { rows: [{ insertId: 0, affectedRows: 1 }] };
+		}
+		if (query.includes("where (false")) return { rows: [] };
+		if (query.includes("from `videos`")) {
+			return { rows: [row(videos, { id: capId, ownerId: videoOwnerId })] };
+		}
+		if (query.includes("from `space_videos`") && alreadyShared) {
+			return {
+				rows: [row(spaceVideos, { id: "share-1", videoId: capId, spaceId })],
+			};
+		}
+		if (query.includes("from `shared_videos`") && organizationShareExists) {
+			return {
+				rows: [
+					row(sharedVideos, {
+						id: "organization-share-1",
+						videoId: capId,
+						organizationId: "org-1",
+					}),
+				],
+			};
+		}
+		if (query.includes("from `organization_members`")) {
+			return { rows: [["org-1"]] };
+		}
+		if (query.includes("from `spaces`")) {
+			if (query.includes("inner join `organizations`")) {
+				if (accessError) throw new Error("Database unavailable");
+				if (!accessExists) return { rows: [] };
+				return {
+					rows: [
+						[
+							spaceId,
+							"org-1",
+							creatorId,
+							"org-owner",
+							organizationRole,
+							spaceRole,
+						],
+					],
+				};
+			}
+			return {
+				rows: [
+					row(spaces, {
+						id: spaceId,
+						organizationId: "org-1",
+						createdById: creatorId,
+						privacy,
+					}),
+				],
+			};
+		}
+		return { rows: [] };
+	});
+});
+
+const writes = () =>
+	mocks.query.mock.calls.filter(([query]) =>
+		/^(insert|update|delete) /.test(query),
+	);
+
+describe("sharing Caps to private spaces", () => {
+	it("rejects an organization member outside the space before changing any shares or visibility", async () => {
+		organizationShareExists = true;
+		await expect(
+			shareCap({ capId, spaceIds: [spaceId], public: true }),
+		).resolves.toMatchObject({ success: false });
+		expect(organizationShareExists).toBe(true);
+		expect(writes()).toEqual([]);
+		const accessQuery = mocks.query.mock.calls.find(([query]) =>
+			query.includes("inner join `organizations`"),
+		);
+		expect(accessQuery?.[0]).toContain("`organization_members`.`userId` = ?");
+		expect(accessQuery?.[0]).toContain("`space_members`.`userId` = ?");
+		expect(accessQuery?.[0]).toContain("`spaces`.`id` = ?");
+		expect(accessQuery?.[0]).toContain("`organizations`.`tombstoneAt` is null");
+		expect(accessQuery?.[1]).toEqual(["caller", "caller", spaceId, 1]);
+	});
+
+	it("rejects a missing space access row without writing", async () => {
+		accessExists = false;
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: false,
+			error: "Unauthorized",
+		});
+		expect(writes()).toEqual([]);
+	});
+
+	it("does not write when the space access query fails", async () => {
+		accessError = true;
+		vi.spyOn(console, "error").mockImplementation(() => {});
+		await expect(
+			shareCap({ capId, spaceIds: [spaceId] }),
+		).resolves.toMatchObject({
+			success: false,
+		});
+		expect(writes()).toEqual([]);
+	});
+
+	it.each(["member", "admin"])("allows a space %s", async (role) => {
+		spaceRole = role;
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: true,
+		});
+		expect(writes()).toHaveLength(1);
+		expect(writes()[0]?.[0]).toMatch(/^insert into `space_videos`/);
+		expect(writes()[0]?.[1]).toEqual(
+			expect.arrayContaining([capId, spaceId, "caller"]),
+		);
+	});
+
+	it("allows the space creator without a space membership row", async () => {
+		creatorId = "caller";
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: true,
+		});
+		expect(writes()).toHaveLength(1);
+	});
+
+	it("allows an organization admin without a space membership row", async () => {
+		organizationRole = "admin";
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: true,
+		});
+		expect(writes()).toHaveLength(1);
+	});
+
+	it("preserves sharing to an organization-visible space", async () => {
+		privacy = "Public";
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: true,
+		});
+		expect(writes()).toHaveLength(1);
+	});
+
+	it("keeps an existing private space share when access was since removed", async () => {
+		alreadyShared = true;
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: true,
+		});
+		expect(writes()).toEqual([]);
+		expect(
+			mocks.query.mock.calls.some(([query]) =>
+				query.includes("inner join `organizations`"),
+			),
+		).toBe(false);
+	});
+
+	it("rejects unauthenticated callers without querying the database", async () => {
+		mocks.user.mockResolvedValue(null);
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: false,
+			error: "Unauthorized",
+		});
+		expect(mocks.query).not.toHaveBeenCalled();
+	});
+
+	it("still rejects another user's video", async () => {
+		videoOwnerId = "another-user";
+		spaceRole = "admin";
+		await expect(shareCap({ capId, spaceIds: [spaceId] })).resolves.toEqual({
+			success: false,
+			error: "Unauthorized",
+		});
+		expect(writes()).toEqual([]);
+		expect(mocks.query).toHaveBeenCalledTimes(1);
+	});
+
+	it("allows clearing sharing without a destination space", async () => {
+		await expect(shareCap({ capId, spaceIds: [] })).resolves.toEqual({
+			success: true,
+		});
+		expect(writes()).toEqual([]);
+	});
+});
diff --git a/apps/web/actions/caps/share.ts b/apps/web/actions/caps/share.ts
index c6ae3969f83..e6e7a7b0e2d 100644
--- a/apps/web/actions/caps/share.ts
+++ b/apps/web/actions/caps/share.ts
@@ -14,6 +14,7 @@ import {
 import type { Organisation, Space, Video } from "@cap/web-domain";
 import { and, eq, inArray } from "drizzle-orm";
 import { revalidatePath } from "next/cache";
+import { getSpaceAccess } from "@/actions/organization/space-authorization";
 
 interface ShareCapParams {
 	capId: Video.VideoId;
@@ -69,6 +70,21 @@ export async function shareCap({
 				),
 			);
 
+		const currentSpaceVideos = await db()
+			.select()
+			.from(spaceVideos)
+			.where(eq(spaceVideos.videoId, capId));
+		const existingSpaceIds = new Set(currentSpaceVideos.map((s) => s.spaceId));
+
+		for (const space of spacesData) {
+			if (space.privacy === "Public" || existingSpaceIds.has(space.id))
+				continue;
+			const access = await getSpaceAccess(user.id, space.id);
+			if (!access || (!access.canManage && !access.spaceRole)) {
+				return { success: false, error: "Unauthorized" };
+			}
+		}
+
 		const organizationIds = directOrgIds;
 
 		const currentSharedOrganizations = await db()
@@ -107,11 +123,6 @@ export async function shareCap({
 
 		const spacesIds = spacesData.map((space) => space.id);
 
-		const currentSpaceVideos = await db()
-			.select()
-			.from(spaceVideos)
-			.where(eq(spaceVideos.videoId, capId));
-
 		const spaceIdsToRemove = currentSpaceVideos
 			.filter((s) => !spacesIds.includes(s.spaceId))
 			.map((s) => s.spaceId);
@@ -126,7 +137,6 @@ export async function shareCap({
 				);
 		}
 
-		const existingSpaceIds = new Set(currentSpaceVideos.map((s) => s.spaceId));
 		const newSpaceEntries = spacesIds
 			.filter((id) => !existingSpaceIds.has(id))
 			.map((spaceId) => ({