diff --git a/app/features/tournament-admin/actions/to.$id.admin.seeds.server.test.ts b/app/features/tournament-admin/actions/to.$id.admin.seeds.server.test.ts
new file mode 100644
index 000000000..47f1253b5
--- /dev/null
+++ b/app/features/tournament-admin/actions/to.$id.admin.seeds.server.test.ts
@@ -0,0 +1,84 @@
+import { describe, expect, test } from "vitest";
+import * as TournamentFactory from "~/db/seed/factories/TournamentFactory";
+import * as TournamentTeamFactory from "~/db/seed/factories/TournamentTeamFactory";
+import * as UserFactory from "~/db/seed/factories/UserFactory";
+import { db } from "~/db/sql";
+import { wrappedAction } from "~/utils/Test";
+import type { adminSeedsActionSchema } from "../tournament-admin-schemas";
+import { action } from "./to.$id.admin.seeds.server";
+
+const seedsAction = wrappedAction<typeof adminSeedsActionSchema>({ action });
+
+const teamRow = (tournamentTeamId: number) =>
+	db
+		.selectFrom("TournamentTeam")
+		.select(["id", "tournamentId", "seed", "startingBracketIdx"])
+		.where("id", "=", tournamentTeamId)
+		.executeTakeFirstOrThrow();
+
+describe("tournament seeds admin action", () => {
+	test("does not touch the seeds of a team of another tournament", async () => {
+		await UserFactory.createAdmin();
+		const attacker = await UserFactory.createRegular();
+		const victimTo = await UserFactory.create();
+		const [player] = await UserFactory.createMany(1);
+
+		// the attacker organizes a tournament of their own
+		const ownTournament = await TournamentFactory.create({
+			authorId: attacker.id,
+		});
+
+		// somebody else's tournament, with a team they seeded themselves
+		const otherTournament = await TournamentFactory.create({
+			authorId: victimTo.id,
+		});
+		const otherTeam = await TournamentTeamFactory.create({
+			tournamentId: otherTournament.id,
+			memberUserIds: [player.id],
+		});
+
+		await seedsAction(
+			{ _action: "UPDATE_SEEDS", seeds: JSON.stringify([otherTeam.id]) as any },
+			{ user: "regular", params: { id: String(ownTournament.id) } },
+		);
+
+		const after = await teamRow(otherTeam.id);
+		expect(after.tournamentId).toBe(otherTournament.id);
+		expect(after.seed, "another tournament's team got seeded").toBeNull();
+	});
+
+	test("does not touch the starting bracket of a team of another tournament", async () => {
+		await UserFactory.createAdmin();
+		const attacker = await UserFactory.createRegular();
+		const victimTo = await UserFactory.create();
+		const [player] = await UserFactory.createMany(1);
+
+		const ownTournament = await TournamentFactory.create({
+			authorId: attacker.id,
+		});
+
+		const otherTournament = await TournamentFactory.create({
+			authorId: victimTo.id,
+		});
+		const otherTeam = await TournamentTeamFactory.create({
+			tournamentId: otherTournament.id,
+			memberUserIds: [player.id],
+		});
+
+		await seedsAction(
+			{
+				_action: "UPDATE_STARTING_BRACKETS",
+				startingBrackets: JSON.stringify([
+					{ tournamentTeamId: otherTeam.id, startingBracketIdx: 0 },
+				]) as any,
+			},
+			{ user: "regular", params: { id: String(ownTournament.id) } },
+		);
+
+		const after = await teamRow(otherTeam.id);
+		expect(
+			after.startingBracketIdx,
+			"another tournament's team got moved to a starting bracket",
+		).toBeNull();
+	});
+});
diff --git a/app/features/tournament-admin/actions/to.$id.admin.seeds.server.ts b/app/features/tournament-admin/actions/to.$id.admin.seeds.server.ts
index ae9c30765..edba548bd 100644
--- a/app/features/tournament-admin/actions/to.$id.admin.seeds.server.ts
+++ b/app/features/tournament-admin/actions/to.$id.admin.seeds.server.ts
@@ -1,6 +1,7 @@
 import type { ActionFunction } from "react-router";
 import * as TournamentRepository from "~/features/tournament/TournamentRepository.server";
 import * as TournamentTeamRepository from "~/features/tournament/TournamentTeamRepository.server";
+import type { Tournament } from "~/features/tournament-bracket/core/Tournament";
 import {
 	clearTournamentDataCache,
 	requireTournamentOrganizer,
@@ -30,6 +31,7 @@ export const action: ActionFunction = async ({ request, params }) => {
 		case "UPDATE_SEEDS": {
 			requireTournamentOrganizer(tournament, user);
 			errorToastIfFalsy(!tournament.hasStarted, "Tournament has started");
+			validateTeamsOfTournament(tournament, data.seeds);
 
 			await TournamentRepository.updateTeamSeeds({
 				tournamentId,
@@ -54,6 +56,10 @@ export const action: ActionFunction = async ({ request, params }) => {
 				),
 				"Invalid starting bracket idx",
 			);
+			validateTeamsOfTournament(
+				tournament,
+				data.startingBrackets.map((t) => t.tournamentTeamId),
+			);
 
 			await TournamentTeamRepository.updateStartingBrackets(
 				data.startingBrackets,
@@ -73,10 +79,9 @@ export const action: ActionFunction = async ({ request, params }) => {
 				"No starting bracket has A/B divisions enabled",
 			);
 
-			const validTeamIds = new Set(tournament.ctx.teams.map((t) => t.id));
-			errorToastIfFalsy(
-				data.abDivisions.every((t) => validTeamIds.has(t.tournamentTeamId)),
-				"Invalid tournament team id",
+			validateTeamsOfTournament(
+				tournament,
+				data.abDivisions.map((t) => t.tournamentTeamId),
 			);
 
 			await TournamentTeamRepository.updateAbDivisions(data.abDivisions);
@@ -93,3 +98,15 @@ export const action: ActionFunction = async ({ request, params }) => {
 
 	return successToast(message);
 };
+
+function validateTeamsOfTournament(
+	tournament: Tournament,
+	tournamentTeamIds: number[],
+) {
+	const validTeamIds = new Set(tournament.ctx.teams.map((t) => t.id));
+
+	errorToastIfFalsy(
+		tournamentTeamIds.every((id) => validTeamIds.has(id)),
+		"Invalid tournament team id",
+	);
+}