diff --git a/apps/api/src/lib/alerts.ts b/apps/api/src/lib/alerts.ts
index f92162d..93dab01 100644
--- a/apps/api/src/lib/alerts.ts
+++ b/apps/api/src/lib/alerts.ts
@@ -40,6 +40,15 @@ const SEND_TIMEOUT_MS = 30_000;
 /** Failed port-22 checks in a row before a host counts as offline. */
 const DOWN_AFTER = 2;
 
+// One text goes to every service: lookalikes and zero-width spaces stop Slack `<!channel>`/`<url|x>`, `@channel` and markdown links without showing escapes in plain-text ones.
+const defuse = (s: string) =>
+    s
+        .replaceAll("<", "‹")
+        .replaceAll(">", "›")
+        .replace(/(?!\n)\p{Cc}/gu, " ")
+        .replace(/(?<![A-Za-z0-9])@/g, "@\u200b")
+        .replaceAll("](", "]\u200b(");
+
 const proxyEnv = () =>
     Object.fromEntries(
         Object.entries(process.env).filter(
@@ -213,7 +222,12 @@ export function alerter({
             .get(String(id));
     const hostName = (e: AuditInput) =>
         (e.name as string | undefined) ?? host(e.host)?.name ?? String(e.host);
-    const who = (e: AuditInput) => String(e.email ?? e.sub ?? "someone");
+    // User-set text (task name, IdP-editable email): a newline could forge alert lines, a URL would autolink.
+    const userText = (s: unknown) =>
+        String(s)
+            .replace(/[\n\u2028\u2029]/g, " ")
+            .replaceAll("://", ":/\u200b/");
+    const who = (e: AuditInput) => userText(e.email ?? e.sub ?? "someone");
 
     function securityMessage(e: AuditInput): Message {
         const lines = [
@@ -230,7 +244,7 @@ export function alerter({
                 ),
             );
         if (e.event === "session_kill" || e.event === "user_signout")
-            lines.push(`User: ${e.user}`);
+            lines.push(`User: ${userText(e.user)}`);
         lines.push(new URL("/admin/audit", appUrl).href);
         return {
             title: `CertShell: ${SECURITY_TITLES[e.event]?.(e, hostName(e))}`,
@@ -262,9 +276,9 @@ export function alerter({
             >("SELECT name, email, sub FROM runs WHERE id = ?")
             .get(runId);
         return {
-            title: `CertShell: task "${run?.name}" failed on ${failed.length} of ${hosts.length} host${hosts.length === 1 ? "" : "s"}`,
+            title: `CertShell: task "${userText(run?.name)}" failed on ${failed.length} of ${hosts.length} host${hosts.length === 1 ? "" : "s"}`,
             body: [
-                `Run by ${run?.email ?? run?.sub}.`,
+                `Run by ${who({ event: "", email: run?.email, sub: run?.sub })}.`,
                 ...failed.map(
                     (h) =>
                         `${h.login}@${h.host_name}: ${h.exit_code === null ? "didn't finish" : `exit ${h.exit_code}`}`,
@@ -314,6 +328,7 @@ export function alerter({
             };
         } else return;
         if (!msg) return;
+        msg = { title: defuse(msg.title), body: defuse(msg.body) };
         for (const row of rows()) {
             const c = channelOf(row);
             if (c.events.includes(event) && matches(c)) void send(row, msg);
diff --git a/apps/api/test/alerts.test.ts b/apps/api/test/alerts.test.ts
index 6156d54..bcbbd7a 100644
--- a/apps/api/test/alerts.test.ts
+++ b/apps/api/test/alerts.test.ts
@@ -4,6 +4,8 @@ import { tmpdir } from "node:os";
 import type { AlertChannel, AlertInput } from "@repo/shared";
 import { alerter } from "../src/lib/alerts";
 import { audit } from "../src/lib/audit";
+import { createSession } from "../src/lib/sessions";
+import { SESSION_COOKIE } from "../src/routes/auth";
 import { hostKeyRejected } from "../src/ssh/terminal";
 import { APP_URL, startApp } from "./helpers/app";
 
@@ -48,6 +50,19 @@ const call = (method: string, path: string, body?: unknown) =>
         body: body === undefined ? undefined : JSON.stringify(body),
     });
 
+const post = async (path: string, body: unknown, cookie = ctx.cookie) =>
+    (await (
+        await ctx.app.request(`${APP_URL}/api/tasks${path}`, {
+            method: "POST",
+            headers: {
+                cookie,
+                origin: APP_URL,
+                "content-type": "application/json",
+            },
+            body: JSON.stringify(body),
+        })
+    ).json()) as { id: number | string };
+
 const input = (over: Partial<AlertInput> = {}): AlertInput => ({
     name: "ops",
     url: "ntfy://ntfy.sh/secret-topic",
@@ -307,6 +322,21 @@ test("terminal event: filtered by host tag or login; empty filter = every termin
     ]);
 });
 
+test("terminal event: mentions and links in the email arrive defused", async () => {
+    await setup();
+    await create({ events: ["terminal"] });
+    audit(ctx.db, {
+        event: "session_start",
+        sub: "u",
+        email: "<!here> _@channel [x](https://evil.test)\nfake\u0007line",
+        principal: "ws:h1:root",
+        session: "s1",
+    });
+    expect((await sent(1))[0]?.args[2]).toBe(
+        "CertShell: ‹!here› _@\u200bchannel [x]\u200b(https:/\u200b/evil.test) fake line opened root@h1",
+    );
+});
+
 test("task_failed: one alert when the last host finishes, only if one failed", async () => {
     await setup({
         command: ({ host, remote }) => ["sh", "-c", remote ?? "", host.id],
@@ -315,32 +345,19 @@ test("task_failed: one alert when the last host finishes, only if one failed", a
         root: "certshell-admins",
     });
     await create({ events: ["task_failed"] });
-    const task = async (script: string) => {
-        const r = await ctx.app.request(`${APP_URL}/api/tasks`, {
-            method: "POST",
-            headers: {
-                cookie: ctx.cookie,
-                origin: APP_URL,
-                "content-type": "application/json",
-            },
-            body: JSON.stringify({
+    const task = async (script: string) =>
+        (
+            await post("", {
                 name: "Upgrade",
                 script,
                 targets: [
                     { host: "h1", login: "root" },
                     { host: "h2", login: "root" },
                 ],
-            }),
-        });
-        return ((await r.json()) as { id: number }).id;
-    };
-    const run = async (id: number) => {
-        const r = await ctx.app.request(`${APP_URL}/api/tasks/${id}/run`, {
-            method: "POST",
-            headers: { cookie: ctx.cookie, origin: APP_URL },
-        });
-        return ((await r.json()) as { id: string }).id;
-    };
+            })
+        ).id;
+    const run = async (id: number | string) =>
+        String((await post(`/${id}/run`, {})).id);
 
     const done = async (runId: string) => {
         for (let i = 0; i < 100; i++) {
@@ -367,6 +384,34 @@ test("task_failed: one alert when the last host finishes, only if one failed", a
     );
 });
 
+test("task_failed: mentions and links in the task name and email arrive defused", async () => {
+    await setup({ command: () => ["sh", "-c", "exit 1"] });
+    await create({ events: ["task_failed"] });
+    const cookie = `${SESSION_COOKIE}=${createSession(ctx.db, {
+        iss: "https://id.test",
+        sub: "user-2",
+        email: "<!channel>\n_@here\u2028https://evil.test",
+        groups: ["certshell-admins"],
+    })}`;
+    const { id } = await post(
+        "",
+        {
+            name: "<!channel> <https://evil.test|x> https://evil.test/login",
+            script: "true",
+            targets: [{ host: "h1", login: "root" }],
+        },
+        cookie,
+    );
+    const runId = (await post(`/${id}/run`, {}, cookie)).id;
+    const [s] = await sent(1);
+    expect(s?.args[2]).toBe(
+        'CertShell: task "‹!channel› ‹https:/\u200b/evil.test|x› https:/\u200b/evil.test/login" failed on 1 of 1 host',
+    );
+    expect(s?.body).toBe(
+        `Run by ‹!channel› _@\u200bhere https:/\u200b/evil.test.\nroot@h1: exit 1\nhttps://ssh.test/tasks/runs/${runId}`,
+    );
+});
+
 test("reboot: alerted when host facts first show the flag, not again until it clears", async () => {
     let reboot = "1";
     await setup({ command: () => ["sh", "-c", `echo reboot=${reboot}`] });