diff --git a/packages/cli/src/lib/dsn/detector.ts b/packages/cli/src/lib/dsn/detector.ts
index 069f47032..59f6fe42b 100644
--- a/packages/cli/src/lib/dsn/detector.ts
+++ b/packages/cli/src/lib/dsn/detector.ts
@@ -24,6 +24,7 @@ import {
   getCachedProjectRoot,
   setCachedProjectRoot,
 } from "../db/project-root-cache.js";
+import { logger } from "../logger.js";
 import {
   extractFirstDsnFromContent,
   scanCodeForDsns,
@@ -37,7 +38,7 @@ import {
 } from "./env-file.js";
 import { isRegularFile } from "./fs-utils.js";
 import { createDetectedDsn, createDsnFingerprint, parseDsn } from "./parser.js";
-import { findProjectRoot, isHomeOrAncestor } from "./project-root.js";
+import { findProjectRoot, shouldSkipDownwardScan } from "./project-root.js";
 import type {
   CachedDsnEntry,
   DetectedDsn,
@@ -45,6 +46,15 @@ import type {
   DsnSource,
 } from "./types.js";
 
+const log = logger.withTag("dsn");
+
+/** Explain at debug level why DSN auto-detection did not scan any files. */
+function logSkippedScan(projectRoot: string, reason: string): void {
+  log.debug(
+    `Skipping DSN file scan: project root ${projectRoot} (${reason}) is the home directory, an ancestor of it, or a shared temp directory`,
+  );
+}
+
 /**
  * Detect DSN with project root detection and caching support.
  *
@@ -67,11 +77,12 @@ export async function detectDsn(cwd: string): Promise<DetectedDsn | null> {
   //    return it immediately - code DSNs take priority)
   const { projectRoot, reason } = await findProjectRoot(cwd);
 
-  // When project root detection fell back to the home directory (or an
-  // ancestor of it), a downward scan would reach OS app-data and credential
-  // directories (~/Library, ~/.ssh, ~/.aws, …). Skip the filesystem scan
-  // entirely in that case and only consult the SENTRY_DSN env var.
-  if (reason === "fallback" && isHomeOrAncestor(projectRoot)) {
+  // When the project root is the home directory, an ancestor of it, or a
+  // shared temp root, a downward scan would reach OS app-data, credential,
+  // and mounted dirs (~/Library, ~/.ssh, /tmp/.mount_*, …). Skip the
+  // filesystem scan entirely and only consult the SENTRY_DSN env var.
+  if (shouldSkipDownwardScan(projectRoot)) {
+    logSkippedScan(projectRoot, reason);
     return detectFromEnv();
   }
 
@@ -156,11 +167,10 @@ export async function detectAllDsns(cwd: string): Promise<DsnDetectionResult> {
     });
   }
 
-  // When project root detection fell back to the home directory (or an
-  // ancestor of it), a downward scan would reach OS app-data and credential
-  // directories (~/Library, ~/.ssh, ~/.aws, …). Skip the filesystem scan
-  // entirely and only report the SENTRY_DSN env var, if any.
-  if (reason === "fallback" && isHomeOrAncestor(projectRoot)) {
+  // Same guard as detectDsn: never scan down from $HOME, its ancestors, or a
+  // shared temp root. Only report the SENTRY_DSN env var, if any.
+  if (shouldSkipDownwardScan(projectRoot)) {
+    logSkippedScan(projectRoot, reason);
     const envDsn = detectFromEnv();
     const all = envDsn ? [envDsn] : [];
     return {
diff --git a/packages/cli/src/lib/dsn/project-root.ts b/packages/cli/src/lib/dsn/project-root.ts
index 40838b496..68d26943e 100644
--- a/packages/cli/src/lib/dsn/project-root.ts
+++ b/packages/cli/src/lib/dsn/project-root.ts
@@ -14,7 +14,7 @@
  */
 
 import { opendir, readFile, stat } from "node:fs/promises";
-import { homedir } from "node:os";
+import { homedir, tmpdir } from "node:os";
 import { isAbsolute, join, relative, resolve } from "node:path";
 import pLimit from "p-limit";
 import picomatch from "picomatch";
@@ -424,6 +424,31 @@ export function isHomeOrAncestor(dir: string): boolean {
   return rel !== "" && !rel.startsWith("..") && !isAbsolute(rel);
 }
 
+/**
+ * Shared system temp roots. Other processes mount private FUSE filesystems
+ * directly under these (AppImage `/tmp/.mount_*`, FreeRDP clipboard dirs),
+ * and stale mounts fail `readdir` with ENOTCONN — so a temp root is never
+ * a project directory worth scanning. Subdirectories are still scanned.
+ */
+function isSharedTempRoot(dir: string): boolean {
+  const resolvedDir = resolve(dir);
+  return [tmpdir(), "/tmp", "/var/tmp", "/private/tmp"].some(
+    (root) => resolve(root) === resolvedDir,
+  );
+}
+
+/**
+ * True when a downward DSN scan from `projectRoot` must be skipped: the
+ * root is `$HOME`, an ancestor of it, or a shared temp root. Applies to
+ * every root-detection reason — a stray `~/package.json` (language marker)
+ * or a dotfiles `~/.git` makes `$HOME` the "project root" just like the
+ * no-marker fallback does, and scanning from there walks into `~/Library`,
+ * cloud-storage mounts, and credential dirs.
+ */
+export function shouldSkipDownwardScan(projectRoot: string): boolean {
+  return isHomeOrAncestor(projectRoot) || isSharedTempRoot(projectRoot);
+}
+
 /**
  * Process one directory level during the walk-up search
  */
diff --git a/packages/cli/test/lib/dsn/home-fallback.test.ts b/packages/cli/test/lib/dsn/home-fallback.test.ts
index 9f89b3c16..ae136f5c2 100644
--- a/packages/cli/test/lib/dsn/home-fallback.test.ts
+++ b/packages/cli/test/lib/dsn/home-fallback.test.ts
@@ -13,13 +13,17 @@ import { mkdirSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
 
-const { fakeHome } = vi.hoisted(() => ({ fakeHome: { path: "" } }));
+const { fakeHome, fakeTmp } = vi.hoisted(() => ({
+  fakeHome: { path: "" },
+  fakeTmp: { path: "" },
+}));
 
 vi.mock("node:os", async (importOriginal) => {
   const actual = await importOriginal<typeof import("node:os")>();
   return {
     ...actual,
     homedir: () => fakeHome.path,
+    tmpdir: () => fakeTmp.path || actual.tmpdir(),
   };
 });
 
@@ -42,6 +46,7 @@ describe("DSN detection from $HOME fallback", () => {
 
   afterEach(() => {
     fakeHome.path = "";
+    fakeTmp.path = "";
     delete process.env.SENTRY_DSN;
   });
 
@@ -117,6 +122,78 @@ describe("DSN detection from $HOME fallback", () => {
     expect(result?.source).toBe("code");
   });
 
+  describe("when a stray project marker sits directly in $HOME", () => {
+    // A ~/package.json or a dotfiles ~/.git makes $HOME the project root
+    // with reason "language" / "vcs" instead of "fallback". The scan must
+    // still be skipped — otherwise it walks ~/Library, cloud-storage and
+    // FUSE mounts, and can pick up an unrelated DSN from anywhere in $HOME.
+    const strayDsn = "https://stray@o444.ingest.sentry.io/444";
+
+    beforeEach(() => {
+      mkdirSync(join(home, "notes"), { recursive: true });
+      writeFileSync(
+        join(home, "notes", "old-demo.ts"),
+        `Sentry.init({ dsn: "${strayDsn}" });`,
+      );
+    });
+
+    test.each([
+      ["language marker", "package.json"],
+      ["build-system marker", "Makefile"],
+    ])("skips the scan with a %s in $HOME", async (_label, marker) => {
+      writeFileSync(join(home, marker), "");
+
+      expect(await detectDsn(home)).toBeNull();
+      expect((await detectAllDsns(home)).all).toHaveLength(0);
+    });
+
+    test("skips the scan with a VCS root at $HOME", async () => {
+      mkdirSync(join(home, ".git"), { recursive: true });
+
+      expect(await detectDsn(home)).toBeNull();
+      expect((await detectAllDsns(home)).all).toHaveLength(0);
+    });
+
+    test("skips the scan when a marker-less subdirectory walks up to $HOME", async () => {
+      writeFileSync(join(home, "package.json"), "{}");
+      const workspace = join(home, ".agent", "workspaces", "w1");
+      mkdirSync(workspace, { recursive: true });
+
+      expect(await detectDsn(workspace)).toBeNull();
+      expect((await detectAllDsns(workspace)).all).toHaveLength(0);
+    });
+  });
+
+  describe("from a shared temp root", () => {
+    test("does not scan the temp root itself", async () => {
+      const tmpRoot = join(getConfigDir(), "tmp");
+      fakeTmp.path = tmpRoot;
+      const mount = join(tmpRoot, ".mount_app");
+      mkdirSync(mount, { recursive: true });
+      writeFileSync(
+        join(mount, "config.ts"),
+        `Sentry.init({ dsn: "https://mnt@o555.ingest.sentry.io/555" });`,
+      );
+
+      expect(await detectDsn(tmpRoot)).toBeNull();
+      expect((await detectAllDsns(tmpRoot)).all).toHaveLength(0);
+    });
+
+    test("still scans a project directory inside the temp root", async () => {
+      const tmpRoot = join(getConfigDir(), "tmp");
+      fakeTmp.path = tmpRoot;
+      const project = join(tmpRoot, "checkout");
+      mkdirSync(join(project, ".git"), { recursive: true });
+      const projectDsn = "https://code@o666.ingest.sentry.io/666";
+      writeFileSync(
+        join(project, "config.ts"),
+        `Sentry.init({ dsn: "${projectDsn}" });`,
+      );
+
+      expect((await detectDsn(project))?.raw).toBe(projectDsn);
+    });
+  });
+
   describe("from an ancestor of $HOME", () => {
     let ancestor: string;