diff --git a/packages/cli/src/result-export.ts b/packages/cli/src/result-export.ts
index 8e65a54da..00606a4a1 100644
--- a/packages/cli/src/result-export.ts
+++ b/packages/cli/src/result-export.ts
@@ -1,7 +1,7 @@
 import { fileReferenceIdentity } from "@hypit/result/node";
 import { randomUUID } from "node:crypto";
 import { lstat, mkdir, rename, rm, writeFile } from "node:fs/promises";
-import { basename, dirname, join, relative, resolve, sep } from "node:path";
+import { basename, dirname, isAbsolute, join, relative, resolve, sep } from "node:path";
 
 import type {
   BuildResultFileRef,
@@ -35,7 +35,9 @@ async function pathExists(path: string): Promise<boolean> {
 function containedPath(root: string, path: string): string {
   const target = resolve(root, path);
   const relation = relative(root, target);
-  assert(relation.length > 0 && relation !== ".." && !relation.startsWith(`..${sep}`),
+  // A cross-drive or UNC target makes relative() return an absolute path on Windows,
+  // which cannot start with ".." — reject it the same way as an escaping one.
+  assert(relation.length > 0 && relation !== ".." && !relation.startsWith(`..${sep}`) && !isAbsolute(relation),
     `Result file ${path} leaves export directory ${root}`);
   return target;
 }
diff --git a/packages/protocol/src/build-id.ts b/packages/protocol/src/build-id.ts
index c6405d8e0..01f9f9cd7 100644
--- a/packages/protocol/src/build-id.ts
+++ b/packages/protocol/src/build-id.ts
@@ -47,6 +47,6 @@ export function assertOrderedBuildId(value: string): void {
 export function assertBuildId(value: string): void {
   assert(value.length > 0 && value === value.trim(), "Build id must be a non-empty unpadded name");
   assert(value !== "." && value !== "..", "Build id must not be a dot directory");
-  assert(!value.includes("/") && !value.includes("\\") && !value.includes("\0"),
+  assert(!value.includes("/") && !value.includes("\\") && !value.includes("\0") && !value.includes(":"),
     "Build id must be one path-free name");
 }
diff --git a/packages/protocol/test/canonical.test.ts b/packages/protocol/test/canonical.test.ts
index ca008ac3e..74a4ec8cd 100644
--- a/packages/protocol/test/canonical.test.ts
+++ b/packages/protocol/test/canonical.test.ts
@@ -35,7 +35,7 @@ test("Resource ids retain their narrow execution-reference syntax", () => {
 
 test("Build ids cannot address a parent or nested path", () => {
   assert.doesNotThrow(() => assertBuildId("build-19"));
-  for (const value of ["..", ".", "episode/19", "episode\\19", " padded "]) {
+  for (const value of ["..", ".", "episode/19", "episode\\19", " padded ", "c:", "d:build"]) {
     assert.throws(() => assertBuildId(value), /Build id/u);
   }
 });
diff --git a/packages/result/src/node/store.ts b/packages/result/src/node/store.ts
index 89382a340..c815aa574 100644
--- a/packages/result/src/node/store.ts
+++ b/packages/result/src/node/store.ts
@@ -207,7 +207,9 @@ export function buildResultDirectory(root: string, build: string): string {
   const bucket = buildResultDateBucket(build)!;
   const target = resolve(directory, bucket, build);
   const relation = relative(directory, target);
-  assert(relation.length > 0 && relation !== ".." && !relation.startsWith(`..${sep}`),
+  // Cross-drive and UNC targets make relative() return an absolute path, which
+  // cannot start with ".." — reject it the same way as an escaping one.
+  assert(relation.length > 0 && relation !== ".." && !relation.startsWith(`..${sep}`) && !isAbsolute(relation),
     `Build ${build} leaves Build Result root ${directory}`);
   return target;
 }
@@ -228,7 +230,7 @@ function containedResultPath(directory: string, path: string): string {
   const root = resolve(directory);
   const absolute = resolve(root, path);
   const relation = relative(root, absolute);
-  assert(relation !== ".." && !relation.startsWith(`..${sep}`),
+  assert(relation !== ".." && !relation.startsWith(`..${sep}`) && !isAbsolute(relation),
     `Build Result path ${path} leaves ${root}`);
   return absolute;
 }
diff --git a/packages/runtime-local/src/config.ts b/packages/runtime-local/src/config.ts
index bf2cf13b0..55a6fc861 100644
--- a/packages/runtime-local/src/config.ts
+++ b/packages/runtime-local/src/config.ts
@@ -1,7 +1,7 @@
 import { fileExecutionLogs } from "./log.js";
 import { constants } from "node:fs";
 import { access, readFile, rm, stat } from "node:fs/promises";
-import { dirname, relative, resolve, sep } from "node:path";
+import { dirname, isAbsolute, relative, resolve, sep } from "node:path";
 
 import {
   loadNodePackageSelection,
@@ -883,7 +883,7 @@ function buildWorkPath(root: string, build: string): string {
   const work = resolve(root, "work");
   const target = resolve(work, build);
   const relation = relative(work, target);
-  if (relation.length === 0 || relation === ".." || relation.startsWith(`..${sep}`)) {
+  if (relation.length === 0 || relation === ".." || relation.startsWith(`..${sep}`) || isAbsolute(relation)) {
     throw new Error(`Build ${build} leaves Runtime work root ${work}`);
   }
   return target;