diff --git a/crates/native-sidecar/tests/builtin_conformance.rs b/crates/native-sidecar/tests/builtin_conformance.rs
index aea2370d7a..2e219e05f6 100644
--- a/crates/native-sidecar/tests/builtin_conformance.rs
+++ b/crates/native-sidecar/tests/builtin_conformance.rs
@@ -69,6 +69,7 @@ const BUILTIN_CONFORMANCE_CASES: &[&str] = &[
     "events",
     "stream",
     "buffer",
+    "global_base64",
     "url",
     "stdlib_polyfill",
     "web_streams",
@@ -3834,6 +3835,45 @@ fn buffer_concat_truncation_matches_host_node() {
     run_isolated_builtin_conformance_test("buffer-concat-truncation");
 }
 
+fn global_base64_conformance_matches_host_node() {
+    assert_conformance(
+        "global_base64",
+        r#"
+function describeError(callback) {
+  try {
+    callback();
+    return { threw: false };
+  } catch (error) {
+    return {
+      threw: true,
+      name: error?.name ?? null,
+      code: error?.code ?? null,
+      message: error?.message ?? null,
+    };
+  }
+}
+
+console.log(JSON.stringify({
+  atobText: atob("aGVsbG8="),
+  atobUnpaddedSingleByte: atob("YQ"),
+  atobUnpaddedTwoBytes: atob("YWI"),
+  atobWhitespace: atob(" YQ== \n"),
+  atobNumberCoercion: atob(1234),
+  btoaText: btoa("hello"),
+  btoaNumberCoercion: btoa(1234),
+  invalidAtob: describeError(() => atob("%%%")),
+  invalidShortAtob: describeError(() => atob("Y")),
+  invalidPaddingAtob: describeError(() => atob("AA=A")),
+  invalidPartialPaddingAtob: describeError(() => atob("YQ=")),
+  invalidOnlyPaddingAtob: describeError(() => atob("==")),
+  invalidUrlSafeDashAtob: describeError(() => atob("AA-A")),
+  invalidUrlSafeUnderscoreAtob: describeError(() => atob("AA_A")),
+  invalidBtoa: describeError(() => btoa("✓")),
+}));
+"#,
+    );
+}
+
 fn mkdtemp_sync_collision_safe_matches_host_node_impl() {
     let cwd = temp_dir("mkdtemp-sync-collision-safe");
     let entrypoint = cwd.join("entry.mjs");
@@ -4874,6 +4914,7 @@ fn run_named_case(case_name: &str) {
         "events" => events_conformance_matches_host_node(),
         "stream" => stream_conformance_matches_host_node(),
         "buffer" => buffer_conformance_matches_host_node(),
+        "global_base64" => global_base64_conformance_matches_host_node(),
         "url" => url_conformance_matches_host_node(),
         "stdlib_polyfill" => stdlib_polyfill_conformance_matches_host_node(),
         "web_streams" => web_streams_conformance_matches_host_node(),
diff --git a/packages/build-tools/bridge-src/builtins/process.ts b/packages/build-tools/bridge-src/builtins/process.ts
index 726e7f66ce..c207d71691 100644
--- a/packages/build-tools/bridge-src/builtins/process.ts
+++ b/packages/build-tools/bridge-src/builtins/process.ts
@@ -1141,6 +1141,9 @@ function setupGlobals() {
   g.Event = Event;
   g.CustomEvent = CustomEvent;
   g.EventTarget = EventTarget;
+  if (typeof g.DOMException === "undefined") {
+    g.DOMException = SandboxDOMException;
+  }
   if (typeof g.Buffer === "undefined") {
     g.Buffer = Buffer3;
   }
@@ -1161,9 +1164,36 @@ function setupGlobals() {
   installBuiltinUtilFormatWithOptions(builtinUtilModule);
   if (typeof g.atob === "undefined" || typeof g.btoa === "undefined") {
     const base64 = require_base64_js();
+    const createInvalidCharacterError = (message = "Invalid character") => {
+      const error = new g.DOMException(message, "InvalidCharacterError");
+      if (error.code === 0) error.code = 5;
+      return error;
+    };
     if (typeof g.atob === "undefined") {
       g.atob = (value) => {
-        const bytes = base64.toByteArray(String(value));
+        // WHATWG forgiving-base64 decode accepts ASCII whitespace and
+        // unpadded input, but rejects the base64url alphabet. base64-js
+        // implements RFC 4648 section 4 instead, so normalize the input and
+        // reject URL-safe characters before handing it over.
+        const input = String(value).replace(/[\t\n\f\r ]+/g, "");
+        const hasPadding = input.includes("=");
+        if (/[^A-Za-z0-9+/=]/.test(input) || /={3,}/.test(input) || /=[^=]/.test(input)) {
+          throw createInvalidCharacterError();
+        }
+        const remainder = input.length % 4;
+        if (remainder === 1) {
+          throw createInvalidCharacterError("The string to be decoded is not correctly encoded.");
+        }
+        if (hasPadding && remainder !== 0) {
+          throw createInvalidCharacterError();
+        }
+        const normalizedInput = !hasPadding && remainder === 2 ? `${input}==` : !hasPadding && remainder === 3 ? `${input}=` : input;
+        let bytes = new Uint8Array(0);
+        try {
+          bytes = base64.toByteArray(normalizedInput);
+        } catch {
+          throw createInvalidCharacterError();
+        }
         let decoded = "";
         for (const byte of bytes) {
           decoded += String.fromCharCode(byte);
@@ -1178,7 +1208,7 @@ function setupGlobals() {
         for (let index = 0; index < input.length; index += 1) {
           const code = input.charCodeAt(index);
           if (code > 255) {
-            throw new TypeError("Invalid character");
+            throw createInvalidCharacterError();
           }
           bytes[index] = code;
         }
@@ -1195,9 +1225,6 @@ function setupGlobals() {
   if (typeof g.CryptoKey === "undefined") {
     g.CryptoKey = SandboxCryptoKey;
   }
-  if (typeof g.DOMException === "undefined") {
-    g.DOMException = SandboxDOMException;
-  }
   if (typeof g.crypto === "undefined") {
     g.crypto = builtinCryptoModule;
   } else {