fix(base64): validate base64 strings (#2779)

This updates the node and browser base64 parsers to validate the
input string before parsing.

packages/util-base64-browser/src/index.spec.ts

@@ -35,4 +35,10 @@ describe("fromBase64", () => {
   it("should convert double padded base64 strings to Uint8Arrays", () => {
     expect(fromBase64(b64DoublePadded)).toEqual(doublePadded);
   });
+
+  describe("should reject invalid base64 strings", () => {
+    it.each(["Rg", "Rg=", "[][]", "-_=="])("rejects '%s'", (value) => {
+      expect(() => fromBase64(value)).toThrowError();
+    });
+  });
 });

packages/util-base64-browser/src/index.ts

@@ -52,6 +52,11 @@ export function fromBase64(input: string): Uint8Array {
     let bitLength = 0;
     for (let j = i, limit = i + 3; j <= limit; j++) {
       if (input[j] !== "=") {
+        // If we don't check for this, we'll end up using undefined in a bitwise
+        // operation, in which it will be treated as 0.
+        if (!(input[j] in alphabetByEncoding)) {
+          throw new TypeError(`Invalid character ${input[j]} in base64 string.`);
+        }
         bits |= alphabetByEncoding[input[j]] << ((limit - j) * bitsPerLetter);
         bitLength += bitsPerLetter;
       } else {

packages/util-base64-node/src/index.spec.ts

@@ -43,4 +43,10 @@ describe("fromBase64", () => {
   it("should throw when given a number", () => {
     expect(() => fromBase64(0xdeadbeefface as any)).toThrow();
   });
+
+  describe("should reject invalid base64 strings", () => {
+    it.each(["Rg", "Rg=", "[][]", "-_=="])("rejects '%s'", (value) => {
+      expect(() => fromBase64(value)).toThrowError();
+    });
+  });
 });

packages/util-base64-node/src/index.ts

@@ -1,12 +1,28 @@
 import { fromArrayBuffer, fromString } from "@aws-sdk/util-buffer-from";
 
+const BASE64_REGEX = /^[A-Za-z0-9+/]*={0,2}$/;
+
 /**
  * Converts a base-64 encoded string to a Uint8Array of bytes using Node.JS's
  * `buffer` module.
  *
  * @param input The base-64 encoded string
  */
 export function fromBase64(input: string): Uint8Array {
+  // Node's buffer module allows padding to be omitted, but we want to enforce
+  // it. So here we ensure that the input represents a number of bits divisible
+  // by 8. Each character represents 6 bits, so after reducing the fraction we
+  // end up mulitplying by 3/4 and checking for a remainder.
+  if ((input.length * 3) % 4 !== 0) {
+    throw new TypeError(`Incorrect padding on base64 string.`);
+  }
+
+  // Node will just ingore invalid characters, so we need to make sure they're
+  // properly rejected.
+  if (!BASE64_REGEX.exec(input)) {
+    throw new TypeError(`Invalid base64 string.`);
+  }
+
   const buffer = fromString(input, "base64");
 
   return new Uint8Array(buffer.buffer, buffer.byteOffset, buffer.byteLength);