From 3a2074b633868b28518d4d989f0f4fefcaa2cb41 Mon Sep 17 00:00:00 2001
From: morluto <76467478+morluto@users.noreply.github.com>
Date: Fri, 23 Oct 2026 18:46:54 +0800
Subject: [PATCH] fix(asn1): decode four-byte BER lengths unsigned

Accumulate length octets arithmetically so a four-octet length with bit 31 set remains a positive unsigned value and is rejected by the containing-range check rather than becoming a negative length.
---
 .../apkSignature/apkV1SignatureBlock.test.ts  | 21 +++++++++++++++++++
 src/domain/asn1Ber.ts                         |  3 ++-
 src/native/pe/PeCertificateParser.test.ts     | 17 +++++++++++++++
 src/native/pe/PeCertificates.fixture.ts       |  3 ++-
 4 files changed, 42 insertions(+), 2 deletions(-)

diff --git a/src/domain/apkSignature/apkV1SignatureBlock.test.ts b/src/domain/apkSignature/apkV1SignatureBlock.test.ts
index 14d091845e..2917190731 100644
--- a/src/domain/apkSignature/apkV1SignatureBlock.test.ts
+++ b/src/domain/apkSignature/apkV1SignatureBlock.test.ts
@@ -244,3 +244,24 @@ it("rejects non-OCTET children inside a constructed signature", () => {
     ).kind,
   ).toBe("malformed-pkcs7");
 });
+
+it.each([
+  ["primitive", 0x04],
+  ["constructed", 0x24],
+] as const)(
+  "rejects a high-bit four-octet BER length on a %s signature field",
+  (_kind, tag) => {
+    const signature = Uint8Array.of(tag, 0x84, 0x80, 0, 0, 0);
+    expect(
+      errorOf(
+        signedDataBlock({
+          certificates: [LEAF_CERTIFICATE],
+          signerInfos: [signerInfo({ names: LEAF_CERTIFICATE, signature })],
+        }),
+      ),
+    ).toEqual({
+      kind: "malformed-pkcs7",
+      detail: "ASN.1 value exceeds its parent.",
+    });
+  },
+);
diff --git a/src/domain/asn1Ber.ts b/src/domain/asn1Ber.ts
index 764865f754..fa66c64b98 100644
--- a/src/domain/asn1Ber.ts
+++ b/src/domain/asn1Ber.ts
@@ -90,7 +90,8 @@ export class Asn1BerReader {
       for (let index = 0; index < width; index += 1) {
         const byte = this.bytes[cursor];
         if (byte === undefined) return fail("Truncated ASN.1 length.");
-        length = (length << 8) | byte;
+        // Bitwise shifts sign-extend a four-octet length with bit 31 set.
+        length = length * 256 + byte;
         cursor += 1;
       }
     }
diff --git a/src/native/pe/PeCertificateParser.test.ts b/src/native/pe/PeCertificateParser.test.ts
index 13e5fd94e0..fe401dfac6 100644
--- a/src/native/pe/PeCertificateParser.test.ts
+++ b/src/native/pe/PeCertificateParser.test.ts
@@ -94,6 +94,23 @@ it("reads BER indefinite constructed ContentInfo and preserves certificate bytes
   );
 });
 
+it.each([
+  ["primitive", 0x04],
+  ["constructed", 0x24],
+] as const)(
+  "rejects a high-bit four-octet BER length on a %s SignerInfo signature",
+  (_kind, tag) => {
+    const payload = pkcs7SignedDataFixture(
+      "REA Fixture",
+      "REA Tests",
+      Buffer.from([tag, 0x84, 0x80, 0, 0, 0]),
+    );
+    expect(() => parse(peCertificateFixture(false, payload))).toThrow(
+      /PKCS#7 signedData is malformed: ASN\.1 value exceeds its parent\./u,
+    );
+  },
+);
+
 it("matches nested indefinite constructed values and rejects invalid EOC forms", () => {
   const nested = readAsn1(
     Buffer.from([0x30, 0x80, 0x30, 0x80, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00]),
diff --git a/src/native/pe/PeCertificates.fixture.ts b/src/native/pe/PeCertificates.fixture.ts
index 4e526435b3..0960d675ea 100644
--- a/src/native/pe/PeCertificates.fixture.ts
+++ b/src/native/pe/PeCertificates.fixture.ts
@@ -66,6 +66,7 @@ const algorithm = (identifier: Buffer): Buffer => seq(identifier, nul());
 export const pkcs7SignedDataFixture = (
   commonName = "REA Fixture",
   organization = "REA Tests",
+  signature = octet(Buffer.from("sig")),
 ): Buffer => {
   const serial = Buffer.from([0x11, 0x22, 0x33, 0x44]);
   const subject = name(commonName, organization);
@@ -92,7 +93,7 @@ export const pkcs7SignedDataFixture = (
     seq(subject, integer(serial)),
     algorithm(SHA256),
     algorithm(SHA256_RSA),
-    octet(Buffer.from("sig")),
+    signature,
   );
   const signedData = seq(
     integer(1),