diff --git a/src/native/pe/PeCertificateAsn1.ts b/src/native/pe/PeCertificateAsn1.ts
index 0b8f760798c..a78b3dd1117 100644
--- a/src/native/pe/PeCertificateAsn1.ts
+++ b/src/native/pe/PeCertificateAsn1.ts
@@ -389,6 +389,31 @@ const requireSubjectPublicKeyInfo = (node: Asn1BerNode): void => {
   requireBitString(key, "SubjectPublicKeyInfo subjectPublicKey");
 };
 
+// RFC 5280 4.1 and 4.2: an Extension is extnID, an optional critical BOOLEAN
+// and an extnValue OCTET STRING. A certificate MUST NOT repeat an extnID.
+const requireExtensions = (extensions: readonly Asn1BerNode[]): void => {
+  const seen = new Set<string>();
+  for (const extension of extensions) {
+    const fields = sequence(extension, "TBSCertificate extension");
+    const extnId = fields[0];
+    const critical = fields.length === 3 ? fields[1] : undefined;
+    const extnValue = fields[fields.length - 1];
+    if (
+      (fields.length !== 2 && fields.length !== 3) ||
+      extnId?.tag !== 0x06 ||
+      (critical !== undefined &&
+        (critical.tag !== 0x01 || critical.content.length !== 1)) ||
+      extnValue?.tag !== 0x04
+    )
+      return fail(
+        "TBSCertificate extension is not an OID, optional BOOLEAN, and OCTET STRING.",
+      );
+    const oid = decodeOid(extnId.content);
+    if (seen.has(oid)) return fail(`TBSCertificate repeats extension ${oid}.`);
+    seen.add(oid);
+  }
+};
+
 // issuerUniqueID [1], subjectUniqueID [2], and extensions [3] are optional
 // and ordered. Anything else is not a TBSCertificate field.
 const finishTbsCertificate = (
@@ -421,6 +446,7 @@ const finishTbsCertificate = (
       body.tag !== 0x30
     )
       return fail("TBSCertificate extensions are not a SEQUENCE.");
+    requireExtensions(body.children);
     cursor += 1;
   }
   if (cursor !== children.length)
diff --git a/src/native/pe/PeCertificateParser.test.ts b/src/native/pe/PeCertificateParser.test.ts
index 66bb62bb269..8e3e8a31e74 100644
--- a/src/native/pe/PeCertificateParser.test.ts
+++ b/src/native/pe/PeCertificateParser.test.ts
@@ -1060,3 +1060,59 @@ it("rejects a certificate whose signatureAlgorithm differs from its TBS signatur
     parseCertificate(certificate(SHA256_ALG)).signature_algorithm_oid,
   ).toBe("2.16.840.1.101.3.4.2.1");
 });
+
+it("rejects malformed and repeated certificate extensions", () => {
+  const name = directoryName(der(0x0c, Buffer.from("Fixture")));
+  const certificate = (...extensions: Buffer[]) =>
+    readAsn1(
+      derSeq(
+        derSeq(
+          der(0xa0, derInt(2)),
+          derInt(1),
+          SHA256_ALG,
+          name,
+          validity,
+          name,
+          spki,
+          der(0xa3, derSeq(...extensions)),
+        ),
+        SHA256_ALG,
+        der(0x03, Buffer.from([0])),
+      ),
+    );
+  const ski = der(0x06, Buffer.from([0x55, 0x1d, 0x0e]));
+  const skiValue = der(0x04, Buffer.from([0x04, 0x01, 0xaa]));
+  const basicConstraints = der(0x06, Buffer.from([0x55, 0x1d, 0x13]));
+  const basicConstraintsValue = der(0x04, Buffer.from([0x30, 0x00]));
+  const critical = (...content: number[]) => der(0x01, Buffer.from(content));
+  for (const extension of [
+    derInt(1),
+    derSeq(ski),
+    derSeq(skiValue),
+    derSeq(ski, derInt(1)),
+    derSeq(ski, derInt(1), skiValue),
+    derSeq(ski, critical(0xff, 0xff), skiValue),
+    derSeq(ski, skiValue, critical(0xff)),
+    derSeq(ski, skiValue, der(0x05, Buffer.alloc(0))),
+  ])
+    expect(() => parseCertificate(certificate(extension))).toThrow(
+      /TBSCertificate extension/u,
+    );
+  expect(() =>
+    parseCertificate(
+      certificate(
+        derSeq(ski, skiValue),
+        derSeq(basicConstraints, basicConstraintsValue),
+        derSeq(ski, skiValue),
+      ),
+    ),
+  ).toThrow(/repeats extension 2\.5\.29\.14/u);
+  expect(
+    parseCertificate(
+      certificate(
+        derSeq(ski, skiValue),
+        derSeq(basicConstraints, critical(0xff), basicConstraintsValue),
+      ),
+    ).subject.text,
+  ).toBe("commonName=Fixture");
+});