fix: Verification of MS countersignatures now takes into account signer

info digest and verifies it against digest of content info (timestamp
info)

The signature of the signed attributes was verified, the digest of
countersigned signature was verified but what was missing is the
verification that the signed attributes are actually signing the
embedded timestamp information. This would allow anyone to take a valid
countersignature, replace content info of the signed data with their own
timestamp info, recalculate hash against the countersigned signature and
it would be verified as OK.

lib/src/modules/pe/authenticode.rs

@@ -345,13 +345,26 @@ impl AuthenticodeParser {
             countersignature.digest_alg = oid_to_str(tst.hash_algorithm.oid());
             countersignature.digest = tst.hashed_message;
 
+            let cs_si_digest = match cs_si
+                .get_signed_attr(&rfc5911::ID_MESSAGE_DIGEST)
+                .map(|value| value.data.as_bytes())
+            {
+                Some(md) => md,
+                None => return Err(ParseError::MissingAuthenticodeDigest),
+            };
+
             countersignature.verified =
                 verify_message_digest(
                     &tst.hash_algorithm,
                     si.signature,
                     tst.hashed_message,
+                ) && verify_message_digest(
+                    &cs_si.digest_algorithm,
+                    sd.content_info.content.as_bytes(),
+                    cs_si_digest,
                 ) && verify_signer_info(cs_si, certificates.as_slice());
 
+
             countersignatures.push(countersignature);
         }
 

lib/src/modules/pe/tests/testdata/130f9b2e65bfceae8d9e7cbe205471fc8ee34c3d9996f77baa3c3ab47a3b3068.out

@@ -689,7 +689,7 @@ signatures:
         not_before: 1513987200 # 2017-12-23 00:00:00 UTC
         not_after: 1868918399 # 2029-03-22 23:59:59 UTC
     countersignatures:
-      - verified: true
+      - verified: false
         sign_time: 1634237249 # 2021-10-14 18:47:29 UTC
         digest: "a6d56d546fb347a798239820467f4871bdd4c2f331548f4fdf5ab961a46c5c45"
         digest_alg: "sha256"