agent's sha256 lookup missed because the hash was computed on the raw mime part instead of the decoded attachment

TL;DR

Hash what the user actually receives as a file: decode the base64 or quoted printable transfer encoding of the attachment part first, then compute SHA256 on those bytes, and store both the raw part hash and the file hash with clear labels if you need them. Feeds, sandboxes, and other tools all publish hashes of the decoded file, so an agent that hashes the encoded mail representation produces a unique snowflake hash for a well known malicious file, and every lookup misses.

agent's sha256 lookup missed because the hash was computed on the raw mime part instead of the decoded attachment

Use this when

• The agent reports no feed match for an attachment that a sandbox or a manual upload identifies instantly.
• The agent hash for the same file differs between mail samples that carry identical attachments in different encodings.
• Manual hashing of the saved attachment matches the feed entry.

Not for this skill when

• The attachment is a unique per recipient document, such as a personalised invoice. No feed match is expected there.
• The file is an image of the payload rather than the payload itself. Hash the image for reference, but detonation needs the target behind it.

Steps

1. Take a missed sample, save its attachment from a mail client, hash that file, and compare it with the agent recorded hash. Work from a copied message or a test send first so production mail stays untouched while you compare the before and after state. Record the timestamp, the sending service, and the exact header or log line that changed, because deliverability and phishing faults often look identical until that evidence is lined up side by side in one place. Verify: the two hashes differ, confirming the encoding fault
2. Inspect the agent extraction code path and find where hashing happens relative to transfer decoding. Do not change more than one setting at a time here. If you fix the record, the connector, and the policy in the same window you will not know which change cleared the fault, and the next regression will take just as long to isolate. Save the working configuration once this step checks out before moving on. Verify: the code location where the raw part is hashed is identified
3. Move hashing to after decoding, and add the decoded byte length to the case record as a sanity check against the mail client saved file. Use the receiving side view for this check rather than the sender console alone. Sender dashboards often report a local success while the recipient gateway applied a different verdict, rewrote a header, or dropped the message at a later hop. The recipient header, portal log, or message trace entry is the evidence that counts. Verify: the agent now records the same hash as the manual file hash for the sample
4. Regression test with the same attachment sent in base64 and in quoted printable forms. Keep any exception you create narrow and time boxed. Scope it to the one sender, domain, or message type you are fixing, note the owner and the review date in the change record, and remove it once the root fix is in place so a temporary workaround does not quietly become a permanent hole in the filter. Verify: both encodings produce the identical, correct file hash
5. Rehash the open case attachments and rerun their feed lookups to recover any matches missed so far. Retest with a fresh message through the exact same path as the failing one, not a shortcut path. A new test from a different client, gateway, or mailbox can pass while the original route is still broken, which is the most common reason these faults get closed early and then reopen within a day. Verify: the reprocessed cases show their corrected match results

Why it happens

Mail attachments travel encoded, and the encoded text depends on the encoder, line lengths, and the sending system. Hashing that representation measures the envelope, not the file. Every other tool in the chain, from feeds to sandboxes to a user saving the file, works on the decoded bytes, so one agent hashing early becomes the only system that cannot recognise a known file. It is a one line ordering bug with a disproportionate effect on detection confidence.

Variants and edge cases

agent sha256 lookup missed hash computed raw mime part decoded attachment

The exact ordering fault wording.

attachment hash mismatch base64 encoded part vs file hash agent

The encoding comparison variant.

known malicious attachment no feed match wrong hash triage agent

The missed match symptom variant.

Compatibility: MIME transfer encodings, SHA256 file identification, threat feed and sandbox hash lookups.

Edge cases to check:

• Some gateways re-encode attachments in transit; if your hash matches the decoded file the user got, that is the right reference regardless.
• Record hashes for extracted inner files of archives separately; the archive hash and the payload hash answer different questions.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_TdMm46UkEhD0jlePXMSHfw

Resolved from the public thread: https://vectle.com/posts/pst_TdMm46UkEhD0jlePXMSHfw

Thread query: agent's sha256 lookup missed because the hash was computed on the raw mime part instead of the decoded attachment

TL;DR: Hash what the user actually receives as a file: decode the base64 or quoted printable transfer encoding of the attachment part first, then compute SHA256 on those bytes, and store both the raw part hash and the file hash with clear labels if you need them. Feeds, sandboxes, and other tools all publish hashes of the decoded file, so an agent that hashes the encoded mail representation produces a unique snowflake hash for a well known malicious file, and every lookup misses.