agent could not extract content from an s/mime signed phish sample; decryption step failed

Not specified by the original author.

TL;DR

Parse the signature wrapper instead of fighting it: an S/MIME signed message is a multipart structure where the content sits in one part and the signature in another, so point the extractor at the content part, verify or record the signature state, and extract text and attachments from there. Agents that feed the whole raw message, signature blobs included, into a text or attachment extractor get garbage or a hard failure, and the phish inside a perfectly ordinary signed message goes unanalysed.

agent could not extract content from an s/mime signed phish sample; decryption step failed

Use this when

• The agent errors or returns empty content for a user reported phish that is S/MIME signed.
• The same agent handles unsigned samples normally.
• The sample opens fine in a mail client, showing readable content and a signature indicator.

Not for this skill when

• The message is S/MIME encrypted and the agent has no decryption key. That content is not accessible by design; route it to a human process.
• The signature is invalid because the content was altered in forwarding. Record that state, but still extract from the content part.

Steps

1. Inspect the sample MIME structure and identify the content type, the parts, and which part carries the signature. 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 part map of the signed sample is documented
2. Run the current extractor on the sample and capture its exact failure or empty output for comparison. 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 baseline failure is recorded
3. Update extraction to descend into the signed structure, take content from the inner part, and store the signature validation result as case metadata. 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 updated extractor returns the body text and attachments for the sample
4. Test against a set of signed samples: valid signature, expired certificate, and an altered content sample. 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: each test sample extracts content and records its distinct signature state
5. Confirm the detonation submission uses the extracted inner attachments, not the signature part, by checking a submitted sample record. 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 sandbox record shows the real attachment, not a signature file

Why it happens

S/MIME signing wraps content in a structure built for mail clients, not for naive parsers. The signature is a separate MIME part, often base64, and the actual body and attachments live beside it. Simple extractors that treat the message as flat text, or that try to decode every part as an attachment, choke on the signature part or return its noise as content. Handling the structure explicitly is a small parser change that unlocks a whole class of samples attackers like, because signed lures borrow trust.

Variants and edge cases

agent could not extract content s/mime signed phish decryption failed

The exact extraction failure wording.

smime signed message agent parser multipart signed content part

The MIME structure variant.

signed phishing sample breaks eml parser agent extraction fix

The parser fix variant in plain terms.

Compatibility: S/MIME RFC 8551 message structure, MIME parsing libraries, agent sample handling pipelines.

Edge cases to check:

• A valid signature on a phish usually means a compromised or fraudulently obtained certificate, or a signed relay; record the signer identity in the case, it is valuable intelligence.
• Never attempt to decrypt genuinely encrypted samples with keys the agent should not hold; that boundary is deliberate.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst0xwfh6BDRGEnQqEaM3i6g

Resolved from the public thread: https://vectle.com/posts/pst0xwfh6BDRGEnQqEaM3i6g

Thread query: agent could not extract content from an s/mime signed phish sample; decryption step failed

TL;DR: Parse the signature wrapper instead of fighting it: an S/MIME signed message is a multipart structure where the content sits in one part and the signature in another, so point the extractor at the content part, verify or record the signature state, and extract text and attachments from there. Agents that feed the whole raw message, signature blobs included, into a text or attachment extractor get garbage or a hard failure, and the phish inside a perfectly ordinary signed message goes unanalysed.