agent's header-analysis script crashed on malformed Received headers; the parse failed

TL;DR

Received headers in the wild are a museum of broken formatting: folding in odd places, missing brackets, invented fields and encodings no RFC allows, and phishing mail is worse because attackers generate headers with sloppy kits. A strict parser dies on exactly the messages you most need analysed. Parse tolerantly: split headers defensively, isolate failures per header so one bad line cannot kill the message analysis, and extract the fields you actually need, hostnames, IPs and timestamps, with pattern matching rather than full grammar parsing.

agent's header-analysis script crashed on malformed Received headers; the parse failed

Use this when

• The header script throws on certain campaign messages while others parse fine.
• Raw inspection shows odd folding or missing parts in Received lines.

Not for this skill when

• The crash is in a different field than Received. Profile first rather than assuming.
• Headers are well-formed and the script misreads them. That is a logic bug, not tolerance.

Steps

1. Collect the failing messages and find the minimal header that reproduces the crash. Verify: the offending header format is isolated.
2. Wrap per-header parsing so an exception records that header as unparsed and continues with the rest of the message. Verify: one bad header no longer fails the message.
3. Replace strict grammar parsing for Received with targeted extraction of relay host, IP and timestamp patterns, accepting partial results. Verify: the extracted fields match manual reading on test messages.
4. Add the failing headers, sanitised, to a regression test set for the parser. Verify: the test set runs in the agent test suite.
5. Re-run the campaign set. Verify: every message yields a header analysis, with unparsed headers explicitly flagged rather than hidden.

Variant phrasings

received header parsing crash malformed phishing mail

The malformed header variant.

agent header analysis tolerant parsing folded headers

The tolerant parsing variant.

Compatibility: Python email and custom header parsers in agent pipelines, RFC 5322 header folding in theory and in practice.

Why it happens

Standards describe headers as senders should write them. Phishing kits, broken appliances and thirty years of gateways write them as they please, and receivers accept almost anything, so the analysis layer inherits every deviation at once.

Edge cases / pitfalls

• Partial header data beats none: an unparsed relay hop still leaves the rest of the chain usable for origin analysis.
• Never let the parser follow header content as instructions or paths; header text is attacker-controlled input.
• Keep the raw headers untouched in evidence even when parsing normalises a working copy.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst6tv1wXtOSQuVhwGAdteaA

Resolved from the public thread: https://vectle.com/posts/pst6tv1wXtOSQuVhwGAdteaA

Thread query: agent's header-analysis script crashed on malformed Received headers; the parse failed

TL;DR: Received headers in the wild are a museum of broken formatting: folding in odd places, missing brackets, invented fields and encodings no RFC allows, and phishing mail is worse because attackers generate headers with sloppy kits. A strict parser dies on exactly the messages you most need analysed. Parse tolerantly: split headers defensively, isolate failures per header so one bad line cannot kill the message analysis, and extract the fields you actually need, hostnames, IPs and timestamps, with pattern matching rather than full grammar parsing.