testing-smtp-smuggling-and-email-spoofing SKILL.md skillA
testing-smtp-smuggling-and-email-spoofing from unboundcompute/security-agent-skills is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing@git:20260819.61df029Pins git:20260819.61df029 in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- unboundcompute/security-agent-skills
- Path
skills/testing-smtp-smuggling-and-email-spoofing/SKILL.md- Versions recorded
- 1
- Latest version
- git:20260819.61df029, committed Aug 19, 2026
- Size
- 8,711 bytes
- Repository stars
- 26
- License
- MIT
Test a mail setup for sender spoofing that survives authentication: SPF, DKIM, and DMARC records that exist but do not align or enforce, subdomains left unprotected, and the end-of-data desync known as SMTP smuggling, where an inbound and an outbound mail server disagree on where one message ends so a second message with a forged, auth-passing sender is smuggled in. Covers policy present but not enforced, alignment gaps between the envelope and header sender, missing subdomain policy, open…
Collected from public GitHub. Every version is saved exactly as it was and never changes. Install it at an exact version with the mdr tool, and every file is checked before it reaches your agent. The main file is checked against the SHA-256 fingerprint recorded here, and the others against the fingerprints GitHub keeps for its source commit. The automatic checks below grade the latest version, and the same checks always give the same file the same grade.
What the file says
# Testing SMTP smuggling and email spoofing: authenticated, and still forged Email authentication is a stack of three records that only stops spoofing if all three are present, aligned, and enforced - and even then only if every server on the delivery path agrees where one message ends and the next begins. The failures are quiet: a policy published but set to take no action, a signature that validates a domain that is not the visible sender, a subdomain with no policy, and the boundary desync where two servers parse the end-of-data sequence differently, letting a forged message ride inside a legitimate one and pass every check. You find them by testing what actually gets delivered and trusted, not by reading the records and assuming they hold. ## When to use - You are auditing a domain's email authentication or a mail server's message handling. - The domain publishes sender-authentication records, or receives mail through more than one hop. - You can send test mail into the path and observe what is accepted and how it is judged. ## Scope check Test mail authentication and delivery only for domains and servers you own or are authorized to …
Read the whole file at its exact version.
How to install
mdr add unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing@sha256:017678ad065e0dd1The line at the top of this page pins the label git:20260819.61df029, which is what most people want. This one pins the sha256 fingerprint of the contents instead, so it survives a label being moved. Either way the exact version is written to mdr.lock, and mdr install downloads and checks those contents again, so it installs them exactly or stops with an error.
[](https://markdownregistry.com/a/art_6fogzie7etko2zj3)
2 badge image fetches in 30 days. What the badge shows.
Versions
The registry has recorded 1 version of this file, committed on Aug 19, 2026.
Audit of the latest version
- pass: Frontmatter block present
- pass: Frontmatter declares a name
- pass: Frontmatter declares a description
- pass: Size between 200 bytes and 200 KB (8711 bytes)
- pass: No zero-width or bidi control characters
- pass: No instruction hidden inside an HTML comment
- pass: No link to an exfiltration or paste host
- pass: No credential-shaped string
- pass: No instruction to send local credentials anywhere
- pass: No text hidden with inline styles
- pass: No prompt-injection phrasing
- pass: No curl or wget piped into a shell
- pass: No recursive delete of root, home or parent
- pass: No instruction to read or print local credentials
- pass: No base64 blob over 200 characters
- pass: No link to a raw IP address
- pass: No script tag
Source
unboundcompute/security-agent-skills · 26 stars · license MIT · pushed 2026-09-08 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_6fogzie7etko2zj3 GET https://markdownregistry.com/api/v1/resolve?ref=unboundcompute/security-agent-skills/testing-smtp-smuggling-and-email-spoofing GET https://markdownregistry.com/api/v1/blob/017678ad065e0dd13104c19f7ba44b758245c546fd56d3377a71dd3a6a175ab2
Your AI assistant does the legwork. You hear about the deals worth your word. Hand yours the standing instructions at modelranch.com and it joins the network that reads files like this one.
Guides: the SKILL.md format and its required fields · what to check before you install a skill · how to pin a skill to an exact version · where to find agent skills · keeping a team on the same skills · which agent file does what.