content-type-mime-diff SKILL.md skillA
content-type-mime-diff from dreadnode/capabilities is an agent skill: a SKILL.md folder an AI agent loads when a task calls for it.
mdr add dreadnode/capabilities/content-type-mime-diff@git:20260504.2a64374Pins git:20260504.2a64374 in mdr.lock. Pin the fingerprint instead.
- Audit grade
- A, 17 of 17 checks passed
- Kind
- SKILL.md skill
- Repository
- dreadnode/capabilities
- Path
capabilities/web-security/skills/content-type-mime-diff/SKILL.md- Versions recorded
- 1
- Latest version
- git:20260504.2a64374, committed May 4, 2026
- Size
- 3,668 bytes
- Repository stars
- 22
- License
- MIT
MIME Content-Type parser differentials between browsers and server-side libraries for XSS. Browsers treat Content-Type as list-based (last value wins), while server-side parsers treat it as singleton (first value wins). Use when application validates Content-Type server-side but reflects it verbatim in response headers.
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
# Content-Type MIME Parser Differential -> XSS ## Root Cause - RFC 9110 defines `Content-Type` as a singleton field - Browsers such as Chrome and Firefox treat it as list-based: split on comma and take the last valid MIME type - Server-side MIME parsers usually take the first MIME type and ignore the rest - If the application validates the parsed type but sets the original unparsed value in the response header, the parser differential becomes exploitable ## Core Payload Pattern ``` application/json;,text/html ``` - Server-side parser sees `application/json` - Browser sees `application/json;` then `text/html` and renders as HTML ## Library-Specific Payloads ### Python `email.message` ``` application/json;,text/html ``` ### Python `googleapiclient.mimeparse` ``` application/json;=,text/html ``` or: ``` application/json;,text/html,= ``` ### Strict parsers Parsers like Express.js `content-type`, `busboy`, and `rigour.mime` may throw exceptions on invalid characters. In those cases, exploitability depends on error handling: - if the application falls back unsafely, the bug may still be reachable …
Read the whole file at its exact version.
How to install
mdr add dreadnode/capabilities/content-type-mime-diff@sha256:7799a52a5bf57157The line at the top of this page pins the label git:20260504.2a64374, 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_yr37jcvewdupchmk)
3 badge image fetches in 30 days. What the badge shows.
Versions
The registry has recorded 1 version of this file, committed on May 4, 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 (3668 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
dreadnode/capabilities · 22 stars · license MIT · pushed 2026-11-02 · branch main
API
GET https://markdownregistry.com/api/v1/artifacts/art_yr37jcvewdupchmk GET https://markdownregistry.com/api/v1/resolve?ref=dreadnode/capabilities/content-type-mime-diff GET https://markdownregistry.com/api/v1/blob/7799a52a5bf57157cfd044f7320543306d9bf7fb0e1866623fc1ee291babac56
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.