Add Kestrel security considerations article#37328
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Publish a new Learn article covering Kestrel's security considerations, configurable limits, and request-parsing behavior, and add a TOC entry under the Kestrel node. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Spell out 'and' in section headings (replacing '&' and '/') so the OPS-generated bookmarks use clean single-dash anchors, and update the in-page links to match. Resolves the bookmark-not-found build warnings. Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
BrennanConroy
requested review from
DeagleGross,
halter73,
tdykstra and
wadepickett
wadepickett
reviewed
Jul 10, 2026
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| - Chunk format: `chunk-size [chunk-ext] CRLF chunk-data CRLF` | ||
| - Maximum chunk prefix length: 10 bytes (supports chunk sizes up to `0x7FFFFFFF`) | ||
| - Chunk extensions are validated—unpaired `\r` or `\n` in extensions cause rejection |
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| - Chunk extensions are validated—unpaired `\r` or `\n` in extensions cause rejection | |
| - Chunk extensions are validated—unpaired `\r` or `\n` in extensions cause rejection. |
Very minor: Complete sentence, so added a period per style guilde.
wadepickett
reviewed
Jul 10, 2026
| ### Request line validation | ||
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| - **Method**: Only valid HTTP token characters are accepted. Invalid token characters in the method cause rejection. | ||
| - **Request target**: Empty paths and paths starting with `?` or `%` are rejected. Null bytes (`0x00`) are rejected because they can cause issues with native servers when forwarded; non-ASCII bytes (>= `0x80`) are rejected during ASCII string conversion. Kestrel's parser scans for delimiters to find target boundaries but doesn't perform a per-byte VCHAR check (RFC 9112 §4), so bytes in `0x01–0x1F` (excluding `\0`, `\n`, `\r`, space) and `0x7F` (DEL) reach `Request.Path` unchanged. The practical risk is log injection—for example, `0x1B` (ESC) in the path can inject ANSI escape sequences into log viewers. Applications can defend by deploying behind a stricter reverse proxy, using a logger provider that escapes control characters, sanitizing logs before displaying them by escaping control characters. |
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| - **Request target**: Empty paths and paths starting with `?` or `%` are rejected. Null bytes (`0x00`) are rejected because they can cause issues with native servers when forwarded; non-ASCII bytes (>= `0x80`) are rejected during ASCII string conversion. Kestrel's parser scans for delimiters to find target boundaries but doesn't perform a per-byte VCHAR check (RFC 9112 §4), so bytes in `0x01–0x1F` (excluding `\0`, `\n`, `\r`, space) and `0x7F` (DEL) reach `Request.Path` unchanged. The practical risk is log injection—for example, `0x1B` (ESC) in the path can inject ANSI escape sequences into log viewers. Applications can defend by deploying behind a stricter reverse proxy, using a logger provider that escapes control characters, sanitizing logs before displaying them by escaping control characters. | |
| - **Request target**: Empty paths and paths starting with `?` or `%` are rejected. Null bytes (`0x00`) are rejected because they can cause issues with native servers when forwarded; non-ASCII bytes (>= `0x80`) are rejected during ASCII string conversion. Kestrel's parser scans for delimiters to find target boundaries but doesn't perform a per-byte VCHAR check (RFC 9112 §4), so bytes in `0x01–0x1F` (excluding `\0`, `\n`, `\r`, space) and `0x7F` (DEL) reach `Request.Path` unchanged. The practical risk is log injection—for example, `0x1B` (ESC) in the path can inject ANSI escape sequences into log viewers. Applications can defend against this with one or more of the following measures: deploying behind a stricter reverse proxy, using a logger provider that escapes control characters, or sanitizing logs before they're displayed. |
Is this meant to be one or more?
wadepickett
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Jul 10, 2026
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Looks great! Approved. See my two suggestions inline.
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@tdykstra, watch in case any timely review merge help needed, since I am not available for a week starting today. |
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Why
Kestrel has a large surface of security-relevant behaviors, configurable limits, and request-parsing decisions that weren't documented anywhere. This publishes that guidance as a proper Learn article so application developers configuring Kestrel in production and security auditors reviewing its threat surface have a single reference.
What
aspnetcore/fundamentals/servers/kestrel/security-considerations.md, a new article covering trust boundaries and reverse-proxy topologies, transport security (TLS/mTLS), HTTP/1.1, HTTP/2, and HTTP/3 limits, request smuggling and parsing validation, forwarded headers, host filtering, rate limiting, WebSockets, named pipes, monitoring/metrics, and a production security checklist.aspnetcore/toc.yml.Notes for reviewers
<xref:...>cross-references to avoid fabricating unverified API doc IDs, per the repo's guidance. These can be linked later once IDs are confirmed.not-latest-version-without-not-supported-content.mdinclude, which is the required variant for articles targeting>= aspnetcore-8.0.monikerRangeis set to>= aspnetcore-8.0; adjust if a different lower bound is preferred.This content is AI-assisted (
ai-usage: ai-assistedin frontmatter).Internal previews