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feat(uploads)!: detectContentType(), and uploads check content types by default - #3019
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Priority: ➖ Normal Merge Risk: ⚪ Minimal · up to No actionable issue identified in the reviewed changes; the PR is mergeable after normal checks. Pre-merge checks ·
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| Check name | Status | Explanation | Resolution |
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| Docstring Coverage |  |
Docstring coverage is 65.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 60 functions across 21 files. (5 skipped:… | Write docstrings for the functions missing them to satisfy the coverage threshold. |
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Check name Status Explanation Linked Issues check Check skipped because no linked issues were found for this pull request. Out of Scope Changes check Check skipped because no linked issues were found for this pull request. Title check The title clearly summarizes the main changes: adding content-type detection and enabling content-type checks by default for uploads. Description check The description directly explains the detector, accepted-type rules, default verification behavior, affected upload flows, canonical MIME types, and opt-out configuration.
Full details: Docstring Coverage
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Docstring coverage is 65.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 60 functions across 21 files. (5 skipped: 5 unsupported.)
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Review comments at @packages/uploads/src/detectContentType.ts:
- Around line 331-334: In the EBML_DOC_TYPE_ID branch, cap size.value before
calling ascii so oversized DocType values are rejected without constructing a
large string. Remove trailing NUL bytes directly from the byte range before
converting it to text, and reject a range that extends beyond bytes.length;
replace the regex trimming in the DocType switch with this bounded parsing.
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.changesets/3019.mddocs/docs/uploads.mddocs/implementation-plans/2026-10-12-detect-content-type.mdpackages/uploads/README.mdpackages/uploads/src/__tests__/detectContentType.test.tspackages/uploads/src/__tests__/plugin.test.tspackages/uploads/src/detectContentType.tspackages/uploads/src/fastify/plugin.tspackages/uploads/src/index.ts
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…ecognised types - detectContentType() recognises OpenDocument (.odt/.ods/.odp, from the head alone) and Office Open XML (.docx/.xlsx/.pptx, from the ZIP central directory). Macro-enabled files (.docm/.xlsm/.pptm) are reported as their own types. A new `tail` option and DETECT_CONTENT_TYPE_TAIL_BYTES (64 KiB) let callers that read only a file's head pass its end too. - Matroska is reported as the registered video/matroska (RFC 9559). - CONTENT_TYPE_ALIASES (image/jpg, video/x-matroska, audio/x-m4a, …) is used everywhere types are compared, and accepted types are stored under their canonical spelling. - acceptedContentType() accepts unrecognised bytes on a claimed type that matches the accept list through a wildcard too, except claimed types the detector knows, and types that can carry script (SCRIPTABLE_CONTENT_TYPES and any +xml type), which must be listed exactly.
confirmUpload() and the S3 webhook read the object's head with a ranged GET (plus its tail for ZIPs) and check it against the type declared when the upload URL was issued. A mismatch fails the row and deletes the object; a match records the canonical spelling of the type. The object's Content-Type needs no rewrite: the presigned URL signs the declared type, and the bytes must be that type. createPresignedUpload() refuses wildcard declared types, and types that can carry script unless the profile lists them exactly. Each takes `verifyContentType` (default true). Storage providers gain an optional readRange(); the S3 provider implements it, and the check falls back to read() for providers that don't.
`verifyContentType` now defaults to true, for POST /upload/fs and the S3 webhook. Pass `verifyContentType: false` to keep trusting the claimed type.
…heir bytes Database uploads are the one place the generated service sees a file's bytes, so it uses acceptedContentType() instead of isMimeTypeAllowed() and stores the type that returns.
Tobbe
changed the title
…an the tail A large .pptx's central directory can be bigger than the 64 KiB tail, so it was refused once checking became the default. findZipCentralDirectory() reports where the directory starts from the end record, and the S3 check reads exactly the missing range and joins it to the tail. Whole files already worked. Directories over MAX_ZIP_DIRECTORY_BYTES (4 MiB) and ZIP64 archives are null.
…ed exactly MACRO_CONTENT_TYPES (the macro-enabled Office Open XML types, and the legacy .doc/.xls/.ppt types) never come in through a wildcard such as application/* or */*, whether detected from the bytes or claimed for bytes the detector doesn't recognise. Macros run in Office on the downloader's machine, and a "documents" profile almost never means them.
…d what S3 checks against
…lookalikes - An ftyp box over 1 KiB is null, so a forged box size can't make the compatible-brand scan walk a whole large upload. - EBML header children whose ID, size or value run past the declared header are null. - End-of-central-directory candidates are tried nearest-first (at most 16) until one leads to a directory that identifies a document, so a lookalike in the archive comment no longer hides the real record.
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Why
@cedarjs/uploadschecks an uploaded file against its profile by the filename and theContent-Typethe browser sent. Both are chosen by whoever uploads the file, so an HTML page uploaded ascat.pngwithContent-Type: image/pngpasses animage/*profile, gets stored as an image and is served back to other users.The only trustworthy signal is the file's own bytes (magic bytes, i.e. content sniffing in the WHATWG MIME Sniffing sense). Checking them is the default, following Cedar's "Secure by default" goal.
What
detectContentType(bytes)to@cedarjs/uploads: tells what kind of file some bytes are from their contents. Small and dependency-free (nofile-type).acceptedContentType(bytes, accept, claimedType?): the type to store, ornullwhen the file doesn't fit the accept list (rules below).verifyContentTypenow defaults totrue(opt out withfalse):POST /upload/fschecks every file against the profile by its bytes and stores it under the accepted type.confirmUpload()and the S3 webhook do a ranged GET of each direct-to-S3 upload's head (plus its tail for ZIPs) and check it against the type declared at presign time. A mismatch fails the row and deletes the object.createPresignedUpload()refuses wildcard declared types, and scriptable types reached only through a wildcard.uploadFileservice (database targets) usesacceptedContentType().image/jpg→image/jpeg,video/x-matroska→video/matroska,audio/x-m4a→audio/mp4, parameters dropped. One alias table (CONTENT_TYPE_ALIASES) is used for profile checks, accept lists and storage-key extensions.Implementation plan:
docs/implementation-plans/2026-10-12-detect-content-type.md.API
Accept rules
accept(wildcards and aliases included) and is returned. The claim is ignored, so a HEIC sent asapplication/octet-streamis accepted asimage/heic.accept, wildcards included, except:text/xml,application/xml, any+xml) must be listed exactly.MACRO_CONTENT_TYPES(.docm/.xlsm/.pptmand their template/add-in/slide-show variants,.xlsb, and the legacyapplication/msword,application/vnd.ms-excel,application/vnd.ms-powerpoint).application/*and*/*never admit them: macros run in Office on the downloader's machine, and a "documents" profile almost never means them.Formats
JPEG, PNG, GIF, WebP, BMP, TIFF, AVIF, HEIC/HEIF (stills and sequences), PDF, MP4, M4A, QuickTime/MOV (ISO-BMFF
ftypbrands), WebM and Matroska (EBMLDocType), and:.odt/.ods/.odp: a ZIP whose first entry is a storedmimetypefile. Decided from the head alone..docx/.xlsx/.pptx: a ZIP with[Content_Types].xmland parts under exactly one ofword/,xl/,ppt/, read from the central directory at the end of the file. Pass{ tail }when you only have the head; without it the head alone returnsnull, never a guess. When the directory is bigger than the 64 KiB tail (large decks),findZipCentralDirectory(tail)gives its offset and size, and the S3 check reads exactly the missing range. Directories over 4 MiB and ZIP64 archives arenull. Files with macros (vbaProject.bin, Excel 4.0 macro sheets) are reported as their.docm/.xlsm/.pptmtypes.Left out on purpose: SVG and other text formats, legacy OLE
.doc/.xls, other ZIPs, ZIP64.Known limits (documented)
Uploadrow has no profile name.Breaking changes, and what to do
Uploads now check each file's bytes by default (
verifyContentType: true). For most apps nothing needs to change: honest uploads of the types a profile allows still pass. These are the cases that behave differently:.jpgstill passes animage/*profile (and is stored asimage/png), but it fails a profile that only allowsimage/jpeg. If your users really do upload such files, widen the profile'sallowedMimeTypes.image/*no longer accepts SVG, andapplication/*or*/*no longer accept HTML, JavaScript, XML, or macro-capable Office files (.docm,.xlsm,.pptm, legacy.doc/.xls/.ppt…). If a profile should accept them, list each type exactly, e.g.['image/*', 'image/svg+xml'], and sanitize SVGs before serving them inline.createPresignedUpload()refuses wildcard declared types. A client asking for a presigned URL must declare a concrete type (image/png, notimage/*). Send the file's real type from the browser (file.type), falling back to a concrete type you allow.confirmUpload()and the S3 webhook now read the object's first 4 KiB (and, for a ZIP, its last 64 KiB) to check it, and a mismatch fails the upload and deletes the object. The api's S3 credentials need read access (s3:GetObject) on the bucket. A customStorageProviderwithoutreadRange()still works, but the check falls back toread(), which loads the whole object: addreadRange()if your files are large.image/jpegrather thanimage/jpg,video/matroskarather thanvideo/x-matroska,audio/mp4rather thanaudio/x-m4a, without parameters. Code that comparesupload.mimeTypeagainst an alias should compare against the canonical type, or usecanonicalContentType()on both sides. Existing rows aren't changed.uploadFileservice before this release keep their old service, which doesn't check bytes. Re-run the uploads setup command, or add theacceptedContentType()call from the new template (packages/cli/src/commands/setup/uploads/templates/uploads.ts.template) to your service.To keep the old behaviour while you sort things out, pass
verifyContentType: falseto the uploads plugin (covers/upload/fsand the S3 webhook),confirmUpload()andcreatePresignedUpload(). Uploads then trust the claimed type, as before. This is meant as a stopgap: without the check, anyone can upload an HTML page ascat.png.