readAndValidateImage caps the byte size of incoming images but the resize()
helper that follows still called image.Decode unconditionally, allocating
pixel memory proportional to the *declared* image dimensions. A ~100 KB
compressed PNG or GIF that declares 65535x65535 px forces image.Decode to
allocate ~17 GB of raster, OOMing the service on a single comment upload
(or on the proxy's CacheExternal path when caching a malicious upstream).
Hardening:
- maxImagePixels = 16 MP constant. Covers any realistic image (~4096x4096)
while bounding peak allocation.
- resize() now runs image.DecodeConfig first (cheap, no pixel allocation)
to read declared width/height before any full decode.
- Multiplication of width × height uses int64 to defeat 32-bit overflow
(GOARCH=386, 32-bit arm): on those targets, int(cfg.Width)*int(cfg.Height)
could wrap below maxImagePixels and bypass the cap. GIF's 16-bit logical
screen and JPEG's 16-bit SOF dimensions both reach this if int-multiplied.
- Bytes exceeding the cap, or non-image input that fails DecodeConfig,
return nil. prepareImage propagates the rejection as a clear error
instead of storing the malformed/oversized data verbatim.
- The no-resize-needed path returns the validated original bytes verbatim
so animated GIFs round-trip without being flattened to a single frame.
The DecodeConfig precheck applies even when MaxWidth/MaxHeight are 0
(resize disabled) — the dimension cap is unconditional defense-in-depth.
Two adjacent fixes surfaced by the new resize contract:
1. readAndValidateImage previously did `data[:512]` without a bounds check,
panicking on any body shorter than 512 bytes. Now bounded with min().
2. image/webp was listed as an allowed format but no WebP decoder was
registered, so DecodeConfig would refuse legitimate WebP uploads. Added
`_ "golang.org/x/image/webp"` (already in go.mod via x/image/draw) so
the registered decoders match the allowlist.
Tests:
- TestService_resizeRejectsDecompressionBomb builds a 14-byte GIF87a header
declaring 65535x65535 and asserts resize() refuses it both at the unit
level and through SaveWithID end-to-end (no store write).
- TestService_SaveWithIDShortPayload regression-tests the short-body panic.
- TestService_SaveWithIDWebP regression-tests WebP round-trip through
prepareImage with the new DecodeConfig requirement.
- TestService_resize subtests updated to assert non-image bytes are now
refused (previously the helper fell back to returning the raw bytes
verbatim, letting malformed content reach the store).
Dmitry VerkhoturovandUmputun
8224626ed4
