Dmitry VerkhoturovandGitHub
0e20861419
* fix(security): reject non-image content-types in image proxy and /picture/ to prevent stored XSS
The /api/v1/img proxy and /api/v1/picture/{user}/{id} endpoints emitted
http.DetectContentType on the served bytes as the response Content-Type. A
controlled upstream serving Content-Type: image/png with an HTML body passed
the upstream check (only the response header was inspected, not the body),
and the body bytes then sniffed back to text/html — so the proxy served the
attacker's HTML from the remark42 origin. Browsers honoured the declared
text/html and executed the response as a document with access to cookies and
CSRF tokens. Affected from v1.6.0 (April 2020) through v1.15.0; verified live
via published docker images.
Layered defense applied to both handlers:
- rest.SafeImgContentType (in backend/app/rest/) validates sniffed content
against a strict allowlist: image/png, image/jpeg, image/gif, image/webp,
image/bmp, image/x-icon. Anything else (HTML, XML, SVG, plain text,
octet-stream, or any future image type the stdlib sniffer may learn) is
rejected with no body echo. SVG is implicitly excluded — it sniffs as
text/xml or text/plain, never image/svg+xml, and SVG can execute scripts
when navigated to top-level. The previous octet-stream → image/* fallback
is gone.
- Per-endpoint Content-Security-Policy override sets
"default-src 'none'; sandbox; frame-ancestors 'none'" on every response
(success, 304, or error). Sandbox neuters scripts even if Content-Type
ever regresses. The same policy is also applied to all /api/v1/* via
apiCSPMiddleware as defense-in-depth.
- Content-Disposition: inline; filename="image" frames the response as a
file rather than a renderable document.
- /picture/ rejection paths set Cache-Control: no-store so 4xx responses
are never cached.
The defense headers and the strict ETag matcher are extracted as
rest.SetImageDefenseHeaders and rest.EtagMatches in the shared rest package
(consumed by both proxy/image and api/rest_public — no package cycle).
The /api/v1/img path additionally bumps the ETag to a versioned `"v2:..."`
so revalidating clients (top-level navigation, Ctrl+R, intermediaries) get
a fresh 200 instead of a 304 against poisoned pre-fix cached HTML.
DELIBERATE TRADEOFF: Cache-Control on /api/v1/img success responses remains
max-age=2592000 (30 days), unchanged from before. An aggressive "force
revalidate on every reuse" policy was prototyped during review but reverted
because the perf cost (a server round-trip on every image view, even with
304 saving the body bytes) outweighed the corner-case mitigation. The
realistic exposure of cache carryover is narrow: cache carryover only
affects users who navigated top-level to an attacker URL pre-fix and still
have it in their local cache — the normal <img> embed path cached text/html
but never executed it. Local browser caches that hold pre-fix bytes
continue to serve them until their 30-day TTL expires or are evicted under
memory pressure. The ETag bump reaches all clients that DO revalidate
during the cached lifetime (Ctrl+R, intermediaries, post-expiry use); for
the rest, exposure self-limits via cache expiry. Operators running a
CDN/edge cache in front of remark42 should purge /api/v1/img after deploy.
The /api/v1/img handler short-circuits on a matching current-version
If-None-Match before any store Load or upstream fetch, returning a bodyless
304 with the defense headers set. Safe because the 304 carries no body and
the client's cached bytes came from a prior validated 200; an attacker
fabricating an etag value can only short-circuit fetches for URLs they
themselves crafted. This avoids upstream DoS amplification when clients
revalidate on hot comment pages.
The /api/v1/img route was moved from the "open routes" group (which uses
middleware.NoCache, stripping If-None-Match from incoming requests) to the
"open routes, cached" group alongside /picture/ and /qr/telegram so the
304 revalidation path is no longer broken upstream of the handler.
The /picture/{user}/{id} endpoint does not need the v2 etag prefix. Upload
validates input format via readAndValidateImage and the serve path
re-validates the stored bytes via rest.SafeImgContentType. Bytes within
the resize dimension limits are preserved verbatim, so the browser defense
relies on the response headers (validated Content-Type + nosniff + strict
CSP + Content-Disposition: inline), not on byte normalization.
Global CSP: font-src data: → font-src 'none'. Audit confirmed no @font-face,
no base64 fonts, no icon-font library in the bundle. Drops an unnecessary
attack surface; no behavioural change.
Tests: TestImage_ContentTypeHandling table-tests a real PNG and attack
shapes (HTML claimed as image/png, image/jpeg, image/gif, image/svg+xml,
image/webp; svg with onload; html fragment; polyglot PNG+HTML), proving
the defense holds across arbitrary upstream Content-Type variation.
Polyglot case is intentionally served as image/png — the browser cannot
execute the trailing HTML when the response type is image/png with nosniff.
TestImage_ContentTypeHandling_CacheHit exercises the cache-hit branch with
attacker bytes preloaded into the store. TestImage_PerRequestRevalidation
alternates upstream PNG/HTML across four proxy calls to prove no trust
accumulates between requests. TestImage_RoutesUsingCachedImage asserts
cache-poisoning is caught at serve time. TestImage_EtagVersioned asserts
the v2 prefix invalidates pre-fix etags AND that the revalidation 304
triggers no store Load. TestImage_RevalidationSkipsIO proves the
short-circuit works even with no upstream reachable. TestSafeImgContentType
covers the allowlist directly. TestRest_LoadPictureDefenseHeaders and
TestRest_LoadPictureRejectsNonImage exercise the /picture/ endpoint.
TestRest_apiCSP covers the strict CSP middleware on JSON API + RSS routes;
TestRest_securityHeaders confirms /web/ HTML pages keep the global CSP.
Verified end-to-end against the dev docker image: the original demo URL
(arbitrary HTML claimed as image/png) now returns 415 application/json with
CSP/nosniff/Content-Disposition set, no XSS in the browser.
* fix(security): set Cache-Control: no-store on image-proxy error paths, sync stale route comment
Addresses two review comments on #2067:
1. Cache-Control: max-age=2592000 and Etag were set before the
load/download/validation block, so 404/400/415 error responses inherited
the 30-day cache TTL and the versioned etag — a transient failure (or an
intentionally triggered 415) would be pinned in browser/intermediary
caches for that TTL, keeping users locked out even after the underlying
cause was resolved. Now: etag is computed but not set as a header until
after validation succeeds; error paths route through sendImageProxyError
which sets Cache-Control: no-store and never sets Etag. The 304
short-circuit still sets both because that path serves the same validated
content the client already has cached.
2. The comment at rest.go:282 still described the prototyped
no-cache/must-revalidate Cache-Control policy that was reverted before
the PR landed. Updated to match the actual 30-day max-age behavior.
Tests: TestImage_ContentTypeHandling now asserts reject paths carry
Cache-Control: no-store and have no Etag header, and accept paths carry
the max-age=2592000 + v2: etag.
Remark42 
Remark42 is a self-hosted, lightweight and simple (yet functional) comment engine, which doesn't spy on users. It can be embedded into blogs, articles, or any other place where readers add comments.
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