Defines **Cover** in the glossary and records ADR-0007, the decision behind #47's fix. ## Why these two files, and why now `CONTEXT.md` named Cover inside the **Series** entry — "facts true regardless of who is reading — title, cover, Latest Chapter" — but never said *what* one is. That gap is the bug. Nothing in the model distinguished "an address on a Site" from "an image a Reader's browser can display", so both clients were left to work it out independently, and one of them got it wrong. kagane serves covers with `cross-origin-resource-policy: same-origin`, the web UI rewrote them to a proxy in its templates, the JSON API did not, and the panel rendered a broken-image glyph. The new entry closes the ambiguity: *an address no client can load is not a Cover, it is a missing one.* ADR-0007 records what follows from that — the backend fetches, stores and serves every Site's cover bytes — plus the alternatives that were rejected and, more importantly, the two places this deliberately departs from existing precedent: - **Destination-class control instead of a host allowlist.** `fetchableSeriesURL` sets the allowlist precedent for `series_url`, and covers do not follow it. Cover hosts are CDNs that move independently of their Site — demonicscans serves its covers from `readermc.org` — so an allowlist would stop producing Covers the day a Site switched CDN, and that failure would look exactly like #47. The resolve-then-classify step is what actually stops the SSRF. - **A public cover route where the kagane proxy is session-gated.** An `<img>` cannot send a bearer token, and it cannot be given one either: the panel's shadow root is `mode: "open"`, so the host page's JavaScript can read any `src` the script sets. Both are security-adjacent departures, which is precisely why they are written down rather than left in a commit message. ## Scope Documentation only — no code, no schema, no behaviour. The implementation is #56–#63. ## Why this should merge promptly rather than sit All eight implementation tickets cite `docs/adr/0007-backend-hosts-cover-bytes.md` as the authority for decisions they must not relitigate, and they are written in the vocabulary this glossary entry defines. An agent picking up #56 reads both from `main`. Until this lands they get a 404 and either invent a rationale or stall — so this PR gates the tickets, not the other way round. Related: #47 (bug), #55 (spec), #54 (deferred admin refetch). Reviewed-on: #64 Co-authored-by: Sulthan Zaki <sultankiki05@gmail.com> Co-committed-by: Sulthan Zaki <sultankiki05@gmail.com>
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# ADR-0007: The backend hosts every Site's Cover bytes
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Date: 2026-08-09
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Status: accepted
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## Decision
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A Cover is the image a Reader's browser can display for a Series. Where the Site
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keeps the picture is the backend's problem, not the client's: the backend fetches
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the bytes, stores them, and serves them from its own origin. No client ever
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renders a third-party URL, and no client ever supplies one.
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Concretely:
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- **Acquisition is server-side.** The Cover is extracted from the same series-page
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fetch that already yields Latest Chapter. It runs once at Series creation rather
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than waiting for the poll queue, so a newly bookmarked Series has both facts in
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seconds instead of up to a queue's depth. The poll fills a blank Cover and never
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overwrites a non-blank one.
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- **Bytes live on a filesystem volume**, content-addressed by the SHA-256 of the
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source URL, sharded `${COVER_DIR}/ab/cd/<sha256>`. The database holds the path
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and content type, not the bytes.
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- **One public route** serves them. No session, no credential.
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- **The wire carries an absolute URL** built from a configured public base, and
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carries `""` until the bytes exist.
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## Why a future reader will find this surprising
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Four of the six Sites let anyone hot-link their covers — `static.comix.to` even
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answers `access-control-allow-origin: *`. Hosting copies looks like work we were
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not obliged to do.
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We were obliged. kagane serves covers with `cross-origin-resource-policy:
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same-origin` behind a JavaScript challenge (measured 2026-08-08), so no `<img>`
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outside kagane.to can load one under any combination of referrer policy and
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`crossorigin` attribute. The first fix for that was a kagane-only proxy applied in
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the web templates — and it produced issue #47, because the JSON API kept emitting
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the raw kagane URL and the userscript rendered it into a broken-image glyph. A
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per-Site exception that only one of two clients knows about is not a fix; it is a
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bug with a delay on it. Uniformity is the property being bought: every client
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renders every Cover the same way, and a Site changing its CORP header or its CDN
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cannot break a client again.
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## Considered options
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**Per-Site exceptions, proxying only what must be proxied.** Cheapest, and what we
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had. Rejected: it is what produced #47, and it requires every current and future
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client to know which Sites are special.
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**A host allowlist for the outbound fetch**, mirroring `fetchableSeriesURL`.
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Rejected in favour of destination-class control — see below.
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**Cover bytes in Postgres `bytea`**, extending the existing `covers` table.
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Rejected: covers are immutable blobs served straight to browsers, which is what a
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filesystem is for. The cost is real and accepted — durability is now two things to
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back up instead of one, against ADR-0001's grain.
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**Per-Reader Cover overrides.** Rejected, consistent with ADR-0003's rejection of
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per-Reader title overrides. A Cover is a fact about the Series.
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## Two deliberate relaxations
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**Destination control is deny-class, not an allowlist.** The outbound fetch
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requires `https`, resolves DNS first and refuses loopback, private, link-local and
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CGNAT addresses, re-checks on every redirect hop, and caps body size and content
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type. It does *not* pin a host set, which is what `fetchableSeriesURL` does for
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`series_url`. Cover hosts are CDNs that move: `demonicscans.org` serves its covers
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from `readermc.org`, a host with no visible relationship to the Site. An allowlist
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would silently stop producing Covers the day a Site switched CDN, and the failure
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would look like this bug. The resolved-IP check is the load-bearing part; without
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it, an attacker-controlled page need only publish a DNS name pointing at
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`127.0.0.1`.
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**The cover route is public, where the kagane proxy was session-gated.** An `<img>`
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in the userscript panel cannot send a bearer token, and it cannot be given one: the
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panel's shadow root is `mode: "open"`, so the host page's own JavaScript can read
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any `src` we set. A credential in an image URL is a credential handed to a
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third-party site. The route serves public artwork from public Sites and its path
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reveals nothing about which Reader holds what. The residual cost is that we can be
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hot-linked by others.
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## Consequences
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- The poll's cover prefetch, today guarded by `sr.Site != "kagane"`, applies to
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every Site in both Libraries. Nothing about Covers is conditioned on `kind`.
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- Only kagane still needs the CDP browser for its bytes. The other five Sites fetch
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over plain TLS — including novelfull, whose HTML answers `cf-mitigated: challenge`
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while its image paths answer 200 with `access-control-allow-origin: *`
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(measured 2026-08-09).
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- Client-side cover scraping is deleted from both userscripts. It could not help: a
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scraped URL has no render path left, and it is absent exactly when a Series is
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created — neither comix nor lightnovelworld exposes a cover on a chapter page,
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which is where a Reader bookmarks mid-read.
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- `PUT /bookmarks/{key}` still accepts a `cover` field and ignores it. This extends
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ADR-0003's "ignored after creation" to "ignored always", and keeps the flat wire
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contract ADR-0004 requires so installed scripts keep working. The field is
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therefore permanently inert rather than pending removal, and says so at the
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decode site.
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- A Cover that fails to load falls back to the placeholder in both clients. The
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broken-image glyph reported in #47 is not a state we render.
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- The existing kagane `covers` rows are dropped rather than migrated; that path
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re-fetches on demand already.
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- Two Sites deserve a note for whoever writes the extractor: asura's `.webp` cover
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URL answers `Content-Type: image/jpeg`, so trust the header; demonic's `og:image`
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carries a raw unencoded space and must be percent-encoded before fetching.
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