CVE-2026-68586: SiYuan: missing authz leaks forbidden doc content via backlink API

**CVE:** This vulnerability corresponds to [CVE-2026-68586](https://nvd.nist.gov/vuln/detail/CVE-2026-68586). ### Summary The backlink API splits into list endpoints (which documents reference a block) and content endpoints (the rendered text of those referencing blocks). The list endpoints apply a publish-access filter, the content endpoints do not. As a result, `/api/ref/getBacklinkDoc` and `/api/ref/getBackmentionDoc` return the rendered DOM of blocks belonging to a publish-forbidden document to an anonymous reader, with no access check. Both content endpoints are gated by `CheckAuth` only, reachable by the publish `RoleReader` token and by the anonymous account when `Publish.Auth.Enable` is `false`. ### Details The asymmetry between the list and content sides is the tell that this is an oversight, not intended behavior: | Endpoint | Returns | Publish-access filter | Route | |---|---|---|---| | `getBacklink` | list (`*Path`) | `FilterPathsByPublishAccess` - present | `CheckAuth` | | `getBacklink2` | list (`*Path`) | `FilterPathsByPublishAccess` - present | `CheckAuth` | | `getBacklinkDoc` | rendered DOM | none | `CheckAuth` | | `getBackmentionDoc` | rendered DOM | none | `CheckAuth` | `model/backlink.go` contains no publish-access reference anywhere, and `Backlink.DOM` is the rendered HTML of the referencing blocks. `getBacklinkDoc(defID, refTreeID)` returns the rendered content of blocks in `refTreeID` - including a publish-forbidden, publish-disabled, or password-protected document with no access check. The filtered list siblings (`getBacklink`/`getBacklink2`) demonstrate that the publish boundary is meant to apply to this data; the content endpoints simply omit it. A reader is not limited to the filtered backlink list, they call `getBacklinkDoc` directly with any `refTreeID`. This also yields a reference-existence oracle: the response reveals whether the forbidden document `refTreeID` references the block `defID`. ### Proof of Concept Reproduced on a local instance (SiYuan running locally, publish mode enabled on port 6808, publish Basic Auth disabled). Setup: a publish-forbidden document `D` (`REFTREEID`) whose body contains the unique marker `SECRET_MARKER_77` and which references a block `DEFID` in a separate known document. **1. Mark the target document publish-forbidden (admin action, the boundary that should block reads):** ``` POST http://127.0.0.1:6806/api/filetree/setPublishAccess Authorization: Token <admin-token> {"id":"REFTREEID","visible":false,"password":"","disable":true} ``` **2. Baseline: the list endpoint correctly hides the forbidden doc from the reader (anonymous, port 6808):** ``` POST http://127.0.0.1:6808/api/ref/getBacklink2 {"id":"DEFID","k":"","mk":""} ``` The returned backlinks do not include the forbidden document `D`, the list side is filtered. **3. Disclosure: the content endpoint returns the forbidden doc's blocks anyway (anonymous, port 6808):** ``` POST http://127.0.0.1:6808/api/ref/getBacklinkDoc {"defID":"DEFID","refTreeID":"REFTREEID","keyword":""} ``` Returns HTTP 200; `data.backlinks[].dom` contains `SECRET_MARKER_77`, the rendered content of the publish-forbidden document, returned to an anonymous reader. `getBackmentionDoc` behaves identically for mention-type references. ### Impact An anonymous reader (publish mode with auth disabled) or any publish `RoleReader`, can read the rendered content of a publish-forbidden document's referencing blocks, defeating a boundary the administrator explicitly configured, and can determine whether a forbidden document references a given block (a reference-existence oracle). **Precondition (stated honestly):** the request requires `refTreeID` (the forbidden document's ID) and `defID` (a block it references). `defID` may be any published/known block, so if the forbidden document references any public content, `defID` is known and only `refTreeID` need be supplied. Block/document IDs for forbidden documents are also obtainable from other `CheckAuth`-only endpoints that lack the publish-access filter (reported separately). This endpoint alone does not enumerate arbitrary documents; it discloses content once an ID is known. Impact is confidentiality-only: content disclosure plus a reference-existence oracle, no modification. No admin role, CSRF token, or write permission is required. Encrypted notebooks are out of scope. ### Suggested fix Apply the same publish-access check the list siblings use. In `getBacklinkDoc`/`getBackmentionDoc`, filter each `Backlink` by its source document's box/path via `CheckPathAccessableByPublishIgnore` plus the publish-password cookie check consistent with `FilterPathsByPublishAccess` in `getBacklink`/`getBacklink2`. Any endpoint returning rendered block DOM should enforce the same publish boundary as the corresponding list endpoint.

An attacker identifies a SiYuan instance running in publish mode (a common self-hosted note-sharing setup) with publish authentication disabled or with a valid RoleReader token. Through reconnaissance of publicly visible published documents, they learn a block ID (defID) that is referenced elsewhere, and separately learn or guess the tree ID (refTreeID) of a document the administrator marked forbidden/password-protected — potentially harvested from other unfiltered CheckAuth-only endpoints noted in the advisory as reported separately. The attacker sends a single unauthenticated POST to /api/ref/getBacklinkDoc (or getBackmentionDoc) with {defID, refTreeID}; the response returns HTTP 200 with the rendered DOM of the forbidden document's referencing blocks in the clear, with no password or access check enforced despite the document being explicitly locked down. The attacker can repeat this against multiple refTreeID values to both exfiltrate content and map which forbidden documents reference which blocks (a reference-existence oracle), without ever touching the correctly-filtered list endpoints that would have hidden the forbidden document.