SiYuan: Cross-boundary content disclosure via getBacklinkDoc/getBackmentionDoc (publish mode): reader-reachable rendered DOM of publish-forbidden docs; sibling list endpoints are filtered

CVE: This vulnerability corresponds to 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:

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.