GHSA-xv9x-fj9g-vj6h

### Details `RemoveDependency` in `routers/web/repo/issue_dependency.go` takes a `removeDependencyID` form parameter identifying the other issue by its global numeric ID, and fetches it with `issues_model.GetIssueByID(ctx, depID)` - no repository or permission check at all. It then calls `issues_model.RemoveIssueDependency(ctx, ctx.Doer, issue, dep, depType)` (`models/issues/dependency.go`), which deletes the dependency join row and then writes a comment referencing the removal, attributed to the calling user, onto the dependency record.

The sibling function in the very same file, `AddDependency`, does this correctly when the two issues are in different repos (which `ALLOW_CROSS_REPOSITORY_DEPENDENCIES`, on by default, permits):

```go if issue.RepoID != dep.RepoID { if !setting.Service.AllowCrossRepositoryDependencies { ... } depRepoPerm, err := access_model.GetDoerRepoPermission(ctx, dep.Repo, ctx.Doer) if !depRepoPerm.CanReadIssuesOrPulls(dep.IsPull) { return // you can't see this dependency } } ```

`RemoveDependency` has no equivalent block at all - it goes straight from resolving `dep` by ID to deleting the link, regardless of which repo `dep` lives in or whether the caller can see it. I confirmed this same code is present in the current latest release, v1.26.4.

### PoC Prerequisites: an account with write access to issues on some repo `ownerA/repoA`, and the global numeric issue ID of an issue in a private repo `repoB` that is (or was) legitimately dependency-linked to one of the attacker's issues in `repoA` (cross-repo dependencies are commonly used between related public/private repos, and `ALLOW_CROSS_REPOSITORY_DEPENDENCIES` defaults to enabled).

```bash curl -s -b "gitea_session=$ATTACKER_SESSION_COOKIE" -X POST \ --data-urlencode "removeDependencyID=<repoB_issue_global_id>" \ --data-urlencode "dependencyType=blockedBy" \ "https://TARGET_HOST/ownerA/repoA/issues/N/dependency/delete" # Expected: the dependency link is deleted and a "removed dependency" comment # authored by the attacker is added to the repoB issue, even though the # attacker has no read access to repoB. ```

### Impact This is a cross-repository IDOR / broken access control issue. An attacker can tamper with issue-tracking state (dependency relationships) and inject an attacker-authored comment into a private repository they cannot otherwise read or write to, crossing a trust boundary the "add" path explicitly enforces. Impact is bounded - it requires an existing dependency link and discloses no repository content - but it is a genuine unauthorized-write primitive across a private-repo boundary.

### Fix Add the same cross-repo permission check used in `AddDependency` (`access_model.GetDoerRepoPermission(ctx, dep.Repo, ctx.Doer).CanReadIssuesOrPulls(dep.IsPull)`) to `RemoveDependency` before allowing the deletion to proceed when `issue.RepoID != dep.RepoID`.

**If possible, please apply for a CVE number when publishing. I would greatly appreciate it.**