IDOR: Cross-Repository Comment Deletion via DeleteComment

Summary

The POST /:owner/:repo/issues/comments/:id/delete endpoint does not verify that the comment belongs to the repository specified in the URL. This allows a repository administrator to delete comments from any other repository by supplying arbitrary comment IDs, bypassing authorization controls.

Vulnerability Details

Field	Value
Affected File	internal/route/repo/issue.go
Affected Function	DeleteComment (lines 955-968)
Secondary File	internal/database/comment.go
Secondary Function	DeleteCommentByID (lines 505-520)

Root Cause

The vulnerability exists due to insufficient authorization validation in the comment deletion flow:

1. Missing Repository Ownership Check in DeleteComment

In internal/route/repo/issue.go, the function retrieves a comment by ID without verifying repository ownership:

func DeleteComment(c *context.Context) {
    comment, err := database.GetCommentByID(c.ParamsInt64(":id"))
    if err != nil {
        c.NotFoundOrError(err, "get comment by ID")
        return
    }

    // Only checks if user is comment poster OR admin of the CURRENT repo (from URL)
    if c.UserID() != comment.PosterID && !c.Repo.IsAdmin() {
        c.NotFound()
        return
    } else if comment.Type != database.CommentTypeComment {
        c.Status(http.StatusNoContent)
        return
    }

    // No verification that comment.IssueID belongs to c.Repo.Repository.ID!
    if err = database.DeleteCommentByID(c.User, comment.ID); err != nil {
        c.Error(err, "delete comment by ID")
        return
    }

    c.Status(http.StatusOK)
}

2. Database Layer Performs No Authorization

In internal/database/comment.go, the deletion function performs no repository validation:

func DeleteCommentByID(doer *User, id int64) error {
    comment, err := GetCommentByID(id)
    if err != nil {
        if IsErrCommentNotExist(err) {
            return nil
        }
        return err
    }

    // Directly deletes without checking repository ownership
    sess := x.NewSession()
    defer sess.Close()
    if err = sess.Begin(); err != nil {
        return err
    }

    if _, err = sess.ID(comment.ID).Delete(new(Comment)); err != nil {
        // ...
    }
    // ...
}

Proof of Concept

Prerequisites

1. Two users: Alice (attacker) and Bob (victim)
2. Alice is admin of alice/attacker-repo
3. Bob has created an issue with a comment on bob/victim-repo
4. Attacker needs to obtain the comment ID from victim's repository (e.g., ID: 42)

HTTP Request

POST /alice/attacker-repo/issues/comments/42/delete HTTP/1.1
Host: gogs.example.com
Cookie: i_like_gogs=<alice_session_token>