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server.go
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server.go
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//
// Challenge server for the "v9" Chromium pwnable of 34C3 CTF.
//
// Copyright (c) 2017 Samuel Groß
//
package main
import (
"bufio"
"bytes"
"context"
"crypto/rand"
"crypto/sha256"
"encoding/binary"
"encoding/hex"
"errors"
"fmt"
"github.com/docker/docker/api/types"
"github.com/docker/docker/api/types/container"
dockerapi "github.com/docker/docker/client"
"log"
"math"
"math/big"
"net"
"net/url"
"os"
"os/exec"
"strconv"
"strings"
"time"
)
type Client = bufio.ReadWriter
const (
host = "0.0.0.0"
port = "1337"
containerTimeout = 120 // in seconds
connectionTimeout = 30 * time.Minute
powHardness = 24 // number of leading zero bits in a sha256 hash
greeting = `Welcome!
In this challenge you are asked to pwn a modified Chromium and read the flag from /flag. You can find the challenge files here: https://34c3ctf.ccc.ac/uploads/v9-cc3610226678027367fef6992639abc1ee6dcaed.tar.gz
This challenge will work as follows:
1. I'll ask you for a proof-of-work
2. I'll ask you for a URL to your exploit and try to access it
3. I'll enqueue your URL
4. Once it's your turn, I'll run Chromium in a fresh container and point it to your URL
5. I'll destroy the container after %d seconds
Current length of the queue: %d
Enjoy!
`
)
var (
workdir string
workQueue chan string
docker *dockerapi.Client
)
func main() {
var err error
workdir, err = os.Getwd()
if err != nil {
log.Fatalf("Could not determine working directory: %v", err)
}
log.Printf("Working directory: %s", workdir)
docker, err = dockerapi.NewEnvClient()
if err != nil {
log.Fatal("Failed to create docker environment: %v", err)
}
workQueue = make(chan string, 1024)
go dockerWorker()
socket, err := net.Listen("tcp", host+":"+port)
if err != nil {
log.Fatalf("Error listening: %v", err)
}
defer socket.Close()
log.Printf("Listening on %v", host+":"+port)
for {
conn, err := socket.Accept()
if err != nil {
log.Fatalf("Error accepting: %v", err)
}
log.Printf("New connection from %v", conn.RemoteAddr())
conn.SetDeadline(time.Now().Add(connectionTimeout))
go handleClient(conn)
}
}
func dockerWorker() {
for {
url := <-workQueue
startContainer(url)
}
}
func startContainer(url string) {
ctx := context.Background()
resp, err := docker.ContainerCreate(ctx, &container.Config{
Image: "saelo/v9",
Cmd: []string{"chromium-browser", "--headless", "--disable-gpu", "--no-sandbox", "--virtual-time-budget=60000", url},
}, nil, nil, "")
if err != nil {
log.Printf("Failed to create container: %v", err)
return
}
if err := docker.ContainerStart(ctx, resp.ID, types.ContainerStartOptions{}); err != nil {
log.Printf("Failed to start container %s: %v", resp.ID, err)
return
}
waitCtx, cancel := context.WithTimeout(ctx, containerTimeout*time.Second)
defer cancel()
statusCh, errCh := docker.ContainerWait(waitCtx, resp.ID, container.WaitConditionNotRunning)
select {
case err := <-errCh:
log.Printf("Container %s timed out: %v", resp.ID, err)
if err := docker.ContainerKill(ctx, resp.ID, "SIGKILL"); err != nil {
log.Printf("Failed to kill container %s: %v", resp.ID, err)
}
case <-statusCh:
}
if err := docker.ContainerRemove(ctx, resp.ID, types.ContainerRemoveOptions{}); err != nil {
log.Printf("Failed to remove container %s: %v", resp.ID, err)
}
}
func randomString(n int) string {
b := make([]byte, (n+1)/2)
if _, err := rand.Read(b); err != nil {
log.Fatal("Failed to obtain random bytes")
}
return hex.EncodeToString(b)[:n]
}
func proofOfWork(client *Client) error {
challenge := randomString(16)
client.WriteString("Proof of work code & solver can be found at https://34c3ctf.ccc.ac/uploads/pow.py\n")
client.WriteString(fmt.Sprintf("Your challenge is %d_%s\n", int(math.Exp2(powHardness)), challenge))
client.WriteString("Your solution: ")
client.Flush()
response, err := client.Reader.ReadString('\n')
if err != nil {
return err
}
solution, err := strconv.ParseUint(strings.TrimSuffix(response, "\n"), 10, 64)
if err != nil {
client.WriteString("That's not a valid number... Please try again")
return err
}
var buf bytes.Buffer
buf.Write([]byte(challenge))
binary.Write(&buf, binary.LittleEndian, solution)
hashBytes := sha256.Sum256(buf.Bytes())
hash := big.NewInt(0)
hash.SetBytes(hashBytes[:])
zeroes := strings.Repeat("0", powHardness)
if !strings.HasPrefix(fmt.Sprintf("%0256b", hash), zeroes) {
client.WriteString("Invalid solution... Please try again")
return errors.New("Invalid solution")
}
return nil
}
func handleClient(conn net.Conn) {
defer conn.Close()
client := bufio.NewReadWriter(bufio.NewReader(conn), bufio.NewWriter(conn))
defer client.Flush()
client.WriteString(fmt.Sprintf(greeting, containerTimeout, len(workQueue)))
client.Flush()
err := proofOfWork(client)
if err != nil {
return
}
client.WriteString("Ok\n")
client.WriteString("Now please give me the URL to your exploit: ")
client.Flush()
rawUrl, err := client.ReadString('\n')
if err != nil {
return
}
url, err := url.Parse(strings.TrimSuffix(rawUrl, "\n"))
if err != nil || !url.IsAbs() {
client.WriteString("Hmm, this doesn't looke like a valid URL to me... Please try again")
return
}
log.Printf("Got URL: %v", url)
// Fetch the URL once to verify it is reachable
cmd := exec.Command("wget",
"-p", // fetch all required files
"-k", // rewrite links
"-P", fmt.Sprintf("%s/attempts/%d/", workdir, time.Now().Unix()), // set directory prefix
url.String())
if err := cmd.Start(); err != nil {
log.Printf("Failed to start wget: %v. This is probably bad...", err)
}
timer := time.AfterFunc(10*time.Second, func() { cmd.Process.Kill() })
if err := cmd.Wait(); err != nil {
client.WriteString("Hmm, it seems I could not access your URL... Please make sure it is reachable and try again")
return
}
timer.Stop()
client.WriteString("Ok\n")
client.WriteString("I will now enqueue your URL. You should get a visit from Chromium soon. Bye!\n")
client.Flush()
workQueue <- url.String()
}