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proposal: cmd/go: define HTTP authentication extension mechanism #26232

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draftcode opened this Issue Jul 5, 2018 · 23 comments

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draftcode commented Jul 5, 2018

Problem

The custom import path mechanism (?go-get=1 and meta tag) works for public URLs, but it doesn't work for auth required URLs. This is because when go-get fetches the URL with ?go-get=1, it uses net/http.DefaultClient, and it doesn't know the credentials it needs to access the URL. A user cannot run go get against private source code hosting service because of this.

Goal

Make go get git.mycompany.com/private-repo work, where https://git.mycompany.com/private-repo requires authentication.

Idea 1 (credential helper)

Introduce a credential helper mechanism like git-credential-helpers. A user specifies a path to a binary via GOGET_CREDENTIAL_HELPER and go get executes that with the import path as an argument. The credential helper binary returns HTTP headers (like "Authorization: Basic blah\n", and go get adds these headers when it tries to fetch go-get=1 URLs.

  • PRO: Straightforward solution to the problem description.
  • CON: Supporting multiple credential helpers becomes complicated. Git's credential helper mechanism supports multiple credential helper, and Git runs each in order. This sometimes makes an unexpected behavior that is hard to debug.

Idea 2 (go-get helper)

Introduce a custom source code fetching mechanism. When go get needs to fetch the source code for the import path git.mycompany.com/private-repo, it looks for the binary go-get-git.mycompany.com based on the host name of the import path. When such binary exists in $PATH, it executes that binary with the import path and a destination in $GOPATH (for example, go-get-git.mycompany.com git.mycompany.com/private-repo $GOPATH/src/git.mycompany.com/private-repo). The binary is responsible for fetching the source code to the specified $GOPATH location.

  • PRO: As a side effect, this make go get work with VCSs other than git/hg/svn/bzr.
  • CON: I'm not sure how this works with go get -f or go get -insecure.

@gopherbot gopherbot added this to the Proposal milestone Jul 5, 2018

@gopherbot gopherbot added the Proposal label Jul 5, 2018

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draftcode commented Jul 5, 2018

Related issues: #16315 #11032

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rsc commented Jul 5, 2018

The original vgo prototype just read $HOME/.netrc. I'd rather do something like that than shell out to a whole separate program.

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draftcode commented Jul 6, 2018

.netrc works for HTTP basic auth with unlimited lifetime credentials. But it doesn't work for other types, like:

  • OAuth2 access tokens. OAuth2 access tokens usually have a limited lifetime, and it needs to be issued dynamically. Because of this, this cannot be in a static file.
  • Cookie based auth. netrc as a format is not a format that represent an HTTP cookie. Alternatively, Netscape's cookie file format (the same format as .gitcookies) can be used. This cannot support dynamically generated credentials as well. We actually have Git repositories that are behind dynamically generated cookie-based auth at Google (BeyondCorp at Google: https://cloud.google.com/beyondcorp/).
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rsc commented Jul 9, 2018

Note that even "access denied" pages can have meta tags - go get does not require a 200 response, exactly for this kind of thing. So git.mycompany.com could serve the tags unauthenticated as one workaround.

Are there any more general "HTTP auth credential helpers" in the world besides git-credential-helper? How do other systems deal with this? If there's something standard to hook into (like .netrc) then that's better than designing our own.

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draftcode commented Jul 9, 2018

If the access denied page contains the meta tag, it can be used as a prober. Let's say git.mycompany.com has an not-yet announced new product, collaborating with another company other-tech-company.com, and they host a Git repository at git.mycompany.com/ng-product/collab-other-tech-company. An attacker can probe if a repository exists by looking at the go-get's meta tag, and they can learn mycompany.com will have a business relationship other-tech-company.com to create a new product (and maybe benefit from this insider info).

As far as I know, there's no generic mechanism that majority of people use for storing and obtaining credentials through API. The closest thing I can think of is ssh-agent. Or if it's OS X, OS X Keychain.

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rsc commented Jul 23, 2018

It's only a prober if you check that the thing exists before serving the meta-tag.

$ curl 'https://rsc.io/asdgihopajdfklgadfglhifsdfj?go-get=1'
<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
<meta name="go-import" content="rsc.io/asdgihopajdfklgadfglhifsdfj git https://github.com/rsc/asdgihopajdfklgadfglhifsdfj">
<meta http-equiv="refresh" content="0; url=https://godoc.org/rsc.io/asdgihopajdfklgadfglhifsdfj">
</head>
<body>
Redirecting to docs at <a href="https://godoc.org/rsc.io/asdgihopajdfklgadfglhifsdfj">godoc.org/rsc.io/asdgihopajdfklgadfglhifsdfj</a>...
</body>
</html>
r$ 
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rsc commented Jul 23, 2018

Clearly we could use a better idea here but I'd like to have just one thing that's standard instead of inventing our own (that is, I want something like .netrc).

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draftcode commented Jul 27, 2018

It's only a prober if you check that the thing exists before serving the meta-tag.

I cannot interpret this. The problem is that an attacker can use this to extract the repository existence by using this meta-tag. If the page checks if the repository exists and change the response, it can be used as a prober.

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rsc commented Aug 13, 2018

If the page checks if the repository exists and change the response, it can be used as a prober.

Then don't check, and don't change the response.

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bradfitz commented Aug 13, 2018

Are there any more general "HTTP auth credential helpers" in the world besides git-credential-helper?

https://docs.docker.com/engine/reference/commandline/login/#credentials-store

https://github.com/GoogleCloudPlatform/docker-credential-gcr

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draftcode commented Aug 13, 2018

Then don't check, and don't change the response.

So the page should not do any auth, and should not contain meta tag? I thought you're suggesting to add a meta tag and in the next response you're saying not to add meta tag?

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draftcode commented Aug 13, 2018

An example leakage: https://play.golang.org/p/rCYzVWQSQni

We want to avoid this. By adding a meta-tag to the response as rsc suggests, the repository existence is leaked.

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bradfitz commented Aug 13, 2018

@draftcode, I assumed what was meant as you should advertise it for all URLs, including /android/device/g000gle/foo-bar-not-exist , regardless of whether it actually exists.

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draftcode commented Aug 13, 2018

I assumed what was meant as you should advertise it for all URLs, including /android/device/g000gle/foo-bar-not-exist , regardless of whether it actually exists.

Note that the third part in meta tag should be a git repository URL. This means, if a repository exists, the third part should be a valid Git repository path. In the example, I used example.com/android/device/google/marlin-kernel/foo/bar as an example package path. The valid meta tag for this is <meta name="go-import" content="example.com git http://example.com/android/device/google/marlin-kernel">, not <meta name="go-import" content="example.com git http://example.com/android/device/google/marlin-kernel/foo/bar">. Because of this, I can tell whether a repository exists by adding paths that wouldn't exist. When I make a request, if meta tag's returned path is modified, it means the repository exists.

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draftcode commented Aug 13, 2018

An example that for this: https://play.golang.org/p/OWb6zRUD02r

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rsc commented Aug 14, 2018

@draftcode I am saying that if you pick a trivial rule like "example.com/x/y/z/w redirects to the repo at git.example.com/x/y", then you can implement it with no Auth check and no repo validity check, so that it responds to any possible x/y/z/w with the same answer. Then there is no leakage. That was the point of my curl example above: there is no actual package at rsc.io/asdgihopajdfklgadfglhifsdfj but the curl still reads back meta tags for it.

If you need example.com/x/y/z/w to redirect to different git servers based on the exact values of x,y,z,w then that's more difficult of course and leakage is hard to avoid.

This is all just a suggested workaround. What I'd really like is to find out about some plausibly general (not git-specific, not docker-specific) semi-standard way to interact with credential helpers. Maybe none exists and we have to come up with something. But that would be my last choice.

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draftcode commented Aug 14, 2018

If you need example.com/x/y/z/w to redirect to different git servers based on the exact values of x,y,z,w then that's more difficult of course and leakage is hard to avoid.

OK. We've considered that option when we investigated how GitHub deals with this, and found out that GitHub won't allow slashes in a repository name, so they can do what you've said. We cannot do that, so filed this bug.

What I'd really like is to find out about some plausibly general (not git-specific, not docker-specific) semi-standard way to interact with credential helpers. Maybe none exists and we have to come up with something. But that would be my last choice.

So far, I've shown what Git does for a similar problem. @bradfitz showed what Docker and GCP does for a similar problem (ADC now works only for service accounts, so it's a bit different). If there's a some standard way to get a cred, considering the size of these tools' community, there should be some implementation of that, but it seems there's no such thing. In fact, Docker created a credential helper mechanism following what Git did. From these, such standard, if exists, is something that Git and Docker communities at least are not aware of and are not using. @rsc, @bradfitz, and I are also not aware of such generic way that would be called as "semi-standard", it seems.

@rsc rsc changed the title from proposal: make go-get work for private source code hosting services to proposal: cmd/go: define HTTP authentication extension mechanism Aug 20, 2018

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rsc commented Aug 20, 2018

Given that there doesn't seem to be an agreed-upon standard, I guess the next step is for someone to propose a design that Go should use. I looked at git credential helpers but stopped looking when I saw bash snippets.

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draftcode commented Aug 26, 2018

Questions on the requirements:

  • My impression is that @rsc strongly prefers using a file instead of executing a command and using stdin/stdout. Is this a hard requirement?
    • If this is a requirement, this means that Go cannot support OS-standard credential management mechanisms.
    • Also this makes it hard to protect credentials. If this is done in stdin/stdout, the credential manager side can do a necessary re-auth.
  • This is just "defining a protocol between Go's toolchain and a user-defined credential manager", right? We do not want to create a password manager like LastPass for this purpose.
  • With a user-defined credential manager, users should be able to:
    • Add an HTTP header (including cookies)
    • Add an SSL client certificate
    • Anything else?
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josharian commented Aug 26, 2018

My impression is that @rsc strongly prefers using a file instead of executing a command and using stdin/stdout. Is this a hard requirement?

He objected to bash snippets. It is possible (I do not know) that executing a binary rather than a shell might sit better with him.

If it were indeed command execution (a la EDITOR), then e.g. those of us who use 1password could use their command line support. I think macOS keychain has similar support.

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rsc commented Sep 19, 2018

Command-line execution seems necessary, since you want to lock things down and give cmd/go access to just one password, not your whole password set. Josh, what did you have in mind? Want to propose a starting point design?

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dmitris commented Sep 25, 2018

It would be great if we could achieve the stated above goal ("Make go get git.mycompany.com/private-repo work, where https://git.mycompany.com/private-repo requires authentication") without forcing the users to perform custom modifications to their ~/.netrc or ~/.gitconfig files (or installing extra binaries) - at least in the (I believe) common cases of GitHub [Enterprise] with SSH authentication (from the ssh agent).

Assuming the server returns a meta tag such as <meta name="go-import" content="git.mycompany.com/org/repo git https://git.mycompany.com/org/repo.git">, could the go tool try the following:

  • shell out git ls-remote -q https://git.mycompany.com/org/repo.git first (as now)
  • if it gets a response with an auth prompt: Username for 'https://git.mycompany.com':, try next the command with the corresponding ssh URL: git ls-remote -q ssh://git@git.mycompany.com/org/repo.git - and if it works, remember / cache that protocol for other git operations with the server (at least, within the same go invocation). (The git user must be standard for GitHub as it is indicated on https://help.github.com/articles/testing-your-ssh-connection/ and https://help.github.com/enterprise/2.14/user/articles/testing-your-ssh-connection/). This "retrial" step can be limited to the cases where server replies with Server: Github.com HTTP header ([1], [2]).

I believe this would make possible to use git with the SSH auth without having everyone to add the insteadOf stanzas in ~/.gitconfig on every server / VM or putting passwords in ~/.netrc etc. (To avoid this, we currently have to use a redirect server on an equivalent of go.mycompany.com that sends back <meta name="go-import" content="git.mycompany.com/org/repo git ssh://git@git.mycompany.com/org/repo"> - but it would be so nice to be able to use the "natural" git.mycompany.com/org/repo as imports / package names instead of go.mycompany.com and forcing everyone to learn and use the git.mycompany.com => go.mycompany.com replacement!) Having all Go users in the company to install an additional binary to bridge go and auth secrets would IMO be even more cumbersome (and the security team will likely consider it as yet another thing to worry about...).

I think it is an incredible strength of the Go tool that you can install and use it "right out of the box" - would love to see that feature and stellar user experience preserved and extended! 😄

[1] https://go-review.googlesource.com/c/go/+/33171 (abandoned; issue https://golang.org/issue/17898)
[2] Russ's comment - #24076 (comment)

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draftcode commented Sep 29, 2018

Assuming the server returns a meta tag

@dmitris This proposal is for the servers that cannot return meta tags without an auth (see #26232 (comment)). It seems to me that your problem is not related to this.

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