This document contains descriptions and guidelines for addressing security
vulnerabilities commonly identified in the GitLab codebase. They are intended
to help developers identify potential security vulnerabilities early, with the
goal of reducing the number of vulnerabilities released over time.
**Contributing**
If you would like to contribute to one of the existing documents, or add
guidelines for a new vulnerability type, please open an MR! Please try to
include links to examples of the vulnerability found, and link to any resources
used in defined mitigations. If you have questions or when ready for a review,
please ping `gitlab-com/gl-security/appsec`.
## Permissions
### Description
Application permissions are used to determine who can access what and what actions they can perform.
For more information about the permission model at GitLab, please see [the GitLab permissions guide](permissions.md) or the [EE docs on permissions](../../ee/user/permissions.md).
### Impact
Improper permission handling can have significant impacts on the security of an application.
Some situations may reveal [sensitive data](https://gitlab.com/gitlab-com/gl-infra/production/-/issues/477) or allow a malicious actor to perform [harmful actions](https://gitlab.com/gitlab-org/gitlab/-/issues/8180).
A common vulnerability when permission checks are missing is called [IDOR](https://owasp.org/www-project-web-security-testing-guide/latest/4-Web_Application_Security_Testing/05-Authorization_Testing/04-Testing_for_Insecure_Direct_Object_References) for Insecure Direct Object References.
Each time you implement a new feature/endpoint, whether it is at UI, API or GraphQL level.
### Mitigations
**Start by writing tests** around permissions: unit and feature specs should both include tests based around permissions
- Fine-grained, nitty-gritty specs for permissions are good: it is ok to be verbose here
- Make assertions based on the actors and objects involved: can a user or group or XYZ perform this action on this object?
- Consider defining them upfront with stakeholders, particularly for the edge cases
- Do not forget **abuse cases**: write specs that **make sure certain things can't happen**
- A lot of specs are making sure things do happen and coverage percentage doesn't take into account permissions as same piece of code is used.
- Make assertions that certain actors cannot perform actions
- Naming convention to ease auditability: to be defined, e.g. a subfolder containing those specific permission tests or a `#permissions` block
Be careful to **also test [visibility levels](https://gitlab.com/gitlab-org/gitlab-foss/-/blob/master/doc/development/permissions.md#feature-specific-permissions)** and not only project access rights.
Some example of well implemented access controls and tests:
Unlike other programming languages (e.g. Perl or Python) Regular Expressions are matching multi-line by default in Ruby. Consider the following example in Python:
The Python example will output an empty array (`[]`) as the matcher considers the whole string `foo\nbar` including the newline (`\n`). In contrast Ruby's Regular Expression engine acts differently:
The output of this example is `#<MatchData "bar">`, as Ruby treats the input `text` line by line. In order to match the whole __string__ the Regex anchors `\A` and `\z` should be used.
GitLab specific examples can be found [here](https://gitlab.com/gitlab-org/gitlab/-/issues/36029#note_251262187) and [there](https://gitlab.com/gitlab-org/gitlab/-/issues/33569).
Another example would be this fictional Ruby On Rails controller:
```ruby
class PingController <ApplicationController
def ping
if params[:ip] =~ /^\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}$/
render :text => `ping -c 4 #{params[:ip]}`
else
render :text => "Invalid IP"
end
end
end
```
Here `params[:ip]` should not contain anything else but numbers and dots. However this restriction can be easily bypassed as the Regex anchors `^` and `$` are being used. Ultimately this leads to a shell command injection in `ping -c 4 #{params[:ip]}` by using newlines in `params[:ip]`.
#### Mitigation
In most cases the anchors `\A` for beginning of text and `\z` for end of text should be used instead of `^` and `$`.
Consider the following example application, which defines a check using a regular expression. A user entering `user@aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa!.com` as the email on a form will hang the web server.
```ruby
class Email <ApplicationRecord
DOMAIN_MATCH = Regexp.new('([a-zA-Z0-9]+)+\.com')
validates :domain_matches
private
def domain_matches
errors.add(:email, 'does not match') if email =~ DOMAIN_MATCH
end
```
### Mitigation
GitLab has `Gitlab::UntrustedRegexp` which internally uses the [`re2`](https://github.com/google/re2/wiki/Syntax) library.
By utilizing `re2`, we get a strict limit on total execution time, and a smaller subset of available regex features.
- Try to be as precise as possible in your regex and avoid the `.` if something else can be used (e.g.: Use `_[^_]+_` instead of `_.*_` to match `_text here_`).
An example can be found [in this commit](https://gitlab.com/gitlab-org/gitlab/commit/717824144f8181bef524592eab882dd7525a60ef).
- [The impact of regular expression denial of service (ReDoS) in practice: an empirical study at the ecosystem scale](http://people.cs.vt.edu/~davisjam/downloads/publications/DavisCoghlanServantLee-EcosystemREDOS-ESECFSE18.pdf). This research paper discusses approaches to automatically detect ReDoS vulnerabilities.
- [Freezing the web: A study of redos vulnerabilities in JavaScript-based web servers](https://www.usenix.org/system/files/conference/usenixsecurity18/sec18-staicu.pdf). Another research paper about detecting ReDoS vulnerabilities.
- Reading internal services, including cloud service metadata.
- The latter can be a serious problem, because an attacker can obtain keys that allow control of the victim's cloud infrastructure. (This is also a good reason
- When the application makes any outbound connection
### Mitigations
In order to mitigate SSRF vulnerabilities, it is necessary to validate the destination of the outgoing request, especially if it includes user-supplied information.
The preferred SSRF mitigations within GitLab are:
1. Only connect to known, trusted domains/IP addresses.
1. Use the [GitLab::HTTP](#gitlab-http-library) library
The [GitLab::HTTP](https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/http.rb) wrapper library has grown to include mitigations for all of the GitLab-known SSRF vectors. It is also configured to respect the
`Outbound requests` options that allow instance administrators to block all internal connections, or limit the networks to which connections can be made.
In some cases, it has been possible to configure GitLab::HTTP as the HTTP
For situations in which an allowlist or GitLab:HTTP cannot be used, it will be necessary to implement mitigations directly in the feature. It is best to validate the destination IP addresses themselves, not just domain names, as DNS can be controlled by the attacker. Below are a list of mitigations that should be implemented.
**Important Note:** There are many tricks to bypass common SSRF validations. If feature-specific mitigations are necessary, they should be reviewed by the AppSec team, or a developer who has worked on SSRF mitigations previously.
- Block connections to all localhost addresses
-`127.0.0.1/8` (IPv4 - note the subnet mask)
-`::1` (IPv6)
- Block connections to networks with private addressing (RFC 1918)
-`10.0.0.0/8`
-`172.16.0.0/12`
-`192.168.0.0/24`
- Block connections to link-local addresses (RFC 3927)
-`169.254.0.0/16`
- In particular, for GCP: `metadata.google.internal` -> `169.254.169.254`
- For HTTP connections: Disable redirects or validate the redirect destination
- To mitigate DNS rebinding attacks, validate and use the first IP address received
Cross site scripting (XSS) is an issue where malicious JavaScript code gets injected into a trusted web application and executed in a client's browser. The input is intended to be data, but instead gets treated as code by the browser.
XSS issues are commonly classified in three categories, by their delivery method:
The injected client-side code is executed on the victim's browser in the context of their current session. This means the attacker could perform any same action the victim would normally be able to do through a browser. The attacker would also have the ability to:
- potentially [obtain the victim's session tokens](https://youtu.be/2VFavqfDS6w?t=739)
- perform actions that lead to data loss/theft or account takeover
Much of the impact is contingent upon the function of the application and the capabilities of the victim's session. For further impact possibilities, please check out [the beef project](https://beefproject.com/).
### When to consider?
When user submitted data is included in responses to end users, which is just about anywhere.
### Mitigation
In most situations, a two-step solution can be utilized: input validation and output encoding in the appropriate context.
For any and all input fields, ensure to define expectations on the type/format of input, the contents, [size limits](https://youtu.be/2VFavqfDS6w?t=7582), the context in which it will be output. It's important to work with both security and product teams to determine what is considered acceptable input.
##### Validate input
- Treat all user input as untrusted.
- Based on the expectations you [defined above](#setting-expectations):
- Validate the [input size limits](https://youtu.be/2VFavqfDS6w?t=7582).
- Validate the input using an [allowlist approach](https://youtu.be/2VFavqfDS6w?t=7816) to only allow characters through which you are expecting to receive for the field.
Note that denylists should be avoided, as it is near impossible to block all [variations of XSS](https://owasp.org/www-community/xss-filter-evasion-cheatsheet).
Once you've [determined when and where](#setting-expectations) the user submitted data will be output, it's important to encode it based on the appropriate context. For example:
- Content placed inside HTML elements need to be [HTML entity encoded](https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#rule-1---html-escape-before-inserting-untrusted-data-into-html-element-content).
- Content placed into a JSON response needs to be [JSON encoded](https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#rule-31---html-escape-json-values-in-an-html-context-and-read-the-data-with-jsonparse).
- Content placed inside [HTML URL GET parameters](https://youtu.be/2VFavqfDS6w?t=3494) need to be [URL-encoded](https://cheatsheetseries.owasp.org/cheatsheets/Cross_Site_Scripting_Prevention_Cheat_Sheet.html#rule-5---url-escape-before-inserting-untrusted-data-into-html-url-parameter-values)
- [Additional contexts may require context-specific encoding](https://youtu.be/2VFavqfDS6w?t=2341).
### Additional info
#### Mitigating XSS in Rails
- [XSS Defense in Rails](https://youtu.be/2VFavqfDS6w?t=2442)
- [XSS Defense with HAML](https://youtu.be/2VFavqfDS6w?t=2796)
- [Validating Untrusted URLs in Ruby](https://youtu.be/2VFavqfDS6w?t=3936)
- [RoR Model Validators](https://youtu.be/2VFavqfDS6w?t=7636)
