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If you discover some domain (domain.tld) that is **being used by some service inside the scope** but the **company** has l**o**st the **ownership** of it, you can try to **register** it (if cheap enough) and let know the company. If this domain is receiving some **sensitive information** like a sessions cookie via **GET** parameter or in the **Referer** header, this is for sure a **vulnerability**.
A subdomain of the company is pointing to a **third-party service with a name not registered**. If you can **create** an **account** in this **third party service** and **register** the **name** being in use, you can perform the subdomain take over.
There are several tools with dictionaries to check for possible takeovers:
Subdomain takeover checks are included in BBOT's default subdomain enumeration. Signatures are pulled directly from [https://github.com/EdOverflow/can-i-take-over-xyz](https://github.com/EdOverflow/can-i-take-over-xyz).
### Subdomain Takeover Generation via DNS Wildcard
When DNS wildcard is used in a domain, any requested subdomain of that domain that doesn't have a different address explicitly will be **resolved to the same information**. This could be an A ip address, a CNAME...
For example, if `*.testing.com` is wilcarded to `1.1.1.1`. Then, `not-existent.testing.com` will be pointing to `1.1.1.1`.
However, if instead of pointing to an IP address, the sysadmin point it to a **third party service via CNAME**, like a **github subdomain** for example (`sohomdatta1.github.io`). An attacker could **create his own third party page** (in Gihub in this case) and say that `something.testing.com` is pointing there. Because, the **CNAME wildcard** will agree the attacker will be able to **generate arbitrary subdomains for the domain of the victim pointing to his pages**.
You can find an example of this vulnerability in the CTF write-up: [https://ctf.zeyu2001.com/2022/nitectf-2022/undocumented-js-api](https://ctf.zeyu2001.com/2022/nitectf-2022/undocumented-js-api)
Subdomain takeover is essentially DNS spoofing for a specific domain across the internet, allowing attackers to set A records for a domain, leading browsers to display content from the attacker's server. This **transparency** in browsers makes domains prone to phishing. Attackers may employ [_typosquatting_](https://en.wikipedia.org/wiki/Typosquatting) or [_Doppelganger domains_](https://en.wikipedia.org/wiki/Doppelg%C3%A4nger) for this purpose. Especially vulnerable are domains where the URL in a phishing email appears legitimate, deceiving users and evading spam filters due to the domain's inherent trust.
SSL certificates, if generated by attackers via services like [_Let's Encrypt_](https://letsencrypt.org/), add to the legitimacy of these fake domains, making phishing attacks more convincing.
Browser transparency also extends to cookie security, governed by policies like the [Same-origin policy](https://en.wikipedia.org/wiki/Same-origin_policy). Cookies, often used to manage sessions and store login tokens, can be exploited through subdomain takeover. Attackers can **gather session cookies** simply by directing users to a compromised subdomain, endangering user data and privacy.
Another aspect of subdomain takeover involves email services. Attackers can manipulate **MX records** to receive or send emails from a legitimate subdomain, enhancing the efficacy of phishing attacks.
Further risks include **NS record takeover**. If an attacker gains control over one NS record of a domain, they can potentially direct a portion of traffic to a server under their control. This risk is amplified if the attacker sets a high **TTL (Time to Live)** for DNS records, prolonging the duration of the attack.
Attackers might exploit unclaimed CNAME records pointing to external services that are no longer used or have been decommissioned. This allows them to create a page under the trusted domain, further facilitating phishing or malware distribution.
1.**Removing vulnerable DNS records** - This is effective if the subdomain is no longer required.
2.**Claiming the domain name** - Registering the resource with the respective cloud provider or repurchasing an expired domain.
3.**Regular monitoring for vulnerabilities** - Tools like [aquatone](https://github.com/michenriksen/aquatone) can help identify susceptible domains. Organizations should also revise their infrastructure management processes, ensuring that DNS record creation is the final step in resource creation and the first step in resource destruction.
For cloud providers, verifying domain ownership is crucial to prevent subdomain takeovers. Some, like [GitLab](https://about.gitlab.com/2018/02/05/gitlab-pages-custom-domain-validation/), have recognized this issue and implemented domain verification mechanisms.
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<summary><strong>Learn AWS hacking from zero to hero with</strong><ahref="https://training.hacktricks.xyz/courses/arte"><strong>htARTE (HackTricks AWS Red Team Expert)</strong></a><strong>!</strong></summary>
* If you want to see your **company advertised in HackTricks** or **download HackTricks in PDF** Check the [**SUBSCRIPTION PLANS**](https://github.com/sponsors/carlospolop)!
* Discover [**The PEASS Family**](https://opensea.io/collection/the-peass-family), our collection of exclusive [**NFTs**](https://opensea.io/collection/the-peass-family)
* **Join the** 💬 [**Discord group**](https://discord.gg/hRep4RUj7f) or the [**telegram group**](https://t.me/peass) or **follow** me on **Twitter** 🐦 [**@carlospolopm**](https://twitter.com/carlospolopm)**.**
* **Share your hacking tricks by submitting PRs to the** [**HackTricks**](https://github.com/carlospolop/hacktricks) and [**HackTricks Cloud**](https://github.com/carlospolop/hacktricks-cloud) github repos.
