Bypassing access controls using email address parsing discrepancies
The Bypassing Access Controls Using Email Address Parsing Discrepancies vulnerability is a sophisticated example of how inconsistencies between different parsing mechanisms can undermine otherwise well-intentioned access control restrictions. In this lab, the application attempts to restrict account registration to users with email addresses belonging to a trusted internal domain. The intention is to ensure that only legitimate users associated with the organization can create accounts and access privileged features. However, the application validates email addresses using one interpretation method while the underlying email delivery system parses them differently. This discrepancy creates an opportunity for attackers to craft specially encoded email addresses that pass validation but deliver confirmation emails to attacker-controlled inboxes.
The attack begins by identifying the registration restriction enforced by the application. When opening the registration page and attempting to create an account using an arbitrary external email address such as:
foo@exploit-server.net
the application immediately rejects the registration attempt and displays an error indicating that the email domain must belong to:
ginandjuice.shop
This confirms that the application performs server-side validation on the email domain during registration. The restriction appears straightforward: only email addresses ending in @ginandjuice.shop are accepted. At first glance, this seems like an effective way to limit account creation to trusted users.
The next phase of the attack involves investigating whether the validation logic can be bypassed using email encoding tricks. Email addresses and email headers can support multiple encoding schemes defined in RFC standards. One relevant mechanism is encoded-word syntax, which allows portions of text to be represented using alternate character sets and encodings. These encodings are commonly used in email headers to support non-ASCII characters.
The attacker first tests a common encoded-word format using ISO-8859-1 and Q encoding. The following email address is submitted:
=?iso-8859-1?q?=61=62=63?=foo@ginandjuice.shop
This decodes to:
abcfoo@ginandjuice.shop
The application rejects the registration and displays:
Registration blocked for security reasons.
This indicates that the application has specific defenses in place to detect and block common encoded-word manipulation attempts.
Next, the attacker tries the same approach using UTF-8 encoded-word syntax:
=?utf-8?q?=61=62=63?=foo@ginandjuice.shop
Again, the application blocks the request with the same error. This suggests that the validation logic specifically recognizes common encoded-word patterns such as ISO-8859-1 and UTF-8 and treats them as suspicious.
At this point, the attacker considers less commonly used encodings. One such encoding is UTF-7, an older and rarely used Unicode transformation format. UTF-7 is often overlooked because it is uncommon in modern systems. The attacker submits:
=?utf-7?q?&AGEAYgBj-?=foo@ginandjuice.shop
Unlike the previous attempts, this request does not trigger the security warning. The application accepts the input without complaining.
This observation is crucial because it reveals that the validation logic does not recognize UTF-7 encoded-word syntax as potentially dangerous. This creates a parser discrepancy: the validator likely checks the raw string for a trusted domain suffix, while the email handling library or mail server later decodes the UTF-7 content and interprets the address differently.
The attacker now exploits this discrepancy by crafting a malicious email address that appears to satisfy the domain restriction while redirecting the confirmation email to an attacker-controlled mailbox.
The crafted email is:
=?utf-7?q?attacker&AEA-[YOUR-EXPLOIT-SERVER-ID]&ACA-?=@ginandjuice.shop
This string uses UTF-7 encoding for special characters:
&AEA-decodes to@&ACA-decodes to a space
After decoding, the address becomes:
attacker@[YOUR-EXPLOIT-SERVER-ID] ?=@ginandjuice.shop
To the application’s validator, the raw string still appears to end with:
@ginandjuice.shop
and therefore passes the domain restriction.
However, the mail server interprets the decoded address differently. It treats:
attacker@[YOUR-EXPLOIT-SERVER-ID]
as the actual recipient.
This means the confirmation email is sent to the attacker’s inbox rather than a legitimate company-controlled mailbox.
After submitting the malicious registration request, the attacker opens the Email client provided by the lab and observes that the confirmation email has been successfully delivered. This confirms that the crafted address bypassed validation and redirected the message.
The attacker clicks the confirmation link in the email to activate the account. At this point, the account is fully registered and treated by the application as belonging to a trusted domain user.
Once logged in, the attacker gains access to features reserved for internal users. The application trusts the registered email domain and grants elevated privileges based on that assumption.
One of these features is access to the Admin panel.
From the admin panel, the attacker can browse the list of users and perform administrative actions. In this case, deleting the user:
carlos
solves the lab.
Technically, the root cause of this vulnerability lies in inconsistent input interpretation across system components. The registration validation layer likely performs a simple string comparison or regex match on the raw input. It checks whether the string ends in:
@ginandjuice.shop
without normalizing or decoding the email address first.
The mail processing library, however, decodes encoded-word syntax according to RFC rules before interpreting the address. This means the validator and mail server are not evaluating the same logical input.
This mismatch creates an attack surface where specially crafted inputs can satisfy validation checks while behaving differently downstream.
In real-world systems, such discrepancies can have severe consequences. Attackers could:
bypass domain-based registration restrictions
create accounts in trusted environments
receive password reset emails intended for internal users
gain access to sensitive administrative functionality
impersonate legitimate employees
The impact depends on how much trust the application places in email domains.
A secure implementation should normalize and decode email addresses before performing validation. All access control checks should operate on the canonical representation of the input.
For example, the application should:
Decode any encoded-word syntax
Normalize Unicode representations
Parse the email address according to RFC-compliant rules
Validate the resulting canonical domain
Only after these steps should the address be accepted or rejected.
Additionally, applications should consider rejecting uncommon or unsupported encodings like UTF-7 entirely unless there is a legitimate business need.
This vulnerability highlights an important principle in security engineering:
When multiple systems interpret the same input differently, attackers can exploit the discrepancy to bypass controls.
The application’s access control mechanism was not technically “broken” in the traditional sense. It did perform validation. The flaw existed because the validation logic and the mail delivery system did not agree on what the input meant.
This is why parser discrepancies are particularly dangerous. They exploit assumptions made by developers rather than weaknesses in cryptography or authentication itself.
The lab also demonstrates the importance of understanding RFC edge cases and unusual standards behavior. Attackers often search for obscure parsing inconsistencies because developers typically defend only against common patterns.
In this case, the application blocked ISO-8859-1 and UTF-8 encoded-word attacks but failed to consider UTF-7.
That small oversight completely undermined the registration restriction and allowed full privilege escalation.
Ultimately, this vulnerability demonstrates that secure input validation requires consistency across all layers of an application stack. Every component must interpret and process user input the same way. If they do not, attackers can manipulate the differences to bypass restrictions and gain unauthorized access.
Comments