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# Bypassing Authentication via Encryption Oracle
Bypassing Authentication via Encryption Oracle notes for PortSwigger Web Security Academy labs, including exploitation workflow, payload testing, Burp Suite validation, and remediation context.
### Authentication Bypass via Encryption Oracle
**Description of lab**
This lab has a logical flaw exposing a \*\*server-side encryption \*\*oracle. By exploiting this weakness, it is possible to forge a valid encrypted value in order to bypass authentication, access the admin panel and delete user **carlos**.
Identifier provided:
* User: `wiener`
* Password: `peter`
**Initial observation**
On the login page, the **Stay logged in** option is available.
When used, a cookie encrypted in **Base64** is generated and stored on the customer side.
```bash
echo '0HE6oMYJL//S1fLwttqR7WMF3EX4KJmvfFRI53qpVnk=' | base64 -d
# The decoded value is binary ciphertext, not readable text.
```
### Identification of Encryption Oracle
When a valid comment is submitted, no specific response is observed.
However, if a comment causes an application error (e.g. an invalid email address), the server:
* returns an error message displayed on the screen;
{% code overflow="wrap" %}
```bash
csrf=cFMQUjAyzETtEr8TLsoir7V5p4C3m4ZG&postId=4&comment=test&name=test&email=test&website=https://localhost
```
{% endcode %}
* generates a new cookie `notification`\*\*, encrypted.
The content displayed is for example:
This shows that:
* the server \*\*decrypts the content of the `notification` cookie;
The server therefore acts as a **oracle of encryption and decryption**.
* the deciphered text is **reinjected in HTML response**.
### Correlation with the Cookie
By injecting the encrypted value of the cookie \*\*
The observed format is:
This clearly corresponds to:
* a user name;
* a timestamp.
#### Forgery of an Administrator Identity
The objective is to have the server encrypt the following string:
```bash
administrator:1766913113564
```
To do this, this value is injected into the vulnerable field (invalid comment/email), causing:
* display the error message containing this string;
the generation of a new `notification` cookie, including the text injected.
{% code overflow="wrap" %}
```bash
Set-Cookie: notification=VQbs3ex4CbvKjTm0LM5wuZa79pnAzKI9MEsa1O0MbX%2fJpfl9baj6yL3ryxh4u6nyiWHVwxP7%2fVDPyft%2f1qAbbQ%3d%3d; HttpOnly
```
{% endcode %}
### Encryption Algorithm Identification
By arbitrarily truncating the encrypted value, the server returns the following error
```bash
PTBLGtTtDG1/yaX5fW2o%2bsi968sYeLup8olh1cMT%2b/1Qz8n7f9agG20%3d
```
`Input length must be multiple of 16 when decrypting with padded cipher`
This clearly indicates the use of **AES with padding**, operating in blocks of **16 bytes**.
#### Cipher Block Manipulation
The problem encountered is the presence of invalid padding characters at the end of the message. / To remedy this, the injected chain is preceded by filling characters to align the blocks correctly:
```
xxxxxxxxxadministrator:1766913113564
```
A new encrypted cookie is then generated.
VQbs3ex4CbvKjTm0LM5wuZJqwlVboMZ2IzNuiTfVH6iHj9RZUT59d5kGoS64tVE5wWiEfmp6VbJG%2bb%2fNW5%2bblA%3d%3d
### Cutting and Recomposing the Cookie
By deleting the first **two encrypted blocks** (corresponding to filling), only blocks containing:
This final encrypted value is accepted by the server.
```
h4/UWVE%2bfXeZBqEuuLVROcFohH5qelWyRvm/zVufm5Q%3d
```
#### Contouring Authentication
Final steps:
1. Delete the existing session cookie.
2. Replace the value of the cookie \*\*
3. Refresh the page.
---
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