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Advisory ID: SYSS-2015-022
Product: Adyen HPP API
Vendor: Adyen
Affected Version(s): HPP API with SHA-1
Tested Version(s): HPP API with SHA-1
Vulnerability Type: Cryptographic Issues (CWE-310)
Risk Level: High
Solution Status: Fixed
Vendor Notification: 2015-04-16
Solution Date:  2015-07-03
Public Disclosure: 2016-05-02
CVE Reference: Not assigned
Author of Advisory: Franz G. Jahn, SySS GmbH, https://www.syss.de/advisories/

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Overview:
Adyen provides a payment service which can be used by various shop
systems like Magento, Demandware or Prestashop. Using an hosted payment
page (HPP), the customer will be redirected to a payment page of Adyen
during the checkout process, afterwards he will be redirected to the
shop itself. The redirect URL contains several parameters which indicate
the status of the payment as well as a signature which can be used to
validate the submitted parameters.

By using signatures which are not suitable to authenticate the payment,
the API allows to bypass the payment process.

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Vulnerability Details:

Depending on the used payment type, up to four different signatures are
used during the payment process. Three of them are generated by the shop
system and submitted to the adyen payment page, the fourth one is
generated by Adyen and submitted as parameter merchantSig to the shop
system to authenticated the other parameters which indicate the status
of the payment. All four signatures are generated in the same way by
using a shared secret which is known to the shop system and to the
hosted payment page.

The way the signatures are build is described in the API manual[1].
Furthermore, several example implementations[2,3,4] which demonstrate
the interaction between the shops and the payment platform are provided
by Adyen. However, the way the signatures are build contains a flaw
which might be abused to skip the whole payment process.

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Solution:

Use new HMAC payment setup (SHA-256) [5]

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Disclosure Timeline:

2015-04-09: Vulnerability discovered
2015-04-16: Vulnerability reported to vendor
2015-04-28: Vulnerability confirmed by vendor
2015-07-03: Vendor provides an alternative HMAC payment setup and
            deprecates old SHA-1 based HMAC
2016-04-21: Vendor releases updated Magento plugin which uses the new
            payment setup
2016-05-02: Public release of security advisory.

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References:

[1] Adyen, HPP API Manual
    https://www.adyen.com/dam/jcr:d00b49c2-1ad9-4617-a6ed-0013ce7670ce/HPP%20API%20Manual.pdf
[2] Adyen, PHP Example Implementation
    https://github.com/Adyen/php/
[3] Adyen, ASP Example Implementation
    https://github.com/Adyen/asp
[4] Adyen, Java Example Implementation
    https://github.com/Adyen/java
[5] Adyen, New SHA-256 based HMAC payment setup
    https://docs.adyen.com/developers/hpp-manual#hmacpaymentsetupsha256
[6] SySS GmbH, SySS Responsible Disclosure Policy
    https://www.syss.de/en/news/responsible-disclosure-policy/

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Credits:


Security vulnerability found by Franz G. Jahn of the SySS GmbH.
E-Mail: franz.jahn@syss.de
Public Key: https://www.syss.de/fileadmin/dokumente/Materialien/PGPKeys/Franz_Jahn.asc
Key ID: 0xD06A14DE
Key Fingerprint: 2D26 E435 DF7F 572D A0C9 1DE8 D5A6 1496 D06A 14DE

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Disclaimer:

The information provided in this security advisory is provided "as is" 
and without warranty of any kind. Details of this security advisory may
be updated in order to provide as accurate information as possible. The
latest version of  this security advisory is available on the SySS Web
site.

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Copyright:

Creative Commons - Attribution (by) - Version 3.0
URL: http://creativecommons.org/licenses/by/3.0/deed.en

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