SAP E-Recruiting Email Verification Bypass

`SEC Consult Vulnerability Lab Security Advisory < 20170912-0 >  
=======================================================================  
title: Email verification bypass  
product: SAP E-Recruiting  
vulnerable version: 605, 606, 616, 617  
fixed version: see SAP security note number 2507798  
impact: medium  
homepage: https://www.sap.com  
found: 2017-07-12  
by: Marc Nimmerrichter (Office Vienna)  
SEC Consult Vulnerability Lab  
  
An integrated part of SEC Consult  
Berlin - Frankfurt/Main - Montreal - Moscow  
Singapore - Vienna (HQ) - Vilnius - Zurich  
  
https://www.sec-consult.com  
  
=======================================================================  
  
Vendor description:  
-------------------  
"SAP E-Recruiting" has recruitment and succession planning instruments that  
will help your company find new employees, employ them in positions that suit  
their capabilities, promote their professional development, and retain them in  
the long term.  
As well as enabling you to handle your companyas applicant tracking activities,  
"SAP E-Recruiting" ensures that you drive up-to-date human resources management,  
by proactively maintaining contact with applicants, potential candidates, and  
consequently, with your employees.  
  
Source:  
https://help.sap.com/saphelp_erp60_sp/helpdata/en/73/8bcf535b804808e10000000a174cb4/frameset.htm  
  
  
Business recommendation:  
------------------------  
Email address verification during the applicant registration can be bypassed.  
Businesses using the vulnerable component are advised to estimate the impact of  
insufficient email address verification on their business processes and react  
accordingly. It is recommended to install a patched version as soon as possible.  
  
  
Vulnerability overview/description:  
-----------------------------------  
When an external applicant registers to the E-Recruiting application, he/she  
receives a link by email to confirm access to the provided email address.  
However, this measure can be bypassed and attackers can register and confirm  
email addresses that they do not have access to.  
  
An attacker could register email addresses not belonging to him/her. This could  
have a business impact, because business processes might rely on a verified  
email address. Furthermore, since an email address can be registered only once,  
an attacker could prevent other legitimate users from registering to the  
E-Recruiting application.  
  
  
Proof of concept:  
-----------------  
The email verification link contains the "param" HTTP GET parameter with base64  
encoded data. When decoded, this data contains the parameters  
"candidate_hrobject" and "corr_act_guid". candidate_hrobject is an incremental  
user ID. corr_act_guid is a random value that needs to be provided during the  
email verification. However, this value is not bound to the current  
registration, which means that the value of a previous registration can be  
reused. Since candidate_hrobject is incremental, it can be guessed by an  
attacker. An attacker who wants to register with an email address not belonging  
to him/her, could simply do the following:  
  
1. Register with his own email address  
2. Directly afterwards register with someone else's email address  
3. Read the current value of candidate_hrobject in the confirmation  
link from the first registration  
4. Increment this value by 1  
5. Send the new value in the HTTP GET request, use the corr_act_guidparameter  
from the first registration  
6. If this did not work: go back to step 4 to try the next ID  
(maybe other people registered in between the two registrations)  
  
This attack works because there is no per-registration nonce in the  
confirmation link.  
  
  
Vulnerable / tested versions:  
-----------------------------  
The vulnerability was found in the following release of E-Recruiting (ERECRUIT):  
Release: 617  
  
According to the vendor, the following versions are affected:  
Release: 605, 606, 616, 617  
  
  
Vendor contact timeline:  
------------------------  
2017-07-12: Contacted vendor via encrypted email with vulnerability description  
and Responsible Disclosure Policy attached at [email protected]  
2017-07-13: Vendor confirmed the receipt of the email  
2017-07-25: Vendor confirmed the vulnerability  
2017-07-31: Contacted vendor to ask for patch release date and versions affected  
2017-08-01: Vendor stated they are working on the fix and requested "adequate  
time". Link to SAP Responsible Disclosure Policy was provided.  
2017-08-01: Discussing release date, requested planned patch release date and  
versions affected.  
2017-08-02: Vendor stated that the patch cannot be published until 2017-08-31  
and requested more time before advisory publication.  
2017-08-23: Contacted vendor to request current patch status, planned patch  
release date and versions affected.  
2017-08-24: Vendor stated that the patch is planned to be published on  
2017-09-12.  
2017-08-24: Contacted vendor to request versions affected.  
2017-08-28: Vendor provided versions affected.  
2017-08-29: Defined 2017-09-12 as the actual advisory publication date.  
2017-08-30: Vendor confirms advisory publication date.  
2017-09-07: Contacted vendor to confirm patch release date and to request number  
of fixed release / patch.  
2017-09-07: Vendor provided security note number for the patch.  
2017-09-12: Public release of advisory.  
  
  
Solution:  
---------  
A patched version of SAP E-Recruiting should be installed, as the vendor  
has rated this vulnerability with CVSS 6.5 base score.  
  
Please refer to SAP Security Note 2507798 for further information.  
  
  
Workaround:  
-----------  
None  
  
  
Advisory URL:  
-------------  
https://www.sec-consult.com/en/vulnerability-lab/advisories/index.html  
  
  
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~  
  
SEC Consult Vulnerability Lab  
  
SEC Consult  
Berlin - Frankfurt/Main - Montreal - Moscow  
Singapore - Vienna (HQ) - Vilnius - Zurich  
  
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EOF Marc Nimmerrichter / @2017  
  
`