RCE Security Advisory
https://www.rcesecurity.com


1. ADVISORY INFORMATION
=======================
Product:        Acronis Cyber Backup
Vendor URL:     https://www.acronis.com
Type:           Server-Side Request Forgery [CWE-918]
Date found:     2020-07-30
Date published: 2020-09-14
CVSSv3 Score:   8.3 (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:L/A:L)
CVE:            CVE-2020-16171


2. CREDITS
==========
This vulnerability was discovered and researched by Julien Ahrens from
RCE Security.


3. VERSIONS AFFECTED
====================
Acronis Cyber Backup v12.5 Build 16341
and below.


4. INTRODUCTION
===============
Businesses can be at risk of losing important data. Lost data leads to costly
downtime, customer dissatisfaction, regulatory fines, and lost revenue. As a
result, IT pros must meet extremely high expectations. You need to keep the
company running 24-hours a day.

Acronis Cyber Backup delivers the data protection that meets today’s demands. It
keeps your business running, protecting any workload, scaling without limits,
and saving you money.

(from the vendor's homepage)


5. VULNERABILITY DETAILS
========================
All API endpoints running on port 9877 under "/api/ams/" whereof some are
reachable without authentication, do accept an additional custom header called
"Shard":

def get_ams_address(headers):
    if 'Shard' in headers:
        [...]
        return headers.get('Shard')  # Mobile agent >= ABC5.0

The value of this header is afterwards to construct a separate web request send
by the application using a urllib.request.urlopen call:

def make_request_to_ams(resource, method, data=None):
    port = config.CONFIG.get('default_ams_port', '9892')
    uri = 'http://{}:{}{}'.format(get_ams_address(request.headers), port, resource)
    logging.debug('Making request to AMS %s %s', method, uri)
    headers = dict(request.headers)
    del headers['Content-Length']
    if not data is None:
        headers['Content-Type'] = 'application/json'
    req = urllib.request.Request(uri,
                                 headers=headers,
                                 method=method,
                                 data=data)
    resp = None
    try:
        resp = urllib.request.urlopen(req, timeout=wcs.web.session.DEFAULT_REQUEST_TIMEOUT)
    except Exception as e:
        logging.error('Cannot access ams {} {}, error: {}'.format(method, resource, e))
    return resp

This can be abused to conduct SSRF attacks against otherwise unreachable internal hosts
of Acronis services that are bound to localhost such as the "NotificationService" running
on 127.0.0.1:30572 with a request header like:

Shard: localhost:30572/external_email?

For more details, see the referenced blog post.


6. RISK
=======
The vulnerability can be used by an unauthenticated or authenticated attacker
to query otherwise unreachable internal network resources. As demonstrated in
the corresponding blog post, using this vulnerability, it is possible to i.e.
(amongst others) send out fully customized emails or modify the application's
resource settings.


7. SOLUTION
===========
Update to v12.5 Build 16342 


8. REPORT TIMELINE
==================
2020-07-30: Discovery of the vulnerability
2020-07-30: Since the vulnerability is fixed in Cyber Protect: Sent out a
            request to the Vendor to check whether Cyber Backup is EOL and users
            are advised to migrate to Cyber Protect instead.
2020-07-30: CVE requested from MITRE
2020-07-31: MITRE assigns CVE-2020-16171
2020-07-31: Public Disclosure date set to 2020-08-14
2020-08-04: Vendor asks for a 90 days extension
2020-08-04: Extension not granted because there is a fix available already. Public disclosure 
            date set to 2020-09-14
2020-09-05: Asking vendor about the status of the fix
2020-09-08: Vendor states that a fix has been backported to Cyber Backup 12.5 under the 
            reference ABR-202103
2020-09-14: Public disclosure



9. REFERENCES
=============
https://www.rcesecurity.com/2020/09/CVE-2020-16171-Exploiting-Acronis-Cyber-Backup-for-Fun-and-Emails/
https://dl.acronis.com/u/backup/rn/12.5/user/en-US/AcronisBackup12.5_relnotes.htm