1. Advisory Information

Title: SAP Note Assistant Insecure handling of SAP Notes signature vulnerability
Advisory ID: CORE-2017-0011
Advisory URL: http://www.coresecurity.com/advisories/SAP-Note-Assistant-Insecure-handling-of-SAP-Notes-signature-vulnerability
Date published: 2017-11-30
Date of last update: 2017-11-27
Vendors contacted: SAP
Release mode: Coordinated release

2. Vulnerability Information

Class: Improper Verification of Cryptographic Signature [CWE-347], Improper Limitation of a Pathname to a Restricted Directory [CWE-22]
Impact: Code execution
Remotely Exploitable: Yes
Locally Exploitable: Yes
CVE Name: CVE-2017-16691

3. Vulnerability Description

SAP distributes program fixes in so called SAP Notes. A component, called SAP Note Assistant [1] is available to assist on managing and installing SAP Notes on SAP Netweaver Application Server systems. In September 2017, a new functionality [2][3] was introduced in SAP Note Assistant that enabled the tool to validate the signature of SAP Notes archive files and thus increase the security of the SAP Notes installation process.

A vulnerability was found in the way the signature validation is performed that could led to privilege escalation scenarios.

4. Vulnerable Packages

  • SAP Note Assistant with security note 2408073

Other products and versions might be affected, but they were not tested.

5. Vendor Information, Solutions and Workarounds

SAP published the following Security Notes:

  • 2546220

As a workaround, we suggest performing a signature validation on SAP Note archive files before extracting them or uploading them to SAP Note Assistant. This can be achieved by using the SAPCAR tool "-tvV" flags. If extraction of untrusted or potentially insecure archive files is requires it's recommended to extract them on a temporary directory using the "-flat" option.

6. Credits

This vulnerability was discovered and researched by Martin Gallo from Core Security Consulting Services. The publication of this advisory was coordinated by Alberto Solino from Core Advisories Team.

7. Technical Description / Proof of Concept Code

[CVE-2017-16691]: According to the documentation in SAP Note 2408073 [4], the process that SAP Note Assistant performs in order to verify the signature of uploaded SAR files is as follows:

1. The SAP Note file is copied into an application server directory for temporary files ($(DIR_TRANS)/tmp) (e.g. /usr/sap/trans/tmp).

2. Checking for signature type (SAP software) and the signature issuer (SAP Trust Community),

However, it was found that the process for verifying the signature of the SAP Note archive file is implemented using the SAPCAR tool [5] in a single step by extracting the files from the archive file to the temporary directory and verifying the signature. This is performed by using the "-xvV" flags of the SAPCAR tool.

As the SAPCAR tool extracts relative and absolute paths by default [6], it would be possible for an attacker to craft a SAR archive file that contains filenames pointing to directories outside the temporary directory (e.g. "../../../home/<sid>adm/.ssh/.authorized_keys"). The SAP Note Assistant will extract the files, overwriting files outside the temporary directory, and then perform the signature validation.

The following code using the pysap library [7] can be used to tamper with the example archive file provided in SAP Note 2408073 [3] ("0002424539_00.SAR") and include such type of relative filenames:

$ echo "This is a file that is not signed and will be written outside the temporary directory" >tamper.txt
$ pip install pysap
$ python
Python 2.7.12 (default, Jul  1 2016, 15:12:24)
[GCC 5.4.0 20160609] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> from pysap.SAPCAR import *
>>> ar = SAPCARArchive("0002424539_00.SAR", "r+")
>>> ar.add_file("tamper.txt", "../tamper.txt")
>>> ar.write()
>>> ar.files
{'../tamper.txt': <pysap.SAPCAR.SAPCARArchiveFile at 0x7f501f678a90>,
 './/0002424539_00.ZIP': <pysap.SAPCAR.SAPCARArchiveFile at 0x7f501f678f50>,
 'SIGNATURE.SMF': <pysap.SAPCAR.SAPCARArchiveFile at 0x7f501f678f10>}
>>> exit
          

It's also worth noticing that the deletion of the files in the temporary directory by SAP Note Assistant only takes into account the archive file, the signature file and the zip file. Other files not expected to be included in the archive file are not deleted.

In addition, it's worth mentioning that signature validation doesn't prevent vulnerabilities affecting the SAPCAR tool (such as [8]) to be triggered, as the signature file need to be extracted from the archive file in order to the validation to be performed.

8. Report Timeline

  • 2017-10-05: Core Security sent an initial notification to SAP, including a draft advisory.
  • 2017-10-06: SAP confirmed the reception of the advisory and assigned the incident ticket 1770463888 for tracking this issue. They will answer back once the team analyze the report.
  • 2017-11-13: SAP informed the availability of a patch for the reported issue that will be published in October 14th, with security note 2546220. SAP will be disclosing the advisory in February 14th, 2018.
  • 2017-11-14: Core Security acknowledged SAP's email and expressed concerns about the coordinated vulnerability process handled by SAP, since there was no communication whatsoever, just a 1-day notice about availability of the patch. Core Security requested the CVE-ID assigned to this issue in order to publish the advisory as soon as possible.
  • 2017-11-21: Core Security asked again for the CVE-ID assigned to the reported vulnerability in order to publish the advisory.
  • 2017-11-22: SAP answered saying they have planned to assign CVE IDs to vulnerabilities reported starting in December Patch Day, and said if we are still in need of a CVE ID to please let them know.
  • 2017-11-22: Core Security answered saying SAP is already listed as CNAs, meaning they are responsible of issuing CVE-IDs for the vulnerabilities reported. Also answered that yes, we are still in need of a CVE-ID.
  • 2017-11-30: Advisory CORE-2017-0011 published.

9. References

[1] https://support.sap.com/en/my-support/knowledge-base/note-assistant.html
[2] https://blogs.sap.com/2017/09/12/enable-note-assistant-to-support-digitally-signed-sap-notes/
[3] https://launchpad.support.sap.com/#/notes/0002408073
[4] https://launchpad.support.sap.com/applications/nnfv2/services/bsp/sap/support/sapnotes/public/services/attachment.htm?iv_key=012006153200001657472016&iv_version=0003&iv_guid=6EAE8B27FE511ED7A0B488C1A6E2A0C7
[5] https://launchpad.support.sap.com/#/softwarecenter/template/products/_APP=00200682500000001943&_EVENT=DISPHIER&HEADER=N&FUNCTIONBAR=Y&EVENT=TREE&TMPL=INTRO_SWDC_SP_AD&V=MAINT&REFERER=CATALOG-PATCHES&ROUTENAME=products/By%20Category%20-%20Additional%20Components
[6] https://www.coresecurity.com/corelabs-research/publications/deep-dive-sap-archive-file-formats
[7] https://github.com/CoreSecurity/pysap
[8] https://www.coresecurity.com/advisories/sap-sapcar-heap-based-buffer-overflow-vulnerability

10. About CoreLabs

CoreLabs, the research center of Core Security, is charged with anticipating the future needs and requirements for information security technologies. We conduct our research in several important areas of computer security including system vulnerabilities, cyber attack planning and simulation, source code auditing, and cryptography. Our results include problem formalization, identification of vulnerabilities, novel solutions and prototypes for new technologies. CoreLabs regularly publishes security advisories, technical papers, project information and shared software tools for public use at: http://corelabs.coresecurity.com.

11. About Core Security

Core Security provides companies with the security insight they need to know who, how, and what is vulnerable in their organization. The company's threat-aware, identity & access, network security, and vulnerability management solutions provide actionable insight and context needed to manage security risks across the enterprise. This shared insight gives customers a comprehensive view of their security posture to make better security remediation decisions. Better insight allows organizations to prioritize their efforts to protect critical assets, take action sooner to mitigate access risk, and react faster if a breach does occur.

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12. Disclaimer

The contents of this advisory are copyright (c) 2017 Core Security and (c) 2017 CoreLabs, and are licensed under a Creative Commons Attribution Non-Commercial Share-Alike 3.0 (United States) License: http://creativecommons.org/licenses/by-nc-sa/3.0/us/