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Title:       EGI SVG 'ALERT' [TLP:WHITE] linux kernel 'use-after-free' flaw in XFRM [EGI-SVG-CVE-2017-16939]  

Date:        2018-02-23
Updated:     2018-03-19 changed to [TLP:WHITE}

Affected software and risk

Vulnerability concerning Linux kernel XFRM for sites having unprivileged user namespaces enabled.

Package : Linux Kernel
CVE ID  : CVE-2017-16939
Bug ID  : 

NOTE: Only applicable for sites having unprivileged user namespaces enabled.

A vulnerability has been found in the Linux kernel concerning XFRM.  
A 'use-after-free' flaw has been found which may in some circumstances lead to privilege 
escalation. [R 1], [R 2], [R 3]  If unprivileged user namespaces are used on a service to 
which users have shell access (e.g. worker nodes supporting fully unprivileged creation of containers), 
then the vulnerability would be exploitable there. 
This is most likely to be significant in EGI in relation to usage of Singularity [R 4].

Actions required/recommended

RedHat 7 and its derivatives: kernel update is NOT yet available, but vulnerable sites should urgently, 
as mitigation, disable at least user _net_ namespaces [R 10]:

    root@host # echo "user.max_net_namespaces = 0" > /etc/sysctl.d/90-max_net_namespaces.conf
    root@host # sysctl -p /etc/sysctl.d/90-max_net_namespaces.conf

Note: network namespaces are an essential aspect to many Docker (and other container runtime) deployments, 
and the site may have to choose between safely running unprivileged Singularity and using the default Docker configuration. Docker can avoid network namespaces by passing the `--net=host` option to its command line clients.

RedHat 6 and its derivatives: not affected. 

For Debian or Ubuntu: fixed via the kernel update for Meltdown/Spectre vulnerabilities. 

If anyone becomes aware of any situation where this vulnerability has a significant impact on the EGI 
infrastructure then please inform EGI SVG.

Component installation information

For RedHat see [R 2]  fix not available at time of writing, but see mitigation above

For Ubuntu see [R 5] 

For Debian see [R 6]

For Scientific linux see [R 7] fix not available at time of writing, but see mitigation above

For CentOS see [R 9] fix not available at time of writing, but see mitigation above


** WHITE information - Unimited distribution 
 - see for distribution restrictions***  

Minor updates may be made without re-distribution to the sites


Comments or questions should be sent to svg-rat  at

If you find or become aware of another vulnerability which is relevant to EGI you may report it by e-mail to  

report-vulnerability at
the EGI Software Vulnerability Group will take a look according to the procedure defined in [R 8]  

Note that this has been updated and the latest version approved by the Operations Management Board in November 2017


[R 1] 

[R 2] 

[R 3]

[R 4]

[R 5]  

[R 6]  
[R 7]

[R 8]

[R 9]

[R 10]


SVG was alerted to this vulnerability by Vincent Brillault.  Vincent Brillault, 
Mischa Salle and Barbara Krasovec carried out the investigations.  

Yyyy-mm-dd  [EGI-SVG-CVE-2017-16939] 

2017-12-06 SVG alerted to this issue by Vincent Brillault
2017-12-07 Investigation on impact on EGI carried out by Vincent Brillault, Mischa Salle and Barbara Krasovec
2017-12-15 Decision to send an 'alert'
2017-12-18 Alert drafted
2017-12-19 OSG sent an alert - Copied to EGI by Dave Dykstra
2018-02-23 Alert sent to EGI sites
2018-03-19 Changed to TLP:WHITE and placed on wiki


This advisory has been prepared as part of the effort to fulfil EGI SVG's purpose 
"To minimize the risk to the EGI infrastructure arising from software vulnerabilities"

The risk is that assessed by the group, according to the EGI SVG issue handling procedure [R 8]  
in the context of how the software is used in the EGI infrastructure. 
It is the opinion of the group, we do not guarantee it to be correct. 
The risk may also be higher or lower in other deployments depending on how the software is used.   

Others may re-use this information provided they:-

1) Respect the provided TLP classification

2) Credit the EGI Software Vulnerability Group

On behalf of the EGI SVG,