etbe – Russell Coker

Centralisation and Corporations

An advantage of a medium to large company is that it permits specialisation. For example I’m currently working in the IT department of a medium sized company and because we have standardised hardware (Dell Latitude and Precision laptops, Dell Precision Tower workstations, and Dell PowerEdge servers) and I am involved in fixing all Linux compatibility issues on that I can fix most problems in a small fraction of the time that I would take to fix on a random computer. There is scope for a lot of debate about the extent to which companies should standardise and centralise things. But for computer problems which can escalate quickly from minor to serious if not approached in the correct manner it’s clear that a good deal of centralisation is appropriate.

For people doing technical computer work such as programming there’s a large portion of the employees who are computer hobbyists who like to fiddle with computers. But if the support system is run well even they will appreciate having computers just work most of the time and for a large portion of the failures having someone immediately recognise the problem, like the issues with NVidia drivers that I have documented so that first line support can implement workarounds without the need for a lengthy investigation.

A big problem with email in the modern Internet is the prevalence of Phishing scams. The current corporate approach to this is to send out test Phishing email to people and then force computer security training on everyone who clicks on them. One problem with this is that attackers only need to fool one person on one occasion and when you have hundreds of people doing something on rare occasions that’s not part of their core work they will periodically get it wrong. When every test Phishing run finds several people who need extra training it seems obvious to me that this isn’t a solution that’s working well. I will concede that the majority of people who click on the test Phishing email would probably realise their mistake if asked to enter the password for the corporate email system, but I think it’s still clear that this isn’t a great solution.

Let’s imagine for the sake of discussion that everyone in a company was 100% accurate at identifying Phishing email and other scam email, if that was the case would the problem be solved? I believe that even in that hypothetical case it would not be a solved problem due to the wasted time and concentration. People can spend minutes determining if a single email is legitimate. On many occasions I have had relatives and clients forward me email because they are unsure if it’s valid, it’s great that they seek expert advice when they are unsure about things but it would be better if they didn’t have to go to that effort. What we ideally want to do is centralise the anti-Phishing and anti-spam work to a small group of people who are actually good at it and who can recognise patterns by seeing larger quantities of spam. When a spam or Phishing message is sent to 600 people in a company you don’t want 600 people to individually consider it, you want one person to recognise it and delete/block all 600. If 600 people each spend one minute considering the matter then that’s 10 work hours wasted!

The Rationale for Human Filtering

For personal email human filtering usually isn’t viable because people want privacy. But corporate email isn’t private, it’s expected that the company can read it under certain circumstances (in most jurisdictions) and having email open in public areas of the office where colleagues might see it is expected. You can visit gmail.com on your lunch break to read personal email but every company policy (and common sense) says to not have actually private correspondence on company systems.

The amount of time spent by reception staff in sorting out such email would be less than that taken by individuals. When someone sends a spam to everyone in the company instead of 500 people each spending a couple of minutes working out whether it’s legit you have one person who’s good at recognising spam (because it’s their job) who clicks on a “remove mail from this sender from all mailboxes” button and 500 messages are deleted and the sender is blocked.

Delaying email would be a concern. It’s standard practice for CEOs (and C*Os at larger companies) to have a PA receive their email and forward the ones that need their attention. So human vetting of email can work without unreasonable delays. If we had someone checking all email for the entire company probably email to the senior people would never get noticeably delayed and while people like me would get their mail delayed on occasion people doing technical work generally don’t have notifications turned on for email because it’s a distraction and a fast response isn’t needed. There are a few senders where fast response is required, which is mostly corporations sending a “click this link within 10 minutes to confirm your password change” email. Setting up rules for all such senders that are relevant to work wouldn’t be difficult to do.

How to Solve This

Spam and Phishing became serious problems over 20 years ago and we have had 20 years of evolution of email filtering which still hasn’t solved the problem. The vast majority of email addresses in use are run by major managed service providers and they haven’t managed to filter out spam/phishing mail effectively so I think we should assume that it’s not going to be solved by filtering. There is talk about what “AI” technology might do for filtering spam/phishing but that same technology can product better crafted hostile email to avoid filters.

An additional complication for corporate email filtering is that some criteria that are used to filter personal email don’t apply to corporate mail. If someone sends email to me personally about millions of dollars then it’s obviously not legit. If someone sends email to a company then it could be legit. Companies routinely have people emailing potential clients about how their products can save millions of dollars and make purchases over a million dollars. This is not a problem that’s impossible to solve, it’s just an extra difficulty that reduces the efficiency of filters.

It seems to me that the best solution to the problem involves having all mail filtered by a human. A company could configure their mail server to not accept direct external mail for any employee’s address. Then people could email files to colleagues etc without any restriction but spam and phishing wouldn’t be a problem. The issue is how to manage inbound mail. One possibility is to have addresses of the form it+russell.coker@example.com (for me as an employee in the IT department) and you would have a team of people who would read those mailboxes and forward mail to the right people if it seemed legit. Having addresses like it+russell.coker means that all mail to the IT department would be received into folders of the same account and they could be filtered by someone with suitable security level and not require any special configuration of the mail server. So the person who read the is mailbox would have a folder named russell.coker receiving mail addressed to me. The system could be configured to automate the processing of mail from known good addresses (and even domains), so they could just put in a rule saying that when Dell sends DMARC authenticated mail to is+$USER it gets immediately directed to $USER. This is the sort of thing that can be automated in the email client (mail filtering is becoming a common feature in MUAs).

For a FOSS implementation of such things the server side of it (including extracting account data from a directory to determine which department a user is in) would be about a day’s work and then an option would be to modify a webmail program to have extra functionality for approving senders and sending change requests to the server to automatically direct future mail from the same sender. As an aside I have previously worked on a project that had a modified version of the Horde webmail system to do this sort of thing for challenge-response email and adding certain automated messages to the allow-list.

The Change

One of the first things to do is configuring the system to add every recipient of an outbound message to the allow list for receiving a reply. Having a script go through the sent-mail folders of all accounts and adding the recipients to the allow lists would be easy and catch the common cases.

But even with processing the sent mail folders going from a working system without such things to a system like this will take some time for the initial work of adding addresses to the allow lists, particularly for domain wide additions of all the sites that send password confirmation messages. You would need rules to direct inbound mail to the old addresses to the new style and then address a huge amount of mail that needs to be categorised. If you have 600 employees and the average amount of time taken on the first day is 10 minutes per user then that’s 100 hours of work, 12 work days. If you had everyone from the IT department, reception, and executive assistants working on it that would be viable. After about a week there wouldn’t be much work involved in maintaining it. Then after that it would be a net win for the company.

The Benefits

If the average employee spends one minute a day dealing with spam and phishing email then with 600 employees that’s 10 hours of wasted time per day. Effectively wasting one employee’s work! I’m sure that’s the low end of the range, 5 minutes average per day doesn’t seem unreasonable especially when people are unsure about phishing email and send it to Slack so multiple employees spend time analysing it. So you could have 5 employees being wasted by hostile email and avoiding that would take a fraction of the time of a few people adding up to less than an hour of total work per day.

Then there’s the training time for phishing mail. Instead of having every employee spend half an hour doing email security training every few months (that’s 300 hours or 7.5 working weeks every time you do it) you just train the few experts.

In addition to saving time there are significant security benefits to having experts deal with possibly hostile email. Someone who deals with a lot of phishing email is much less likely to be tricked.

Will They Do It?

They probably won’t do it any time soon. I don’t think it’s expensive enough for companies yet. Maybe government agencies already have equivalent measures in place, but for regular corporations it’s probably regarded as too difficult to change anything and the costs aren’t obvious. I have been unsuccessful in suggesting that managers spend slightly more on computer hardware to save significant amounts of worker time for 30 years.

The Announcement

Late last month there was an announcement of a “severity 9.9 vulnerability” allowing remote code execution that affects “all GNU/Linux systems (plus others)” [1]. For something to affect all Linux systems that would have to be either a kernel issue or a sshd issue. The announcement included complaints about the lack of response of vendors and “And YES: I LOVE hyping the sh1t out of this stuff because apparently sensationalism is the only language that forces these people to fix”.

He seems to have a different experience to me of reporting bugs, I have had plenty of success getting bugs fixed without hyping them. I just report the bug, wait a while, and it gets fixed. I have reported potential security bugs without even bothering to try and prove that they were exploitable (any situation where you can make a program crash is potentially exploitable), I just report it and it gets fixed. I was very dubious about his ability to determine how serious a bug is and to accurately report it so this wasn’t a situation where I was waiting for it to be disclosed to discover if it affected me. I was quite confident that my systems wouldn’t be at any risk.

Analysis

Not All Linux Systems Run CUPS

When it was published my opinion was proven to be correct, it turned out to be a series of CUPS bugs [2]. To describe that as “all GNU/Linux systems (plus others)” seems like a vast overstatement, maybe a good thing to say if you want to be a TikTok influencer but not if you want to be known for computer security work.

For the Debian distribution the cups-browsed package (which seems to be the main exploitable one) is recommended by cups-daemon, as I have my Debian systems configured to not install recommended packages by default that means that it wasn’t installed on any of my systems. Also the vast majority of my systems don’t do printing and therefore don’t have any part of CUPS installed.

CUPS vs NAT

The next issue is that in Australia most home ISPs don’t have IPv6 enabled and CUPS doesn’t do the things needed to allow receiving connections from the outside world via NAT with IPv4. If inbound port 631 is blocked on both TCP and USP as is the default on Australian home Internet or if there is a correctly configured firewall in place then the network is safe from attack. There is a feature called uPnP port forwarding [3] to allow server programs to ask a router to send inbound connections to them, this is apparently usually turned off by default in router configuration. If it is enabled then there are Debian packages of software to manage this, the miniupnpc package has the client (which can request NAT changes on the router) [4]. That package is not installed on any of my systems and for my home network I don’t use a router that runs uPnP.

The only program I knowingly run that uses uPnP is Warzone2100 and as I don’t play network games that doesn’t happen. Also as an aside in version 4.4.2-1 of warzone2100 in Debian and Ubuntu I made it use Bubblewrap to run the game in a container. So a Remote Code Execution bug in Warzone 2100 won’t be an immediate win for an attacker (exploits via X11 or Wayland are another issue).

MAC Systems

Debian has had AppArmor enabled by default since Buster was released in 2019 [5]. There are claims that AppArmor will stop this exploit from doing anything bad.

To check SE Linux access I first use the “semanage fcontext” command to check the context of the binary, cupsd_exec_t means that the daemon runs as cupsd_t. Then I checked what file access is granted with the sesearch program, mostly just access to temporary files, cupsd config files, the faillog, the Kerberos cache files (not used on the Kerberos client systems I run), Samba run files (might be a possibility of exploiting something there), and the security_t used for interfacing with kernel security infrastructure. I then checked the access to the security class and found that it is permitted to check contexts and access-vectors – not access that can be harmful.

The next test was to use sesearch to discover what capabilities are granted, which unfortunately includes the sys_admin capability, that is a capability that allows many sysadmin tasks that could be harmful (I just checked the Fedora source and Fedora 42 has the same access). Whether the sys_admin capability can be used to do bad things with the limited access cupsd_t has to device nodes etc is not clear. But this access is undesirable.

So the SE Linux policy in Debian and Fedora will stop cupsd_t from writing SETUID programs that can be used by random users for root access and stop it from writing to /etc/shadow etc. But the sys_admin capability might allow it to do hostile things and I have already uploaded a changed policy to Debian/Unstable to remove that. The sys_rawio capability also looked concerning but it’s apparently needed to probe for USB printers and as the domain has no access to block devices it is otherwise harmless. Below are the commands I used to discover what the policy allows and the output from them.

# semanage fcontext -l|grep bin/cups-browsed
/usr/bin/cups-browsed                              regular file       system_u:object_r:cupsd_exec_t:s0 
# sesearch -A -s cupsd_t -c file -p write
allow cupsd_t cupsd_interface_t:file { append create execute execute_no_trans getattr ioctl link lock map open read rename setattr unlink write };
allow cupsd_t cupsd_lock_t:file { append create getattr ioctl link lock open read rename setattr unlink write };
allow cupsd_t cupsd_log_t:file { append create getattr ioctl link lock open read rename setattr unlink write };
allow cupsd_t cupsd_runtime_t:file { append create getattr ioctl link lock open read rename setattr unlink write };
allow cupsd_t cupsd_rw_etc_t:file { append create getattr ioctl link lock open read rename setattr unlink write };
allow cupsd_t cupsd_t:file { append create getattr ioctl link lock open read rename setattr unlink write };
allow cupsd_t cupsd_tmp_t:file { append create getattr ioctl link lock open read rename setattr unlink write };
allow cupsd_t faillog_t:file { append getattr ioctl lock open read write };
allow cupsd_t init_tmpfs_t:file { append getattr ioctl lock read write };
allow cupsd_t krb5_host_rcache_t:file { append create getattr ioctl link lock open read rename setattr unlink write }; [ allow_kerberos ]:True
allow cupsd_t print_spool_t:file { append create getattr ioctl link lock open read relabelfrom relabelto rename setattr unlink write };
allow cupsd_t samba_var_t:file { append getattr ioctl lock open read write };
allow cupsd_t security_t:file { append getattr ioctl lock open read write };
allow cupsd_t security_t:file { append getattr ioctl lock open read write }; [ allow_kerberos ]:True
allow cupsd_t usbfs_t:file { append getattr ioctl lock open read write };
# sesearch -A -s cupsd_t -c security
allow cupsd_t security_t:security check_context; [ allow_kerberos ]:True
allow cupsd_t security_t:security { check_context compute_av };
# sesearch -A -s cupsd_t -c capability
allow cupsd_t cupsd_t:capability net_bind_service; [ allow_ypbind ]:True
allow cupsd_t cupsd_t:capability { audit_write chown dac_override dac_read_search fowner fsetid ipc_lock kill net_bind_service setgid setuid sys_admin sys_rawio sys_resource sys_tty_config };
# sesearch -A -s cupsd_t -c capability2
allow cupsd_t cupsd_t:capability2 { block_suspend wake_alarm };
# sesearch -A -s cupsd_t -c blk_file

Conclusion

This is an example of how not to handle security issues. Some degree of promotion is acceptable but this is very excessive and will result in people not taking security announcements seriously in future. I wonder if this is even a good career move by the researcher in question, will enough people believe that they actually did something good in this that it outweighs the number of people who think it’s misleading at best?

• [1] https://threadreaderapp.com/thread/1838169889330135132.html
• [2] https://tinyurl.com/26rjd5ex
• [3] https://tinyurl.com/2ckyvpyq
• [4] https://packages.debian.org/sid/miniupnpc
• [5] https://wiki.debian.org/AppArmor/HowToUse