11 ms·

I must be missing something obvious here. Why can’t skype just hard code the locations and hashes of the dll files it needs to load? Why is Skype loading random

I must be missing something obvious here. Why can’t skype just hard code the locations and hashes of the dll files it needs to load? Why is Skype loading random DLLs from user accessible folders? I must be misunderstanding how Windows programs use DLLs and why it needs to just search for them. Edit: @jwilk in the comments here pointed to a better article about the security vulnerability [1]. Based on the technical details, there seems to be no reason why Microsoft could not issue a very quick fix loading the DLL from the secure location. Important quote in my eyes: “The engineers provided me with an update on this case. They've reviewed the code and were able to reproduce the issue, but have determined that the fix will be implemented in a newer version of the product rather than a security update. The team is planning on shipping a newer version of the client, and this current version will slowly be deprecated. The installer would need a large code revision to prevent DLL injection, but all resources have been put toward development of the new client.” In other words, it seems Microsoft/Skype doesn’t care about security at all. A couple of lines to fix the bug, in theory, but Microsoft is too busy to do it. Doesn’t make sense, unless a new version would have been released shortly, but it has been 4 months between that email and disclosure. [1]: http://seclists.org/fulldisclosure/2018/Feb/33 http://seclists.org/fulldisclosure/2018/Feb/33

stevekemp 9y ago
This is due to the way that Windows loads DLLs, there is a defined search order and the current working-directory is one of the locations. There have been a lot of write-ups on this topic on Raymond Chen's blog, here is a good example: https://blogs.msdn.microsoft.com/oldnewthing/20101110-00/ https://blogs.msdn.microsoft.com/oldnewthing/20101110-00/
- photonios 9y ago
  That doesn't mean you cannot load a DLL manually from a static location: https://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx https://msdn.microsoft.com/en-us/library/windows/desktop/ms6...
  - enzanki_ars 9y ago
    I was just about to come back here and mention that. The article and comments that @stevekemp linked to provided a lot of insight, but as you mentioned, there are ways to prevent exploits of that nature, which the comments in the article point to also. Something still seems to be missing from my understanding of what Skype is doing...
    - chaz6 9y ago
      The cynic in me thinks this is already known and is in use by a "strategic partner" as a vector. The next version will only make it harder to find and exploit.
      - Tiki 9y ago
        Your comment reminded me of this: https://notepad-plus-plus.org/news/notepad-7.3.3-fix-cia-hacking-issue.html https://notepad-plus-plus.org/news/notepad-7.3.3-fix-cia-hac... I doubt this was intentional at first, but the fact that they don't want to fix it is very fishy.
        saurik 9y ago
        That is a very different issue, as there the concept is that the original file in the original location is being replaced. As is stated in that post, at that point the person could just as easily replace the program itself with a modified version of the entire program. That is not a security issue in the software.
- mannykannot 9y ago
  If that were all there is to it, would this not be an issue for all Windows applications, not just Skype? Is it just Skype that puts the current working directory in the list, or at least in a position where it can be used to preempt the loading of the correct DLL? The link you provide as an example suggests that this sort of thing might be done to facilitate testing, but again, that doesn't seem to be a compelling reason for not fixing it now - 'ship what you test' is a good principle, but it does not preempt 'fix known security holes.'
  - Svip 9y ago
    Well, I guess not all Windows applications are run as the SYSTEM account, but the Skype SKY-tmp is? I'd assume, at least, that programs intended to run as the SYSTEM account does a better job of loading DLLs from secure locations.
    - notriddle 9y ago
      In other words, the Linux equivalent would be a setuid executable that doesn't properly ignore LD_PRELOAD_PATH.
      - user5994461 9y ago
        Well, the system executable should not be run as root from /tmp obviously.
  - rhodysurf 9y ago
    Its an issue with all windows applications but not all windows applications have the same elevated permissions that skype seems to
  - mherdeg 9y ago
    It is an issue for all Windows applications. There was a month when 50%+ of the traffic on full-disclosure@ was just one person repeatedly announcing newly discovered problems with various Windows software that all involved search-path problems ("DLL hijacking" or related issues). There's at least one author who is super keen on using the phrase "binary planting" to refer to a similar class of attacks.
- leeter 9y ago
  You can actually control most of this using an application manifest. The advantage being you can prevent any non-"known" dll from loading from any folder it's not supposed to. Moreover you can ensure that it's embedded in the binary so if someone tries to modify the manifest it breaks the cryptographic signature on the binary. To anyone going "but what if they replace the binary!" well then they've already gotten past the air-tight hatchway.
brazzledazzle 9y ago
It would be interesting to see Google Project Zero’s take on it. It’s a shame they didn’t discover it because I’m sure the back and forth between them and the Skype devs would have been really interesting.
- _jal 9y ago
  PZ almost certainly is aware of at least the general issue - Windows DDL hijacking posts hit the Full Disclosure list pretty much weekly. It is a very well-known issue, at least among those who subscribe to security lists, approaching dead-horse status.
cptskippy 9y ago
> A couple of lines to fix the bug, in theory Yes that's all it takes. No code reviews, security review, integration testing, quality assurance testing, compatibility testing, or validation. None of the work on the installer or updater. No release notes or other communication to the community. Just a couple lines of code. Easily done before morning coffee.
- mosselman 9y ago
  Luckily 'they don't care about security at all'. They just looked into it for fun when the report came in.
- dspillett 9y ago
  > code reviews, security review, integration testing, quality assurance testing, compatibility testing, Valid counter-criticism of the "just a couple of lines of code" line generally, but four months is a long time since disclosure and that is a lot of morning coffees.
  - cptskippy 9y ago
    I wasn't defending Microsoft, just responding to the trivialization the op made. According to the article, Microsoft is dedicating resources to a new client which won't exhibit the flaw. So I guess that means either they examined the flaw and determined it wasn't a high enough risk to warrant patching, or they don't care. Dealer's choice I guess. Maybe the number of people using Skype these days is insignificant? I tried to use it about a year ago and noped the hell away after 5 minutes.
- enzanki_ars 9y ago
  As others have already noted, yes, it’s a pain ship fixes in a company setting, and I fully understand that. On the oth r hand, 4 months for no fix is inexcusable.
- vorpalhex 9y ago
  > No code reviews, security review, integration testing, quality assurance testing, compatibility testing, or validation. If this was a small startup, I can absolutely see those being hurdles. This is Microsoft on a product they've had for 20+ years that is a major part of their platform - those things should be mostly automated and well oiled machines. A mature enterprise level company has no excuses for "QA is hard!" or "We can't validate patches!" for a product they sell. I don't expect them to get this out in ten minutes, but they should be able to manage a patch in 24 - 72 hours for a fairly critical and relatively easily solved security bug.
sametmax 9y ago
More probably this is something they need for a reason we don't know about but won't disclose.
Amezarak 9y ago
> Why is Skype loading random DLLs from user accessible folders? %SYSTEMROOT%/Temp doesn't seem to be user-readable at all, so I'm having trouble understanding how you write anything there without already having escalated permissions.
- enzanki_ars 9y ago
  I need to test this later as I only have access to an unprivileged account at the moment, but based on comments online (https://stackoverflow.com/a/11917816 https://stackoverflow.com/a/11917816), the folder is write only for unprivileged users (FILE_ADD_FILE).
  - Amezarak 9y ago
    You're correct, I was able to write a file without privilege escalation, and then I verified it was there by escalating. Thanks. If anyone is wondering, it's a little bit tricky to check because without the read permission, Windows Explorer can't see what the permissions are.
mtgx 9y ago
> Why is Skype loading random DLLs from user accessible folders? Good question. Almost every time I want to use some new security tool - like say AppLocker - Skype craps out on me, because it seems to be so badly programmed, and it's all over the place in Windows. It's why I stopped using the native app completely, and only use the web version whenever I still need to use Skype.

stevekemp 9y ago

This is due to the way that Windows loads DLLs, there is a defined search order and the current working-directory is one of the locations. There have been a lot of write-ups on this topic on Raymond Chen's blog, here is a good example: https://blogs.msdn.microsoft.com/oldnewthing/20101110-00/ https://blogs.msdn.microsoft.com/oldnewthing/20101110-00/

photonios 9y ago
That doesn't mean you cannot load a DLL manually from a static location: https://msdn.microsoft.com/en-us/library/windows/desktop/ms684175(v=vs.85).aspx https://msdn.microsoft.com/en-us/library/windows/desktop/ms6...
- enzanki_ars 9y ago
  I was just about to come back here and mention that. The article and comments that @stevekemp linked to provided a lot of insight, but as you mentioned, there are ways to prevent exploits of that nature, which the comments in the article point to also. Something still seems to be missing from my understanding of what Skype is doing...
  - chaz6 9y ago
    The cynic in me thinks this is already known and is in use by a "strategic partner" as a vector. The next version will only make it harder to find and exploit.
    - Tiki 9y ago
      Your comment reminded me of this: https://notepad-plus-plus.org/news/notepad-7.3.3-fix-cia-hacking-issue.html https://notepad-plus-plus.org/news/notepad-7.3.3-fix-cia-hac... I doubt this was intentional at first, but the fact that they don't want to fix it is very fishy.
      - saurik 9y ago
        That is a very different issue, as there the concept is that the original file in the original location is being replaced. As is stated in that post, at that point the person could just as easily replace the program itself with a modified version of the entire program. That is not a security issue in the software.
mannykannot 9y ago
If that were all there is to it, would this not be an issue for all Windows applications, not just Skype? Is it just Skype that puts the current working directory in the list, or at least in a position where it can be used to preempt the loading of the correct DLL? The link you provide as an example suggests that this sort of thing might be done to facilitate testing, but again, that doesn't seem to be a compelling reason for not fixing it now - 'ship what you test' is a good principle, but it does not preempt 'fix known security holes.'
- Svip 9y ago
  Well, I guess not all Windows applications are run as the SYSTEM account, but the Skype SKY-tmp is? I'd assume, at least, that programs intended to run as the SYSTEM account does a better job of loading DLLs from secure locations.
  - notriddle 9y ago
    In other words, the Linux equivalent would be a setuid executable that doesn't properly ignore LD_PRELOAD_PATH.
    - user5994461 9y ago
      Well, the system executable should not be run as root from /tmp obviously.
- rhodysurf 9y ago
  Its an issue with all windows applications but not all windows applications have the same elevated permissions that skype seems to
- mherdeg 9y ago
  It is an issue for all Windows applications. There was a month when 50%+ of the traffic on full-disclosure@ was just one person repeatedly announcing newly discovered problems with various Windows software that all involved search-path problems ("DLL hijacking" or related issues). There's at least one author who is super keen on using the phrase "binary planting" to refer to a similar class of attacks.
leeter 9y ago
You can actually control most of this using an application manifest. The advantage being you can prevent any non-"known" dll from loading from any folder it's not supposed to. Moreover you can ensure that it's embedded in the binary so if someone tries to modify the manifest it breaks the cryptographic signature on the binary. To anyone going "but what if they replace the binary!" well then they've already gotten past the air-tight hatchway.

brazzledazzle 9y ago

It would be interesting to see Google Project Zero’s take on it. It’s a shame they didn’t discover it because I’m sure the back and forth between them and the Skype devs would have been really interesting.

_jal 9y ago
PZ almost certainly is aware of at least the general issue - Windows DDL hijacking posts hit the Full Disclosure list pretty much weekly. It is a very well-known issue, at least among those who subscribe to security lists, approaching dead-horse status.

cptskippy 9y ago

> A couple of lines to fix the bug, in theory Yes that's all it takes. No code reviews, security review, integration testing, quality assurance testing, compatibility testing, or validation. None of the work on the installer or updater. No release notes or other communication to the community. Just a couple lines of code. Easily done before morning coffee.

mosselman 9y ago
Luckily 'they don't care about security at all'. They just looked into it for fun when the report came in.
dspillett 9y ago
> code reviews, security review, integration testing, quality assurance testing, compatibility testing, Valid counter-criticism of the "just a couple of lines of code" line generally, but four months is a long time since disclosure and that is a lot of morning coffees.
- cptskippy 9y ago
  I wasn't defending Microsoft, just responding to the trivialization the op made. According to the article, Microsoft is dedicating resources to a new client which won't exhibit the flaw. So I guess that means either they examined the flaw and determined it wasn't a high enough risk to warrant patching, or they don't care. Dealer's choice I guess. Maybe the number of people using Skype these days is insignificant? I tried to use it about a year ago and noped the hell away after 5 minutes.
enzanki_ars 9y ago
As others have already noted, yes, it’s a pain ship fixes in a company setting, and I fully understand that. On the oth r hand, 4 months for no fix is inexcusable.
vorpalhex 9y ago
> No code reviews, security review, integration testing, quality assurance testing, compatibility testing, or validation. If this was a small startup, I can absolutely see those being hurdles. This is Microsoft on a product they've had for 20+ years that is a major part of their platform - those things should be mostly automated and well oiled machines. A mature enterprise level company has no excuses for "QA is hard!" or "We can't validate patches!" for a product they sell. I don't expect them to get this out in ten minutes, but they should be able to manage a patch in 24 - 72 hours for a fairly critical and relatively easily solved security bug.

sametmax 9y ago

More probably this is something they need for a reason we don't know about but won't disclose.

Amezarak 9y ago

> Why is Skype loading random DLLs from user accessible folders? %SYSTEMROOT%/Temp doesn't seem to be user-readable at all, so I'm having trouble understanding how you write anything there without already having escalated permissions.

enzanki_ars 9y ago
I need to test this later as I only have access to an unprivileged account at the moment, but based on comments online (https://stackoverflow.com/a/11917816 https://stackoverflow.com/a/11917816), the folder is write only for unprivileged users (FILE_ADD_FILE).
- Amezarak 9y ago
  You're correct, I was able to write a file without privilege escalation, and then I verified it was there by escalating. Thanks. If anyone is wondering, it's a little bit tricky to check because without the read permission, Windows Explorer can't see what the permissions are.

mtgx 9y ago

> Why is Skype loading random DLLs from user accessible folders? Good question. Almost every time I want to use some new security tool - like say AppLocker - Skype craps out on me, because it seems to be so badly programmed, and it's all over the place in Windows. It's why I stopped using the native app completely, and only use the web version whenever I still need to use Skype.