Posts

  • Ghost Potato

    Halloween has come and gone, and yet NTLM reflection is back from the dead to haunt MSRC once again. This post describes a deceptively simple bug that has existed in Windows for 15 years.

    NTLM reflection is still possible through a highly reliable timing attack. The attack works by abusing the logic responsible for its mitigation, a widely speculated challenge cache. Attackers can purge this cache by deliberately failing an authentication attempt and doing so removes all challenge entries older than 5 minutes.

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  • DEF CON 27 Workshop Slide Deck

    Now that we finished our workshop “Constructing Kerberos Attacks with Delegation Primitives” at DEF CON 27, we can share the slide deck:

    Constructing Kerberos Attacks with Delegation Primitives.pdf

    It was fun spreading the gospel, and we thank all the attendees for their participation.

  • Gone to the Dogs

    Just in time for our DEF CON workshop “Constructing Kerberos Attacks with Delegation Primitives”, Microsoft failed to meet the disclosure deadline, and so we publish another primitive that can be abused to achieve Windows Local Privilege Escalation (LPE). It affects all domain-joined Windows 10 hosts by default, as well as Windows Server 2016 and Windows Server 2019 that have the WebDAV Redirector feature installed.

    This attack is very similar to the LPE attack chain that we disclosed in “Wagging the Dog”. Actually, it is identical except for the primitive used to initiate the attack chain.

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  • A Surprising LSASS Bug

    When lunch conversations at work take on a mischievous tone, all sorts of strange ideas come forth. This time, as Elad Shamir (@elad_shamir) was present, talk of course turned to his recent work on Kerberos, Wagging the Dog (or as I prefer to call it, “Screwing the Pooch”, its original title, which was eventually vetoed by a person far more sensible than either of us).

    My fuzzing targets had gone stale, and I was on the lookout for new openings. Someone mentioned constrained delegation and began describing the flow of Kerberos messages. Well, what about Kerberos messages? How many researchers have in fact explored this attack surface for memory corruption? The lunch gang reasoned that the number could probably be placed in the hundreds if not thousands.

    Owing to cockiness, I decided to investigate anyway.

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  • Linux Privilege Escalation via LXD & Hijacked UNIX Socket Credentials

    Linux systems running LXD are vulnerable to privilege escalation via multiple attack paths, two of which are published in my “lxd_root” GitHub repository. This blog will go into the details of what I think is a very interesting path - abusing relayed UNIX socket credentials to speak directly to systemd’s private interface.

    Ubuntu 19.04 Server edition comes with the LXD snap installed by default. The only requirement for this exploit in a fresh install of Ubuntu is access to a user account that is a member of the lxd group.

    Privilege escalation via LXD in general has been a known issue since 2016, with a simple method described in theory in a GitHub issue and also in a practical implementation in a security blog by @reboare.

    I believe I am the first to describe exploitation using stolen socket credentials, which also works with unprivileged containers.

    Before I came across these issues, nothing in the official LXD documentation existed to warn users that the lxd group was dangerous. Anyone following the official guidelines to configure LXD would have added their account into this group before deploying their first container. I opened a bug with Canonical to express my concerns - you can read the full thread here. The LXD team quickly made adjustments to the documentation, which now clearly states that this group should only be given to those trusted with root access.

    As always, interacting with the Canonical folks via their bug tracker was a really pleasant experience. I’d like to thank them for their time and for the thoughtful consideration they gave my ideas. I highly recommend other security researchers bring items directly to them in this manner.

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  • Privilege Escalation in Ubuntu Linux (dirty_sock exploit)

    In January 2019, I discovered a privilege escalation vulnerability in default installations of Ubuntu Linux. This was due to a bug in the snapd API, a default service. Any local user could exploit this vulnerability to obtain immediate root access to the system.

    Two working exploits are provided in the dirty_sock repository:

    1. dirty_sockv1: Uses the ‘create-user’ API to create a local user based on details queried from the Ubuntu SSO.
    2. dirty_sockv2: Sideloads a snap that contains an install-hook that generates a new local user.

    Both are effective on default installations of Ubuntu. Testing was mostly completed on 18.10, but older verions are vulnerable as well.

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  • Wagging the Dog: Abusing Resource-Based Constrained Delegation to Attack Active Directory

    Back in March 2018, I embarked on an arguably pointless crusade to prove that the TrustedToAuthForDelegation attribute was meaningless, and that “protocol transition” can be achieved without it. I believed that security wise, once constrained delegation was enabled (msDS-AllowedToDelegateTo was not null), it did not matter whether it was configured to use “Kerberos only” or “any authentication protocol”.

    I started the journey with Benjamin Delpy’s (@gentilkiwi) help modifying Kekeo to support a certain attack that involved invoking S4U2Proxy with a silver ticket without a PAC, and we had partial success, but the final TGS turned out to be unusable. Ever since then, I kept coming back to it, trying to solve the problem with different approaches but did not have much success. Until I finally accepted defeat, and ironically then the solution came up, along with several other interesting abuse cases and new attack techniques.

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  • Internal Monologue Attack - Retrieving NTLM Hashes without Touching LSASS (Repost)

    Mimikatz is a well-regarded post-exploitation tool, which allows adversaries to extract plain text passwords, NTLM hashes and Kerberos tickets from memory, as well as perform attacks such as pass-the-hash, pass-the-ticket or build a golden ticket. Arguably, the primary use of Mimikatz is retrieving user credentials from LSASS process memory for use in post exploitation lateral movement.

    Recently, Microsoft has introduced Credential Guard in Windows 10 Enterprise and Windows Server 2016, which uses virtualization-based security to isolate secrets, and it is very effective in preventing Mimikatz from retrieving hashes directly from memory. Also, Mimikatz has become a prime target of most endpoint protection solutions, and they are very aggressive in their efforts to detect and prevent it. Although these efforts are bound to fail, they are increasingly becoming a nuisance.

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  • Launch!

    Welcome to our new blog, where we will occasionally write about our research projects when we discover an interesting vulnerability or a new TTP, and if we are at liberty to publish them.

    We hope you find our posts valuable and enjoy reading them.

    We will publish an interesting post at the end of the month. Stay tuned!