├── LICENSE └── README.md /LICENSE: -------------------------------------------------------------------------------- 1 | GNU GENERAL PUBLIC LICENSE 2 | Version 2, June 1991 3 | 4 | Copyright (C) 1989, 1991 Free Software Foundation, Inc., 5 | 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA 6 | Everyone is permitted to copy and distribute verbatim copies 7 | of this license document, but changing it is not allowed. 8 | 9 | Preamble 10 | 11 | The licenses for most software are designed to take away your 12 | freedom to share and change it. By contrast, the GNU General Public 13 | License is intended to guarantee your freedom to share and change free 14 | software--to make sure the software is free for all its users. This 15 | General Public License applies to most of the Free Software 16 | Foundation's software and to any other program whose authors commit to 17 | using it. (Some other Free Software Foundation software is covered by 18 | the GNU Lesser General Public License instead.) You can apply it to 19 | your programs, too. 20 | 21 | When we speak of free software, we are referring to freedom, not 22 | price. Our General Public Licenses are designed to make sure that you 23 | have the freedom to distribute copies of free software (and charge for 24 | this service if you wish), that you receive source code or can get it 25 | if you want it, that you can change the software or use pieces of it 26 | in new free programs; and that you know you can do these things. 27 | 28 | To protect your rights, we need to make restrictions that forbid 29 | anyone to deny you these rights or to ask you to surrender the rights. 30 | These restrictions translate to certain responsibilities for you if you 31 | distribute copies of the software, or if you modify it. 32 | 33 | For example, if you distribute copies of such a program, whether 34 | gratis or for a fee, you must give the recipients all the rights that 35 | you have. You must make sure that they, too, receive or can get the 36 | source code. And you must show them these terms so they know their 37 | rights. 38 | 39 | We protect your rights with two steps: (1) copyright the software, and 40 | (2) offer you this license which gives you legal permission to copy, 41 | distribute and/or modify the software. 42 | 43 | Also, for each author's protection and ours, we want to make certain 44 | that everyone understands that there is no warranty for this free 45 | software. If the software is modified by someone else and passed on, we 46 | want its recipients to know that what they have is not the original, so 47 | that any problems introduced by others will not reflect on the original 48 | authors' reputations. 49 | 50 | Finally, any free program is threatened constantly by software 51 | patents. We wish to avoid the danger that redistributors of a free 52 | program will individually obtain patent licenses, in effect making the 53 | program proprietary. To prevent this, we have made it clear that any 54 | patent must be licensed for everyone's free use or not licensed at all. 55 | 56 | The precise terms and conditions for copying, distribution and 57 | modification follow. 58 | 59 | GNU GENERAL PUBLIC LICENSE 60 | TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION 61 | 62 | 0. This License applies to any program or other work which contains 63 | a notice placed by the copyright holder saying it may be distributed 64 | under the terms of this General Public License. The "Program", below, 65 | refers to any such program or work, and a "work based on the Program" 66 | means either the Program or any derivative work under copyright law: 67 | that is to say, a work containing the Program or a portion of it, 68 | either verbatim or with modifications and/or translated into another 69 | language. (Hereinafter, translation is included without limitation in 70 | the term "modification".) Each licensee is addressed as "you". 71 | 72 | Activities other than copying, distribution and modification are not 73 | covered by this License; they are outside its scope. The act of 74 | running the Program is not restricted, and the output from the Program 75 | is covered only if its contents constitute a work based on the 76 | Program (independent of having been made by running the Program). 77 | Whether that is true depends on what the Program does. 78 | 79 | 1. You may copy and distribute verbatim copies of the Program's 80 | source code as you receive it, in any medium, provided that you 81 | conspicuously and appropriately publish on each copy an appropriate 82 | copyright notice and disclaimer of warranty; keep intact all the 83 | notices that refer to this License and to the absence of any warranty; 84 | and give any other recipients of the Program a copy of this License 85 | along with the Program. 86 | 87 | You may charge a fee for the physical act of transferring a copy, and 88 | you may at your option offer warranty protection in exchange for a fee. 89 | 90 | 2. You may modify your copy or copies of the Program or any portion 91 | of it, thus forming a work based on the Program, and copy and 92 | distribute such modifications or work under the terms of Section 1 93 | above, provided that you also meet all of these conditions: 94 | 95 | a) You must cause the modified files to carry prominent notices 96 | stating that you changed the files and the date of any change. 97 | 98 | b) You must cause any work that you distribute or publish, that in 99 | whole or in part contains or is derived from the Program or any 100 | part thereof, to be licensed as a whole at no charge to all third 101 | parties under the terms of this License. 102 | 103 | c) If the modified program normally reads commands interactively 104 | when run, you must cause it, when started running for such 105 | interactive use in the most ordinary way, to print or display an 106 | announcement including an appropriate copyright notice and a 107 | notice that there is no warranty (or else, saying that you provide 108 | a warranty) and that users may redistribute the program under 109 | these conditions, and telling the user how to view a copy of this 110 | License. (Exception: if the Program itself is interactive but 111 | does not normally print such an announcement, your work based on 112 | the Program is not required to print an announcement.) 113 | 114 | These requirements apply to the modified work as a whole. If 115 | identifiable sections of that work are not derived from the Program, 116 | and can be reasonably considered independent and separate works in 117 | themselves, then this License, and its terms, do not apply to those 118 | sections when you distribute them as separate works. But when you 119 | distribute the same sections as part of a whole which is a work based 120 | on the Program, the distribution of the whole must be on the terms of 121 | this License, whose permissions for other licensees extend to the 122 | entire whole, and thus to each and every part regardless of who wrote it. 123 | 124 | Thus, it is not the intent of this section to claim rights or contest 125 | your rights to work written entirely by you; rather, the intent is to 126 | exercise the right to control the distribution of derivative or 127 | collective works based on the Program. 128 | 129 | In addition, mere aggregation of another work not based on the Program 130 | with the Program (or with a work based on the Program) on a volume of 131 | a storage or distribution medium does not bring the other work under 132 | the scope of this License. 133 | 134 | 3. You may copy and distribute the Program (or a work based on it, 135 | under Section 2) in object code or executable form under the terms of 136 | Sections 1 and 2 above provided that you also do one of the following: 137 | 138 | a) Accompany it with the complete corresponding machine-readable 139 | source code, which must be distributed under the terms of Sections 140 | 1 and 2 above on a medium customarily used for software interchange; or, 141 | 142 | b) Accompany it with a written offer, valid for at least three 143 | years, to give any third party, for a charge no more than your 144 | cost of physically performing source distribution, a complete 145 | machine-readable copy of the corresponding source code, to be 146 | distributed under the terms of Sections 1 and 2 above on a medium 147 | customarily used for software interchange; or, 148 | 149 | c) Accompany it with the information you received as to the offer 150 | to distribute corresponding source code. (This alternative is 151 | allowed only for noncommercial distribution and only if you 152 | received the program in object code or executable form with such 153 | an offer, in accord with Subsection b above.) 154 | 155 | The source code for a work means the preferred form of the work for 156 | making modifications to it. For an executable work, complete source 157 | code means all the source code for all modules it contains, plus any 158 | associated interface definition files, plus the scripts used to 159 | control compilation and installation of the executable. However, as a 160 | special exception, the source code distributed need not include 161 | anything that is normally distributed (in either source or binary 162 | form) with the major components (compiler, kernel, and so on) of the 163 | operating system on which the executable runs, unless that component 164 | itself accompanies the executable. 165 | 166 | If distribution of executable or object code is made by offering 167 | access to copy from a designated place, then offering equivalent 168 | access to copy the source code from the same place counts as 169 | distribution of the source code, even though third parties are not 170 | compelled to copy the source along with the object code. 171 | 172 | 4. You may not copy, modify, sublicense, or distribute the Program 173 | except as expressly provided under this License. Any attempt 174 | otherwise to copy, modify, sublicense or distribute the Program is 175 | void, and will automatically terminate your rights under this License. 176 | However, parties who have received copies, or rights, from you under 177 | this License will not have their licenses terminated so long as such 178 | parties remain in full compliance. 179 | 180 | 5. You are not required to accept this License, since you have not 181 | signed it. However, nothing else grants you permission to modify or 182 | distribute the Program or its derivative works. These actions are 183 | prohibited by law if you do not accept this License. Therefore, by 184 | modifying or distributing the Program (or any work based on the 185 | Program), you indicate your acceptance of this License to do so, and 186 | all its terms and conditions for copying, distributing or modifying 187 | the Program or works based on it. 188 | 189 | 6. Each time you redistribute the Program (or any work based on the 190 | Program), the recipient automatically receives a license from the 191 | original licensor to copy, distribute or modify the Program subject to 192 | these terms and conditions. You may not impose any further 193 | restrictions on the recipients' exercise of the rights granted herein. 194 | You are not responsible for enforcing compliance by third parties to 195 | this License. 196 | 197 | 7. If, as a consequence of a court judgment or allegation of patent 198 | infringement or for any other reason (not limited to patent issues), 199 | conditions are imposed on you (whether by court order, agreement or 200 | otherwise) that contradict the conditions of this License, they do not 201 | excuse you from the conditions of this License. If you cannot 202 | distribute so as to satisfy simultaneously your obligations under this 203 | License and any other pertinent obligations, then as a consequence you 204 | may not distribute the Program at all. For example, if a patent 205 | license would not permit royalty-free redistribution of the Program by 206 | all those who receive copies directly or indirectly through you, then 207 | the only way you could satisfy both it and this License would be to 208 | refrain entirely from distribution of the Program. 209 | 210 | If any portion of this section is held invalid or unenforceable under 211 | any particular circumstance, the balance of the section is intended to 212 | apply and the section as a whole is intended to apply in other 213 | circumstances. 214 | 215 | It is not the purpose of this section to induce you to infringe any 216 | patents or other property right claims or to contest validity of any 217 | such claims; this section has the sole purpose of protecting the 218 | integrity of the free software distribution system, which is 219 | implemented by public license practices. Many people have made 220 | generous contributions to the wide range of software distributed 221 | through that system in reliance on consistent application of that 222 | system; it is up to the author/donor to decide if he or she is willing 223 | to distribute software through any other system and a licensee cannot 224 | impose that choice. 225 | 226 | This section is intended to make thoroughly clear what is believed to 227 | be a consequence of the rest of this License. 228 | 229 | 8. If the distribution and/or use of the Program is restricted in 230 | certain countries either by patents or by copyrighted interfaces, the 231 | original copyright holder who places the Program under this License 232 | may add an explicit geographical distribution limitation excluding 233 | those countries, so that distribution is permitted only in or among 234 | countries not thus excluded. In such case, this License incorporates 235 | the limitation as if written in the body of this License. 236 | 237 | 9. The Free Software Foundation may publish revised and/or new versions 238 | of the General Public License from time to time. Such new versions will 239 | be similar in spirit to the present version, but may differ in detail to 240 | address new problems or concerns. 241 | 242 | Each version is given a distinguishing version number. If the Program 243 | specifies a version number of this License which applies to it and "any 244 | later version", you have the option of following the terms and conditions 245 | either of that version or of any later version published by the Free 246 | Software Foundation. If the Program does not specify a version number of 247 | this License, you may choose any version ever published by the Free Software 248 | Foundation. 249 | 250 | 10. If you wish to incorporate parts of the Program into other free 251 | programs whose distribution conditions are different, write to the author 252 | to ask for permission. For software which is copyrighted by the Free 253 | Software Foundation, write to the Free Software Foundation; we sometimes 254 | make exceptions for this. Our decision will be guided by the two goals 255 | of preserving the free status of all derivatives of our free software and 256 | of promoting the sharing and reuse of software generally. 257 | 258 | NO WARRANTY 259 | 260 | 11. BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY 261 | FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN 262 | OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES 263 | PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED 264 | OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 265 | MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS 266 | TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE 267 | PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, 268 | REPAIR OR CORRECTION. 269 | 270 | 12. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING 271 | WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR 272 | REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, 273 | INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING 274 | OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED 275 | TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY 276 | YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER 277 | PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE 278 | POSSIBILITY OF SUCH DAMAGES. 279 | 280 | END OF TERMS AND CONDITIONS 281 | 282 | How to Apply These Terms to Your New Programs 283 | 284 | If you develop a new program, and you want it to be of the greatest 285 | possible use to the public, the best way to achieve this is to make it 286 | free software which everyone can redistribute and change under these terms. 287 | 288 | To do so, attach the following notices to the program. It is safest 289 | to attach them to the start of each source file to most effectively 290 | convey the exclusion of warranty; and each file should have at least 291 | the "copyright" line and a pointer to where the full notice is found. 292 | 293 | 294 | Copyright (C) 295 | 296 | This program is free software; you can redistribute it and/or modify 297 | it under the terms of the GNU General Public License as published by 298 | the Free Software Foundation; either version 2 of the License, or 299 | (at your option) any later version. 300 | 301 | This program is distributed in the hope that it will be useful, 302 | but WITHOUT ANY WARRANTY; without even the implied warranty of 303 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 304 | GNU General Public License for more details. 305 | 306 | You should have received a copy of the GNU General Public License along 307 | with this program; if not, write to the Free Software Foundation, Inc., 308 | 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA. 309 | 310 | Also add information on how to contact you by electronic and paper mail. 311 | 312 | If the program is interactive, make it output a short notice like this 313 | when it starts in an interactive mode: 314 | 315 | Gnomovision version 69, Copyright (C) year name of author 316 | Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 317 | This is free software, and you are welcome to redistribute it 318 | under certain conditions; type `show c' for details. 319 | 320 | The hypothetical commands `show w' and `show c' should show the appropriate 321 | parts of the General Public License. Of course, the commands you use may 322 | be called something other than `show w' and `show c'; they could even be 323 | mouse-clicks or menu items--whatever suits your program. 324 | 325 | You should also get your employer (if you work as a programmer) or your 326 | school, if any, to sign a "copyright disclaimer" for the program, if 327 | necessary. Here is a sample; alter the names: 328 | 329 | Yoyodyne, Inc., hereby disclaims all copyright interest in the program 330 | `Gnomovision' (which makes passes at compilers) written by James Hacker. 331 | 332 | , 1 April 1989 333 | Ty Coon, President of Vice 334 | 335 | This General Public License does not permit incorporating your program into 336 | proprietary programs. If your program is a subroutine library, you may 337 | consider it more useful to permit linking proprietary applications with the 338 | library. If this is what you want to do, use the GNU Lesser General 339 | Public License instead of this License. 340 | -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | # Attacking And Defending The GCPMetadata API 2 | This repo gives an overview of some GCP metadata API attack and defend patterns 3 | 4 | This is complementary to a presentation that I recently did with @matter_of_cat at bsidessf on this subject. Video available here [https://www.youtube.com/watch?v=z5hPU3g2aZ8](https://www.youtube.com/watch?v=z5hPU3g2aZ8) 5 | 6 | ## Overview 7 | A metadata API in a cloud platform is an internal API resources like VM's that run code can query to obtain information about themselves, and obtain credentials to access the instance identity attached to the resource. 8 | 9 | 10 | 11 | Similar API's exist in other cloud platforms, including AWS. 12 | 13 | ## AWS Metadata API Recap/Overview 14 | In AWS for a long time simple get requests could be use to fetch instance identity credentials. This lead to a string of SSRF vulnerabilities, where when services would send simple get requests on behalf of a user, the user could proxy requests to the metadata API endpoint and fetch credentials. 15 | 16 | 17 | 18 | This is the vulnerability that lead to the [Capital One data breach](https://edition.cnn.com/2019/07/29/business/capital-one-data-breach/index.html) 19 | 20 | ### AWS Protection 21 | AWS rolled out [IMDSv2](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/configuring-instance-metadata-service.html), which protects against most SSRF vulnerabilities, by requiring users have a header set on their request. 22 | 23 | # GCP Metadata API 24 | Similar to AWS, there are two versions of the metadata API, v0.1 and v1. v1 requires a header be set, which protects against many SSRF vulnerabilities. 25 | 26 | ## Bypassing the GCP Protections in Cloud Functions 27 | Cloud functions are a serverless offering in GCP that has a metadata available to them. 28 | 29 | Not too long ago, a blog post was released by Google, demonstrating how to run a headless browser in these cloud functions 30 | 31 | 32 | 33 | Because browsers can set headers, untrusted HTML can potentially access the metadata API. They are beholden to the Same Origin Policy, but [DNS rebinding](https://github.com/nccgroup/singularity) tricks [allow](https://www.youtube.com/watch?v=Q0JG_eKLcws) us to [bypass](https://www.youtube.com/watch?v=HfpnloZM61I) this restriction. 34 | 35 | This means customers of Google that stood up headless browsers in the cloud function and allowed untrusted HTML to be rendered exposed the instance credentials to the untrusted HTML. 36 | 37 | ### Identifying Vulnerable Customers 38 | HTTP Triggered Cloud functions expose most of their naming in their naming in their subdomain. They also default to being on the open internet with no authentication. They use the following naming convention: 39 | 40 | ``` 41 | https://-.cloudfunctions.net/ 42 | ``` 43 | 44 | Where the function names default to function-1, function-2, etc... the blog post also suggested naming the function `screenshot`. 45 | 46 | Using this information and a passiveDNS vendor, we can enumerate cloud functions: 47 | 48 | 49 | 50 | At this point to get the last piece, we can guess the path via either trying `/function-X` or `/screenshot` and see if we're presented with a page that matches the one from the blog post. 51 | 52 | Using this technique I was able to identify vulnerable customers. 53 | 54 | _Because in GCP instances have default identities with default permissions, this leads to a compromise of a significant portion of the customer's GCP project. More on this later..._ 55 | 56 | ### Fixing DNS Rebind Attacks 57 | Google paid 1337 for the report, and added Host header validation to protect against future DNS rebind attacks. 58 | 59 | 60 | All that said, SSRF is not the only way to attack the Metadata API, and the rest of this post will serve to show other methods of attack that are specific to the GCP platform. 61 | 62 | ## Google Created Identities (Service Accounts) In GCP 63 | In GCP, Service Accounts are used to provide instances identity and give them privilege. When you enable all the API's in GCP, identities with default permissions bound to your project are created on your behalf. Some of them are attached to instances you control, some of them are attached to instances you do not control. Here's a list of them: 64 | 65 | 66 | 67 | These service accounts fall into two buckets, __Google Managed__ service accounts and __User Managed__ service accounts. 68 | 69 | ### User Managed Service Accounts 70 | The main difference between these two is __User Managed__ service accounts, though created on your behalf, have permissions that are easily visible to you, and are mostly attached to resources you can see and have control over. There are two __User Managed__ service accounts, and 47 __Google Managed__ ones for comparison. 71 | 72 | Here's what the user managed ones look like: 73 | ``` 74 | -compute@developer.gserviceaccount.com 75 | @appspot.gserviceaccount.com 76 | ``` 77 | 78 | Both user managed Service Accounts have a project level Editor role binding. The editor role binding has thousands of permissions and can do things which include, access all the bigquery in your project, access all the buckets in your project, access all the databases in your project, access all the spanner in your project, etc... 79 | 80 | 81 | 82 | These two service accounts are attached to just about all of your resources by default. This means the cloud function example above likely meant the compromise of a lot of data and many resources, because by default the metadata API would have returned a credential that has over 2000 permissions. 83 | 84 | 85 | ### Attacking The Metadata API in GKE 86 | In GKE, the GCP Kubernetes offering, the nodes that power your cluster are just standard GCP VM's you can view in your project. This means, again they are given the default service account with project editor. 87 | 88 | Unlike Cloud Functions, VM's have something called _scopes_ that limit which API's the service account can access, regardless 89 | of the service account's permissions. By default this is the scope applied to VM's: 90 | 91 | 92 | 93 | Note: this leaves read access to storage open, meaning VM's by default can read data out of all buckets in the project. 94 | 95 | Because workloads in GKE all run on underlying VM's that have storage open, all workloads in GKE by default can hit the metadata API, and fetch these credentials. 96 | 97 | Note, nodes likely need storage enabled, so they can fetch from the GCR (Google Container Registry) which is powered by storage. 98 | 99 | #### GKE Metadata Protections 100 | GCP offeres a wide range of offerings to protect against both the K8's and the GCP metadata API: 101 | 102 | 103 | You can read about them here: 104 | 105 | + https://cloud.google.com/kubernetes-engine/docs/how-to/protecting-cluster-metadata#concealment 106 | + https://cloud.google.com/kubernetes-engine/docs/how-to/workload-identity 107 | + https://cloud.google.com/kubernetes-engine/docs/how-to/shielded-gke-nodes 108 | 109 | 110 | Note none are enabled by default, and the only offering that blocks the VM's GCP credentials from being fetched is Workload Identity, and it's incompatible with Metadata Concealment. 111 | 112 | ### GKE Example 113 | Here's a demo where we enabled both Concealment and Shielded Nodes, and where able to access the node's credentials: https://drive.google.com/file/d/1JLNzBjixe_iqPSmOZfR8oE9spdnOVCp8/view 114 | 115 | ## Google Managed Service Accounts 116 | So far we talked about service accounts created on your behalf, that by default get attached to your resources, like Cloud Functions and VM's. 117 | 118 | But what about Google Managed Service Accounts? 119 | 120 | For starters, it's not 100% clear how they're used, because they're mostly attached to infrastructure you can't actually see, to power the cloud. The role bindings for these service accounts are visible, which is how we took the screenshot above, but the roles themselves are often not visible, so we can't always see the underlying permissions. 121 | 122 | Fortunately, a clever trick [@matter_of_cat](https://twitter.com/MatterOfCat) found allows us to see these permissions, via the iam roles copy API, copying the permissions from a role we can't introspect, into a role we can introspect. 123 | 124 | 125 | 126 | Using this technique, let's look at a Google Managed Service account used for [Cloud Build](https://cloud.google.com/cloud-build) 127 | 128 | ``` 129 | @cloudbuild.gserviceaccount.com 130 | ``` 131 | 132 | 133 | 134 | This role has read/write to all Storage, pubsub, Cloud Build, and a few other things in your project. 135 | 136 | ### Attacking The Metadata API in Cloud Build 137 | Cloud Build is an API that monitors a Git Repository, and builds artifacts from the repository, and typically publishes them to GCR for you. 138 | 139 | It's pretty handy to just attach to a repo and have it magically build your container images for you. 140 | 141 | ### Cloud Build Example 142 | Though we can't actually know exactly what's going on here, it is likely when you push to your repo, behind the scenes there's some kind of VM or Cloud Function that isn't visibile in your project doing your build. This VM as it so happens, exposes a metadata API. This means if in the build steps we reach out to the metadata API, we can steal a credential for the Google Managed service account 143 | 144 | 145 | Note, in this demo, I've been granted 0 access to GCP, I was simply added as a collaborator on a repo, that had cloudbuild enabled. By default this is enabled on all branches, so in this demo I'll be stealing this GCP credential from a random branch on a repo I've been added as a contributor to. 146 | 147 | Here's a demo of us doing that: 148 | https://drive.google.com/file/d/1bISilsz1XhsNSzvvrt_WX4iLqbSe_o3y/view?usp=sharing 149 | 150 | And here's the repository we used: 151 | https://github.com/allisonis/fun-cloudbuild 152 | 153 | ## Detection 154 | Fortunately, though some aspects of Google Managed service accounts are not visible in your project, logs from the usage of its credentials are. 155 | 156 | This means we can do things like alert on anomalous behavior such as these credentials being used outside of Google IP ranges: 157 | 158 | 159 | 160 | Note this method is imperfect, because the attacker can likely spin up a VM in GCP and come from an IP similar to the one legitimately used. 161 | 162 | Other anomalous behavior is API specific. For example, we can be pretty sure this cloudbuild service account shouldn't be accessing certain buckets (such as the passwords bucket used in the demo) so we can write custom alerts per API to look for anomalous behavior. 163 | 164 | [Event Threat Detection](https://cloud.google.com/event-threat-detection/docs/concepts-overview) is an API built into Google that is meant to do this for you. It monitors stackdriver and alerts you if it sees anomalous or bad behavior. Unfortunately today it has no detection for these types of attacks, however we can expect they will continue to build this service out and hopefully pick up support for these types of attacks. 165 | 166 | # Conclusion 167 | Though the GCP metadata API at a high level seems similar to AWS, when we dig deeper into it we see it's the default identities GCP gives all of its offerings that really sets its risk profile apart from AWS. 168 | 169 | Though we covered 3 API's in this readme, there are way more than 3 that expose the metadata API, some of which allow you to fetch Google Managed credentials, many of which allow you to fetch user managed credentials. 170 | 171 | The custom header Google has you set protects against a lot of SSRF attacks, however as shown in this writeup, there are still many attacks you can perform against the metadata API that don't require an SSRF vulnerability. 172 | 173 | 174 | ## Recommendations 175 | Here's what we recommend: 176 | 177 | + Across the board descope your default user managed identities 178 | + Don't use default identities 179 | + Make use of project isolation (put cloudbuild in its own project) 180 | + In GKE enable workload identity 181 | + Write custom alerts in stackdriver to detect anomalous usage of Google Managed credentials 182 | --------------------------------------------------------------------------------