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Resolv::getaddresses bug allows one to bypass your SSRF filter. #1

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EdOverflow opened this issue Nov 7, 2017 · 1 comment

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commented Nov 7, 2017

Description

Resolv::getaddresses is OS-dependent, therefore by playing around with different IP formats one can return blank values. This bug can be abused to bypass your exclusion list.

馃捇 Machine 1 馃捇 Machine 2
ruby 2.3.3p222 (2016-11-21) [x86_64-linux-gnu] ruby 2.3.1p112 (2016-04-26) [x86_64-linux-gnu]

馃捇 Machine 1

irb(main):002:0> Resolv.getaddresses("127.0.0.1")
=> ["127.0.0.1"]
irb(main):003:0> Resolv.getaddresses("localhost")
=> ["127.0.0.1"]
irb(main):004:0> Resolv.getaddresses("127.000.000.1")
=> ["127.0.0.1"]

馃捇 Machine 2

irb(main):008:0> Resolv.getaddresses("127.0.0.1")
=> ["127.0.0.1"]
irb(main):009:0> Resolv.getaddresses("localhost")
=> ["127.0.0.1"]
irb(main):010:0> Resolv.getaddresses("127.000.000.1")
=> []

Proof of concept

irb(main):001:0> require 'private_address_check'
=> true
irb(main):002:0> PrivateAddressCheck.resolves_to_private_address?("127.1")
=> false # Bypass

Mitigation

I suggest staying away from Resolv::getaddresses altogether and using the Socket class.

irb(main):002:0> Resolv.getaddresses("127.1")
=> []
irb(main):003:0> Socket.getaddrinfo("127.1", nil).sample[3]
=> "127.0.0.1"
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commented Nov 8, 2017

This issue has been resolved in 58a0d7f.

@EdOverflow EdOverflow closed this Nov 8, 2017

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