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Date: Fri, 7 Sep 2018 09:54:02 +1000
From: Jeremy Choi <jechoi@...hat.com>
To: oss-security@...ts.openwall.com
Subject: perl Crypt::JWT vulnerability
A vulnerability that might be able to cause bypass authentication was
discovered by myself in Perl Crypt::JWT package prior to 0.023(fix -
https://github.com/DCIT/perl-Crypt-JWT/commit/b98a59b42ded9f9e51b2560410106207c2152d6c
).
## Details
(JWT.pm)
606 # key
607 my $key = defined $args{keypass} ? [$args{key}, $args{keypass}] :
$args{key};
608 my $kid = exists $header->{kid} ? $header->{kid} :
$unprotected_header->{kid};
609 if (!defined $key && defined $kid && $args{kid_keys}) {
610 my $k = _kid_lookup($kid, $args{kid_keys}, $alg);
611 $key = $k if defined $k;
612 }
613 # if no key given, try to use 'jwk' value from header
614 $key = $header->{jwk} if !$key && $header->{jwk};
The vulnerability comes from line 614. If no 'kid' is given, 'jwk' will be
used instead. Where 'RS256' is set as alg, it's okay as _prepare_rsa_key()
will be failed. However, if 'HS256' is set, the key from the 'jwk' header
is used for decoding.
...
537 elsif ($alg =~ /^HS(256|384|512)$/) { # HMAC integrity
538 $key = _prepare_oct_key($key);
539 return 1 if $sig eq hmac("SHA$1", $key, $data);
540 }
...
65 sub _prepare_oct_key {
66 my ($key) = @_;
67 croak "JWT: undefined oct key" unless defined $key;
68 if (ref $key eq 'HASH' && $key->{k} && $key->{kty} && $key->{kty} eq
'oct') {
69 return decode_b64u($key->{k});
70 }
71 elsif (!ref $key) {
72 return $key;
73 }
Since the jwk key is a string, it reaches line 72 and then 539 above.
If a project uses Crypt::JWT for its authentication without additional
mitigation, it may allow attackers to bypass authentication by providing a
token by crafting with hmac() with 'HS(256|384|512)'
I'm requesting a CVE ID through DWF.
Thanks
--
Jeremy Choi / Red Hat Product Security
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