dovecot - Apr 2019 - CVE-2019-10691: JSON encoder in Dovecot 2.3 incorrecty assert-crashes when encountering invalid UTF-8 characters.

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we're sharing our latest advisory with you and would like to thank
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Feel free to join our bug bounty programs (open-xchange, dovecot,
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Yours sincerely,
Aki Tuomi
Open-Xchange Oy

Open-Xchange Security Advisory 2019-04-18
Product: Dovecot
Vendor: OX Software GmbH

Internal reference: DOV-3173 (Bug ID)
Vulnerability type: CWE-176
Vulnerable version: 2.3.0 - 2.3.5.1
Vulnerable component: json encoder
Report confidence: Confirmed
Researcher credits: cPanel L.L.C.
Solution status: Fixed by Vendor
Fixed version: 2.3.5.2
Vendor notification: 2019-04-02
Solution date: 2019-04-11
Public disclosure: 2019-04-18
CVE reference: CVE-2019-10691
CVSS: 7.5 (CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
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Vulnerability Details:
JSON encoder in Dovecot 2.3 incorrecty assert-crashes when encountering
invalid UTF-8 characters. This can be used to crash dovecot in two ways.
Attacker can repeatedly crash Dovecot authentication process by logging
in using invalid UTF-8 sequence in username. This requires that auth
policy is enabled.
Crash can also occur if OX push notification driver is enabled and an
email is delivered with invalid UTF-8 sequence in From or Subject header.
In 2.2, malformed UTF-8 sequences are forwarded "as-is", and thus do
not
cause problems in Dovecot itself. Target systems should be checked for
possible problems in dealing with such sequences.
See https://wiki.dovecot.org/Authentication/Policy for details on auth
policy support.

Risk:
Determined attacker can prevent authentication process from staying up
by keeping on attempting to log in with username containing invalid
UTF-8 sequence.
Steps to reproduce:
Configure dovecot with auth_policy_server_url and auth_policy_hash_nonce
set.
Attempt to log in with username containing an invalid UTF-8 sequence
Observe assert-crash in dovecot logs.

Solution:
Operators should update to the latest Patch Release or disable auth
policy support.


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