FreeBSD : py-yaml -- FullLoader (still) exploitable for arbitrary command execution (aae8fecf-888e-11ea-9714-08002718de91)

critical Nessus Plugin ID 136021


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The remote FreeBSD host is missing one or more security-related updates.


Riccardo Schirone ( reports :

In FullLoader python/object/new constructor, implemented by construct_python_object_apply, has support for setting the state of a deserialized instance through the set_python_instance_state method.
After setting the state, some operations are performed on the instance to complete its initialization, however it is possible for an attacker to set the instance' state in such a way that arbitrary code is executed by the FullLoader.

This patch tries to block such attacks in FullLoader by preventing set_python_instance_state from setting arbitrar properties. It implements a blacklist that includes extend method (called by construct_python_object_apply) and all special methods (e.g. __set__,
__setitem__, etc.).

Users who need special attributes being set in the state of a deserialized object can still do it through the UnsafeLoader, which however should not be used on untrusted input. Additionally, they can subclass FullLoader and redefine state_blacklist_regexp to include the additional attributes they need, passing the subclassed loader to yaml.load.


Update the affected packages.

See Also

Plugin Details

Severity: Critical

ID: 136021

File Name: freebsd_pkg_aae8fecf888e11ea971408002718de91.nasl

Version: 1.3

Type: local

Published: 4/28/2020

Updated: 10/15/2021

Dependencies: ssh_get_info.nasl

Risk Information


Risk Factor: Medium

Score: 5.9


Risk Factor: Critical

Base Score: 10

Temporal Score: 7.4

Vector: AV:N/AC:L/Au:N/C:C/I:C/A:C

Temporal Vector: E:U/RL:OF/RC:C


Risk Factor: Critical

Base Score: 9.8

Temporal Score: 8.5

Vector: CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

Temporal Vector: E:U/RL:O/RC:C

Vulnerability Information

CPE: p-cpe:/a:freebsd:freebsd:py27-yaml, p-cpe:/a:freebsd:freebsd:py35-yaml, p-cpe:/a:freebsd:freebsd:py36-yaml, p-cpe:/a:freebsd:freebsd:py37-yaml, p-cpe:/a:freebsd:freebsd:py38-yaml, cpe:/o:freebsd:freebsd

Required KB Items: Host/local_checks_enabled, Host/FreeBSD/release, Host/FreeBSD/pkg_info

Exploit Ease: No known exploits are available

Patch Publication Date: 4/27/2020

Vulnerability Publication Date: 3/2/2020

Reference Information

CVE: CVE-2020-1747

IAVA: 2021-A-0463