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AUSCERT External Security Bulletin Redistribution
curl security update
21 December 2020
AusCERT Security Bulletin Summary
Operating System: Debian GNU/Linux
Impact/Access: Denial of Service -- Remote/Unauthenticated
Provide Misleading Information -- Remote with User Interaction
Access Confidential Data -- Remote with User Interaction
CVE Names: CVE-2020-8286 CVE-2020-8285 CVE-2020-8284
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Debian LTS Advisory DLA-2500-1 firstname.lastname@example.org
https://www.debian.org/lts/security/ Roberto C. S=E1nchez
December 18, 2020 https://wiki.debian.org/LTS
Package : curl
Version : 7.52.1-5+deb9u13
CVE ID : CVE-2020-8284 CVE-2020-8285 CVE-2020-8286
Debian Bug : 977161 977162 977163
Several vulnerabilities were discovered in curl, a command line tool for
transferring data with URL syntax and an easy-to-use client-side URL
When curl performs a passive FTP transfer, it first tries the EPSV
command and if that is not supported, it falls back to using PASV.
Passive mode is what curl uses by default. A server response to a
PASV command includes the (IPv4) address and port number for the
client to connect back to in order to perform the actual data
transfer. This is how the FTP protocol is designed to work. A
malicious server can use the PASV response to trick curl into
connecting back to a given IP address and port, and this way
potentially make curl extract information about services that are
otherwise private and not disclosed, for example doing port scanning
and service banner extractions.
The IP address part of the response is now ignored by default, by
making CURLOPT_FTP_SKIP_PASV_IP default to 1L instead of previously
being 0L. This has the minor drawback that a small fraction of use
cases might break, when a server truly needs the client to connect
back to a different IP address than what the control connection uses
and for those CURLOPT_FTP_SKIP_PASV_IP can be set to 0L. The same
goes for the command line tool, which then might need
--no-ftp-skip-pasv-ip set to prevent curl from ignoring the address
in the server response.
libcurl offers a wildcard matching functionality, which allows a
callback (set with CURLOPT_CHUNK_BGN_FUNCTION) to return information
back to libcurl on how to handle a specific entry in a directory
when libcurl iterates over a list of all available entries. When
this callback returns CURL_CHUNK_BGN_FUNC_SKIP, to tell libcurl to
not deal with that file, the internal function in libcurl then calls
itself recursively to handle the next directory entry. If there's a
sufficient amount of file entries and if the callback returns "skip"
enough number of times, libcurl runs out of stack space. The exact
amount will of course vary with platforms, compilers and other
environmental factors. The content of the remote directory is not
kept on the stack, so it seems hard for the attacker to control
exactly what data that overwrites the stack - however it remains a
Denial-Of-Service vector as a malicious user who controls a server
that a libcurl-using application works with under these premises can
trigger a crash.
The internal function is rewritten to instead and more appropriately
use an ordinary loop instead of the recursive approach. This way,
the stack use will remain the same no matter how many files that are
libcurl offers "OCSP stapling" via the CURLOPT_SSL_VERIFYSTATUS
option. When set, libcurl verifies the OCSP response that a server
responds with as part of the TLS handshake. It then aborts the TLS
negotiation if something is wrong with the response. The same
feature can be enabled with --cert-status using the curl tool. As
part of the OCSP response verification, a client should verify that
the response is indeed set out for the correct certificate. This
step was not performed by libcurl when built or told to use OpenSSL
as TLS backend. This flaw would allow an attacker, who perhaps
could have breached a TLS server, to provide a fraudulent OCSP
response that would appear fine, instead of the real one. Like if
the original certificate actually has been revoked.
The OCSP response checker function now also verifies that the
certificate id is the correct one.
For Debian 9 stretch, these problems have been fixed in version
We recommend that you upgrade your curl packages.
For the detailed security status of curl please refer to
its security tracker page at:
Further information about Debian LTS security advisories, how to apply
these updates to your system and frequently asked questions can be
found at: https://wiki.debian.org/LTS
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