=== Introduction ===================================================
My institution uses Qualys
to scan for vulnerabilities, including on some Debian Linux machines
that I manage. The scanner does some network scans, and also logs in
to each machine to do "authenticated scans".
=== Discovery ======================================================
When I recently updated my machines from Debian11 to Debian12, the
Qualys scanner was no longer able to SSH login, with syslog lines:
sshd: userauth_pubkey: signature algorithm ssh-rsa not in PubkeyAcceptedAlgorithms [preauth]
The ssh-rsa algorithm was removed from the default list in Debian12
(has OpenSSH 9.2, up from 8.4 in Debian11), see e.g.
... disables RSA signatures using the SHA-1 hash algorithm by
default. This change has been made as the SHA-1 hash algorithm
is cryptographically broken ...
I confirmed that Qualys uses (requires) ssh-rsa as public key signing
algorithm: its SSH login to Debian12 suceeds with the SSHD setting
"PubkeyAcceptedAlgorithms +ssh-rsa", and to Debian11 fails with the
opposite "PubkeyAcceptedKeyTypes -ssh-rsa".
=== Issues =========================================================
- Qualys scanner uses insecure ssh-rsa algorithm for pubkey signing
in its attempt of SSH login.
- Modern SSHD servers reject pubkey login with ssh-rsa, so Qualys is
unable to scan up-to-date Linux e.g. Debian12 or RHEL9.
- Qualys does not check the list of pubkey signing algorithms
accepted by SSHD servers, cannot notify about any insecure ones.
=== Vulnerability ==================================================
Any SSHD server that accepts the insecure ssh-rsa algorithm for pubkey
signing is vulnerable. The fact that Qualys had been able to log in to
all Linux machines at my institution, shows that all accept ssh-rsa
and are vulnerable. It is expected that anywhere that Qualys is used,
all Linux machines (except recently updated) are similarly vulnerable.
The vulnerability affects all uses of public key authentication.
Qualys itself facilitates an internal attack, by providing the account
used to do "authenticated scans", forced onto all machines and with
root (sudo) access, with the public key commonly available to any
local admins of any scanned machines. An attack on this account is
both easier and more fruitful; admittedly an attack may be impractical
with currently available computing resources.
=== Fixes needed ===================================================
- Qualys to reconfigure the scanner to use a secure pubkey signing
algorithm for its SSH login attempt. This same fix also enables
Qualys to scan up-to-date Linux e.g. Debian12 or RHEL9.
- Qualys to check the pubkey signing algorithms accepted by SSHD
servers, and notify when insecure ones are in use.
- Administrators of Linux machines to check SSHD settings, ensure
that ssh-rsa is not accepted. This is needed on all SSHD servers,
regardless of whether Qualys is used.
=== Comments =======================================================
It is curious how Qualys:
- uses (requires!) an insecure pubkey signing algorithm, though
better alternatives have been the norm for decades;
- did not notice its inability to do authenticated scans on RHEL9
and similar machines, since over a year ago;
- checks many similar (similarly impractical) SSHD issues, but does
not check pubkey signing; and
- seems to know all about SSH, reporting esoteric issues in its
internals, but still uses it wrongly.
=== Dedication =====================================================
I dedicate this advisory to Luis Fuentes-Cobas, my one-time professor
of Electromagnetism, who taught me logic, deduction and persistence.
Maybe I missed the class about patience.
=== References =====================================================
=== Timeline =======================================================
24 June 2023 Discovered, notified internally within my institution
9 July 2023 Qualys contacted via "community" post
16 July 2023 Qualys contacted via firstname.lastname@example.org
26 July 2023 CVE requested from email@example.com (a CNA partner)
Paul Szabo firstname.lastname@example.org www.maths.usyd.edu.au/u/psz
School of Mathematics and Statistics University of Sydney Australia