Critical Bug in Mozilla’s NSS Crypto Library Potentially Affects Several Other Software

Mozilla has rolled out fixes to address a critical security
weakness in its cross-platform Network Security Services (NSS^[1]) cryptographic library
that could be potentially exploited by an adversary to crash a
vulnerable application and even execute arbitrary code.

Tracked as CVE-2021-43527, the flaw affects NSS versions prior
to 3.73 or 3.68.1 ESR, and concerns a heap overflow^[2]
vulnerability when verifying digital signatures such as DSA^[3]
and RSA-PSS^[4]
algorithms that are encoded using the DER^[5]
binary format. Credited with reporting the issue is Tavis Ormandy
of Google Project Zero, who codenamed it “BigSig^[6].”

“NSS (Network Security Services) versions prior to 3.73 or
3.68.1 ESR are vulnerable to a heap overflow when handling
DER-encoded DSA or RSA-PSS signatures,” Mozilla said^[7]
in an advisory published Wednesday. “Applications using NSS for
handling signatures encoded within CMS, S/MIME, PKCS #7, or PKCS
#12 are likely to be impacted.”

NSS is a collection of open-source cryptographic computer
libraries designed to enable cross-platform development of
client-server applications, with support for SSL v3, TLS, PKCS #5,
PKCS #7, PKCS #11, PKCS #12, S/MIME, X.509 v3 certificates, and
other security standards.

The bug, the consequence of missing bounds check that could
allow the execution of arbitrary attacker-controlled code, is said
to have been exploitable dating all the way back to June 2012, “The
striking thing about this vulnerability is just how simple it is,”
Ormandy said^[8]
in a technical write-up.

While the BigSig shortcoming doesn’t affect Mozilla’s Firefox
web browser itself, email clients, PDF viewers, and other
applications that rely on NSS for signature verification, such as
Red Hat^[9], Thunderbird,
LibreOffice, Evolution, and Evince, are believed to be
vulnerable.

“This is a major memory corruption flaw in NSS, almost any use
of NSS is affected,” Ormandy tweeted^[10]. “If you are a vendor
that distributes NSS in your products, you will most likely need to
update or backport the patch.”