OpenSSL Releases Patches for 2 High-Severity Security Vulnerabilities

The maintainers of OpenSSL have released a fix for two
high-severity security flaws in its software that could be
exploited to carry out denial-of-service (DoS) attacks and bypass
certificate verification.

Tracked as CVE-2021-3449 and CVE-2021-3450, both
the vulnerabilities^[1]
have been resolved in an update (version OpenSSL 1.1.1k) released
on Thursday. While CVE-2021-3449 affects all OpenSSL 1.1.1
versions, CVE-2021-3450 impacts OpenSSL versions 1.1.1h and
newer.

OpenSSL is a software library consisting of cryptographic
functions that implement the Transport Layer Security protocol with
the goal of securing communications sent over a computer
network.

According to an advisory published by OpenSSL, CVE-2021-3449
concerns a potential DoS vulnerability arising due to NULL pointer
dereferencing that can cause an OpenSSL TLS server to crash if in
the course of renegotiation the client transmits a malicious
“ClientHello” message during the handshake^[2]
between the server and a user. The issue was introduced as part of
changes dating back to January 2018.

“If a TLSv1.2 renegotiation ClientHello omits the
signature_algorithms extension (where it was present in the initial
ClientHello), but includes a signature_algorithms_cert extension
then a NULL pointer dereference will result, leading to a crash and
a denial of service attack,” the advisory said^[3].

Nokia, which has been credited with reporting the flaw on March
17, fixed the DoS bug with a one-line code change^[4].

CVE-2021-3450, on the other hand, relates to an
X509_V_FLAG_X509_STRICT flag that enables additional security
checks of certificates present in a certificate chain. While this
flag is not set by default, an error in the implementation meant
that OpenSSL failed to check that “non-CA certificates must not be
able to issue other certificates,” resulting in a certificate
bypass.

As a result, the flaw prevented apps from rejecting TLS
certificates that aren’t digitally signed by a browser-trusted
certificate authority (CA).

“In order to be affected, an application must explicitly set the
X509_V_FLAG_X509_STRICT verification flag and either not set a
purpose for the certificate verification or, in the case of TLS
client or server applications, override the default purpose,”
OpenSSL said.

Benjamin Kaduk from Akamai is said to have reported the issue to
the project maintainers on March 18. The vulnerability was
discovered by Xiang Ding and others at Akamai, with a fix^[5]
put in place by former Red Hat principal software engineer and
OpenSSL developer Tomáš Mráz.

Although neither of the issues affect OpenSSL 1.0.2, it’s also
worth noting that the version has been out of support since January
1, 2020, and is no longer receiving updates. Applications that rely
on a vulnerable version of OpenSSL are advised to apply the patches
to mitigate the risk associated with the flaws.