CVE-2021-20305

A flaw was found in Nettle in versions before 3.7.2, where several Nettle signature verification functions (GOST DSA, EDDSA & ECDSA) result in the Elliptic Curve Cryptography point (ECC) multiply function being called with out-of-range scalers, possibly resulting in incorrect results. This flaw allows an attacker to force an invalid signature, causing an assertion failure or possible validation. The highest threat to this vulnerability is to confidentiality, integrity, as well as system availability.

Weakness

The product uses a broken or risky cryptographic algorithm or protocol.

Affected Software

Extended Description

Cryptographic algorithms are the methods by which data is scrambled to prevent observation or influence by unauthorized actors. Insecure cryptography can be exploited to expose sensitive information, modify data in unexpected ways, spoof identities of other users or devices, or other impacts. It is very difficult to produce a secure algorithm, and even high-profile algorithms by accomplished cryptographic experts have been broken. Well-known techniques exist to break or weaken various kinds of cryptography. Accordingly, there are a small number of well-understood and heavily studied algorithms that should be used by most products. Using a non-standard or known-insecure algorithm is dangerous because a determined adversary may be able to break the algorithm and compromise whatever data has been protected. Since the state of cryptography advances so rapidly, it is common for an algorithm to be considered “unsafe” even if it was once thought to be strong. This can happen when new attacks are discovered, or if computing power increases so much that the cryptographic algorithm no longer provides the amount of protection that was originally thought. For a number of reasons, this weakness is even more challenging to manage with hardware deployment of cryptographic algorithms as opposed to software implementation. First, if a flaw is discovered with hardware-implemented cryptography, the flaw cannot be fixed in most cases without a recall of the product, because hardware is not easily replaceable like software. Second, because the hardware product is expected to work for years, the adversary’s computing power will only increase over time.

Potential Mitigations

• When there is a need to store or transmit sensitive data, use strong, up-to-date cryptographic algorithms to encrypt that data. Select a well-vetted algorithm that is currently considered to be strong by experts in the field, and use well-tested implementations. As with all cryptographic mechanisms, the source code should be available for analysis.
• For example, US government systems require FIPS 140-2 certification [REF-1192].
• Do not develop custom or private cryptographic algorithms. They will likely be exposed to attacks that are well-understood by cryptographers. Reverse engineering techniques are mature. If the algorithm can be compromised if attackers find out how it works, then it is especially weak.
• Periodically ensure that the cryptography has not become obsolete. Some older algorithms, once thought to require a billion years of computing time, can now be broken in days or hours. This includes MD4, MD5, SHA1, DES, and other algorithms that were once regarded as strong. [REF-267]
• Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid.
• Industry-standard implementations will save development time and may be more likely to avoid errors that can occur during implementation of cryptographic algorithms. Consider the ESAPI Encryption feature.

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/20.html
• https://cwe.mitre.org/data/definitions/459.html
• https://cwe.mitre.org/data/definitions/473.html
• https://cwe.mitre.org/data/definitions/475.html
• https://cwe.mitre.org/data/definitions/608.html
• https://cwe.mitre.org/data/definitions/614.html
• https://cwe.mitre.org/data/definitions/97.html

References

• https://bugzilla.redhat.com/show_bug.cgi?id=1942533
• https://lists.debian.org/debian-lts-announce/2021/09/msg00008.html
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MQKWVVMAIDAJ7YAA3VVO32BHLDOH2E63/
• https://security.gentoo.org/glsa/202105-31
• https://security.netapp.com/advisory/ntap-20211022-0002/
• https://www.debian.org/security/2021/dsa-4933
• https://bugzilla.redhat.com/show_bug.cgi?id=1942533
• https://lists.debian.org/debian-lts-announce/2021/09/msg00008.html
• https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/MQKWVVMAIDAJ7YAA3VVO32BHLDOH2E63/
• https://security.gentoo.org/glsa/202105-31
• https://security.netapp.com/advisory/ntap-20211022-0002/
• https://www.debian.org/security/2021/dsa-4933