How severe is CVE-2023-4421?

CVE-2023-4421 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.

Is CVE-2023-4421 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (46th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2023-4421?

CVE-2023-4421 affects Mozilla Nss. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2023-4421?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2023-4421 published?

CVE-2023-4421 was published on 2023-12-12 and last updated on 2026-06-17.

CVE-2023-4421

CVE-2023-4421 is a medium-severity vulnerability in Mozilla Nss with a CVSS 3.x base score of 6.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-203.

Key facts

Severity: Medium (CVSS 3.x base score 6.5)
EPSS exploit prediction: 1% (46th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-203
Affected product: Mozilla Nss
Published: 2023-12-12
Last modified: 2026-06-17

Description

The NSS code used for checking PKCS#1 v1.5 was leaking information useful in mounting Bleichenbacher-like attacks. Both the overall correctness of the padding as well as the length of the encrypted message was leaking through timing side-channel. By sending large number of attacker-selected ciphertexts, the attacker would be able to decrypt a previously intercepted PKCS#1 v1.5 ciphertext (for example, to decrypt a TLS session that used RSA key exchange), or forge a signature using the victim's key. The issue was fixed by implementing the implicit rejection algorithm, in which the NSS returns a deterministic random message in case invalid padding is detected, as proposed in the Marvin Attack paper. This vulnerability affects NSS < 3.61.

Frequently asked questions

What is CVE-2023-4421?: The NSS code used for checking PKCS#1 v1.5 was leaking information useful in mounting Bleichenbacher-like attacks. Both the overall correctness of the padding as well as the length of the encrypted message was leaking through timing side-channel. By sending large number of attacker-selected ciphertexts, the attacker would be able to decrypt a previously intercepted PKCS#1 v1.5 ciphertext (for example, to decrypt a TLS session that used RSA key exchange), or forge a signature using the victim's key. The issue was fixed by implementing the implicit rejection algorithm, in which the NSS returns a deterministic random message in case invalid padding is detected, as proposed in the Marvin Attack paper. This vulnerability affects NSS < 3.61.
How severe is CVE-2023-4421?: CVE-2023-4421 has a CVSS 3.x base score of 6.5, rated medium severity. It is exploitable over network with low attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
Is CVE-2023-4421 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (46th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2023-4421?: CVE-2023-4421 affects Mozilla Nss. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2023-4421?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2023-4421 published?: CVE-2023-4421 was published on 2023-12-12 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:mozilla:nss:*:*:*:*:*:*:*:*

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