How severe is CVE-2021-43203?

CVE-2021-43203 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.

Is CVE-2021-43203 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (55th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2021-43203?

CVE-2021-43203 affects Jetbrains Ktor. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2021-43203?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.

When was CVE-2021-43203 published?

CVE-2021-43203 was published on 2021-11-09 and last updated on 2026-06-17.

CVE-2021-43203

CVE-2021-43203 is a high-severity vulnerability in Jetbrains Ktor with a CVSS 3.x base score of 7.5. 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-287.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 5.0
EPSS exploit prediction: 1% (55th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-287
Affected product: Jetbrains Ktor
Published: 2021-11-09
Last modified: 2026-06-17

Description

In JetBrains Ktor before 1.6.4, nonce verification during the OAuth2 authentication process is implemented improperly.

Frequently asked questions

What is CVE-2021-43203?: In JetBrains Ktor before 1.6.4, nonce verification during the OAuth2 authentication process is implemented improperly.
How severe is CVE-2021-43203?: CVE-2021-43203 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is none, integrity high, and availability none.
Is CVE-2021-43203 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (55th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2021-43203?: CVE-2021-43203 affects Jetbrains Ktor. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2021-43203?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
When was CVE-2021-43203 published?: CVE-2021-43203 was published on 2021-11-09 and last updated on 2026-06-17.

References

https://blog.jetbrains.com/blog/2021/11/08/jetbrains-security-bulletin-q3-2021/

Affected products (1)

cpe:2.3:a:jetbrains:ktor:*:*:*:*:*:*:*:*

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