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% (53rd 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% (53rd 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% (53rd 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:*:*:*:*:*:*:*:*

More vulnerabilities in Jetbrains Ktor

CVE-2019-12736 — Critical (CVSS 9.8): JetBrains Ktor framework before 1.2.0-rc does not sanitize the username provided by the user for the LDAP protocol,…
CVE-2022-29930 — High (CVSS 8.7): SHA1 implementation in JetBrains Ktor Native 2.0.0 was returning the same value. The issue was fixed in Ktor version…
CVE-2023-45612 — High (CVSS 8.6): In JetBrains Ktor before 2.3.5 default configuration of ContentNegotiation with XML format was vulnerable to XXE
CVE-2019-10102 — High (CVSS 8.1): JetBrains Ktor framework (created using the Kotlin IDE template) versions before 1.1.0 were resolving artifacts using…
CVE-2022-48476 — High (CVSS 7.5): In JetBrains Ktor before 2.3.0 path traversal in the `resolveResource` method was possible
CVE-2023-45613 — Medium (CVSS 6.8): In JetBrains Ktor before 2.3.5 server certificates were not verified

All CVEs affecting Jetbrains Ktor →

Other CWE-287 (Improper Authentication) vulnerabilities

CVE-2026-45480 — Critical (CVSS 10.0): Improper authentication in Azure Active Directory allows an unauthorized attacker to elevate privileges over a network.
CVE-2026-46389 — Critical (CVSS 10.0): UDS Identity Config builds the Keycloak configuration image (realm, plugins, theme, truststore, JARs) consumed by UDS…
CVE-2026-10611 — Critical (CVSS 10.0): An authentication bypass vulnerability exists in MISP when LDAP mixed authentication is enabled with OTP enforcement.…
CVE-2026-47280 — Critical (CVSS 10.0): Improper authentication in Azure Resource Manager (ARM) allows an unauthorized attacker to elevate privileges over a…
CVE-2026-42822 — Critical (CVSS 10.0): Improper authentication in Azure Local Disconnected Operations allows an unauthorized attacker to elevate privileges…
CVE-2026-20182 — Critical (CVSS 10.0): May 2026: This security advisory provides the details and fix information for a vulnerability that was discovered and…

Browse all CWE-287 (Improper Authentication) vulnerabilities →