How severe is CVE-2026-27459?

CVE-2026-27459 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2026-27459 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (49th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-27459?

CVE-2026-27459 affects Pyopenssl. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-27459?

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

Does CVE-2026-27459 have an EU (EUVD) identifier?

Yes. CVE-2026-27459 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-12677.

When was CVE-2026-27459 published?

CVE-2026-27459 was published on 2026-03-18 and last updated on 2026-07-01.

CVE-2026-27459

CVE-2026-27459 is a critical-severity vulnerability in Pyopenssl with a CVSS 3.x base score of 9.8. 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-120.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v4: 7.2
EPSS exploit prediction: 1% (49th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-12677
Weakness: CWE-120
Affected product: Pyopenssl
Published: 2026-03-18
Last modified: 2026-07-01

Description

pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to `set_cookie_generate_callback` returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.

Frequently asked questions

What is CVE-2026-27459?: pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to `set_cookie_generate_callback` returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.
How severe is CVE-2026-27459?: CVE-2026-27459 has a CVSS 3.x base score of 9.8, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2026-27459 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (49th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-27459?: CVE-2026-27459 affects Pyopenssl. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-27459?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
Does CVE-2026-27459 have an EU (EUVD) identifier?: Yes. CVE-2026-27459 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-12677.
When was CVE-2026-27459 published?: CVE-2026-27459 was published on 2026-03-18 and last updated on 2026-07-01.

References

Affected products (1)

cpe:2.3:a:pyopenssl:pyopenssl:*:*:*:*:*:*:*:*

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