How severe is CVE-2026-39324?

CVE-2026-39324 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-2026-39324 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (19th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-39324?

CVE-2026-39324 affects Rack Rack-session. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-39324?

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-2026-39324 have an EU (EUVD) identifier?

Yes. CVE-2026-39324 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-19820.

When was CVE-2026-39324 published?

CVE-2026-39324 was published on 2026-04-07 and last updated on 2026-06-17.

CVE-2026-39324

CVE-2026-39324 is a critical-severity vulnerability in Rack Rack-session 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-287.

Key facts

Severity: Critical (CVSS 3.x base score 9.8)
CVSS v4: 9.3
EPSS exploit prediction: 0% (19th percentile)
Actively exploited: Not listed in CISA KEV
EU (EUVD) id: EUVD-2026-19820
Weakness: CWE-287
Affected product: Rack Rack-session
Published: 2026-04-07
Last modified: 2026-06-17

Description

Rack::Session is a session management implementation for Rack. From 2.0.0 to before 2.1.2, Rack::Session::Cookie incorrectly handles decryption failures when configured with secrets:. If cookie decryption fails, the implementation falls back to a default decoder instead of rejecting the cookie. This allows an unauthenticated attacker to supply a crafted session cookie that is accepted as valid session data without knowledge of any configured secret. Because this mechanism is used to load session state, an attacker can manipulate session contents and potentially gain unauthorized access. This vulnerability is fixed in 2.1.2.

Frequently asked questions

What is CVE-2026-39324?: Rack::Session is a session management implementation for Rack. From 2.0.0 to before 2.1.2, Rack::Session::Cookie incorrectly handles decryption failures when configured with secrets:. If cookie decryption fails, the implementation falls back to a default decoder instead of rejecting the cookie. This allows an unauthenticated attacker to supply a crafted session cookie that is accepted as valid session data without knowledge of any configured secret. Because this mechanism is used to load session state, an attacker can manipulate session contents and potentially gain unauthorized access. This vulnerability is fixed in 2.1.2.
How severe is CVE-2026-39324?: CVE-2026-39324 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-2026-39324 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (19th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-39324?: CVE-2026-39324 affects Rack Rack-session. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-39324?: 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-2026-39324 have an EU (EUVD) identifier?: Yes. CVE-2026-39324 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-19820.
When was CVE-2026-39324 published?: CVE-2026-39324 was published on 2026-04-07 and last updated on 2026-06-17.

References

https://github.com/rack/rack-session/security/advisories/GHSA-33qg-7wpp-89cq

Affected products (1)

cpe:2.3:a:rack:rack-session:*:*:*:*:*:ruby:*:*

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