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CRITICALpublic exploit

CVE-2026-39324

CVE-2026-39324 — Rack::Session::Cookie secrets: decrypt failure fallback enables secretless session forgery and Marshal deserialization

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.

Published Updated Sources: cvelistV5, GitHub_M

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the scoring sources call it.

Exploitation

Exploit code

public exploit, none observed

EPSS

0%

chance of exploitation in 30 days

Affects

rack

rack_session

CVSS base

9.3

CRITICAL

CISA SSVC assessment

Three decision points CISA publishes for the CVEs it assesses · SSVC 2.0.3. A stakeholder decision, not a severity score.

CISA

Exploitation

None

none · proof-of-concept · active

Automatable

Yes

can an attacker script all four kill-chain steps

Technical impact

Total

partial · total control of the vulnerable component

Affected scope

The catalog records vendors and products as separate lists, not pairs, so which product belongs to which vendor is not something this page can say.

Vendors (1)

rack

Products (1)

rack session

Every base score collected

Sources score independently and disagree; each row says who scored it and under which version.

ScoreVersionSeverityExpl.ImpactSource
9.3CVSS 4.0CRITICAL——cvelistV5

Weakness & attack patterns

  • CWE-287
  • CWE-345
  • CWE-502
  • CWE-565

Attack patterns reported against this CVE. The ATT&CK techniques below are inferred from its weakness class.

  • T1548Abuse Elevation Control Mechanism
  • T1040Network Sniffing
  • T1185Browser Session Hijacking

Public exploit

Capability, not use: code existing is a different claim from anyone running it.

Indexed by

poc in github

References

1 on the record

Elsewhere on this site

  • rackevery CVE for this vendor
  • CWE-287other pages naming this weakness

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

  • No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
  • No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
  • No exploit packet captures, log samples, or incident case IDs supplied.