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                            CERT-Renater

                Note d'Information No. 2026/VULN359
_____________________________________________________________________

DATE                : 08/04/2026

HARDWARE PLATFORM(S): /

OPERATING SYSTEM(S): Systems running rack-session (RubyGems) versions
                                      prior to 2.1.2.

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

`Rack::Session::Cookie` decrypt failure falls back to accepting
unencrypted cookies.
Critical
ioquatix published GHSA-33qg-7wpp-89cq Apr 7, 2026

Package
rack-session (RubyGems)

Affected versions
>=2, < 2.1.2

Patched versions
2.1.2


Description

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.


Details

When secrets: is configured, Rack::Session::Cookie attempts to decrypt
incoming session cookies using one of the configured encryptors. If
all decrypt attempts fail, the implementation does not reject the
cookie. Instead, it falls back to decoding the cookie using a default
coder.

This fallback path processes attacker-controlled cookie data as trusted
session state. The behavior is implicit and occurs even when encrypted
cookies are expected.

The fallback decoder is applied automatically and does not require the
application to opt into a non-encrypted session format. As a result, a
client can send a specially crafted cookie value that bypasses the
intended integrity protections provided by secrets:.

This issue affects both default configurations and those using
alternative serializers for encrypted payloads.


Impact

Any Rack application using Rack::Session::Cookie with secrets: may be
affected.

Note

Rails applications are typically not affected — Rails uses
ActionDispatch::Session::CookieStore, which is a separate implementation
backed by ActiveSupport::MessageEncryptor and does not share the
vulnerable code path.

An unauthenticated attacker can supply a crafted session cookie that is
accepted as valid session data. This can lead to authentication bypass or
privilege escalation in applications that rely on session values for
identity or authorization decisions.

Depending on application behavior and available runtime components,
processing of untrusted session data may also expose additional risks.


Mitigation

    Update to a patched version ofrack-session that rejects cookies when
decryption fails under the secrets: configuration.

        After updating, rotate session secrets to invalidate existing
session cookies, since attacker-supplied session data may have been
accepted and re-issued prior to the fix.


Severity
Critical
9.3/ 10

CVSS v4 base metrics
Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality High
Integrity High
Availability High
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

CVE ID
CVE-2026-39324

Weaknesses
Weakness CWE-287
Weakness CWE-345
Weakness CWE-502
Weakness CWE-565

Credits

    @sm1ee sm1ee Reporter
    @ioquatix ioquatix Remediation developer
    @jeremyevans jeremyevans Remediation reviewer


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