How severe is CVE-2016-4028?

CVE-2016-4028 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with high attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.

Is CVE-2016-4028 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-2016-4028?

CVE-2016-4028 affects Open-xchange Ox Guard. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2016-4028?

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-2016-4028 published?

CVE-2016-4028 was published on 2016-12-15 and last updated on 2026-06-17.

CVE-2016-4028

CVE-2016-4028 is a high-severity vulnerability in Open-xchange Ox Guard 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-255.

Key facts

Severity: High (CVSS 3.x base score 7.5)
CVSS v2: 3.5
EPSS exploit prediction: 1% (49th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-255
Affected product: Open-xchange Ox Guard
Published: 2016-12-15
Last modified: 2026-06-17

Description

An issue was discovered in Open-Xchange OX Guard before 2.4.0-rev8. OX Guard uses an authentication token to identify and transfer guest users' credentials. The OX Guard API acts as a padding oracle by responding with different error codes depending on whether the provided token matches the encryption padding. In combination with AES-CBC, this allows attackers to guess the correct padding. Attackers may run brute-forcing attacks on the content of the guest authentication token and discover user credentials. For a practical attack vector, the guest users needs to have logged in, the content of the guest user's "OxReaderID" cookie and the value of the "auth" parameter needs to be known to the attacker.

Frequently asked questions

What is CVE-2016-4028?: An issue was discovered in Open-Xchange OX Guard before 2.4.0-rev8. OX Guard uses an authentication token to identify and transfer guest users' credentials. The OX Guard API acts as a padding oracle by responding with different error codes depending on whether the provided token matches the encryption padding. In combination with AES-CBC, this allows attackers to guess the correct padding. Attackers may run brute-forcing attacks on the content of the guest authentication token and discover user credentials. For a practical attack vector, the guest users needs to have logged in, the content of the guest user's "OxReaderID" cookie and the value of the "auth" parameter needs to be known to the attacker.
How severe is CVE-2016-4028?: CVE-2016-4028 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with high attack complexity, requires low privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability high.
Is CVE-2016-4028 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-2016-4028?: CVE-2016-4028 affects Open-xchange Ox Guard. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2016-4028?: 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-2016-4028 published?: CVE-2016-4028 was published on 2016-12-15 and last updated on 2026-06-17.

References

Affected products (1)

cpe:2.3:a:open-xchange:ox_guard:*:rev7:*:*:*:*:*:*

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