CVE-2026-110310: RSA PKCS#1 v1.5 padding oracle via legacy envelope key format downgrade in Amazon S3 Encryption Client for .NET
A remote user who can read encrypted objects in the S3 bucket can force a padding downgrade by altering the padding scheme recorded in an object's metadata to legacy PKCS#1 v1.5. Since this padding scheme is susceptible to the well-known Bleichenbacher attack, the user can use a PKCS#1 v1.5 padding oracle to recover the RSA-wrapped envelope key and decrypt the object.
To remediate this issue, we recommend that users upgrade to version 3.2.2 or 4.0.3. The 1.x and 2.x lines will not be patched, users of those lines need to migrate to 3.2.2 or 4.0.3.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
Amazon S3 Encryption Client for .NETto a version that resolves this vulnerability.Fixed in 3.2.2 - Upgrade
Upgrade
Amazon S3 Encryption Client for .NETto a version that resolves this vulnerability.Fixed in 4.0.3
Event History
Frequently Asked Questions
Who can realistically exploit this issue?
An attacker needs the ability to read encrypted objects in the affected S3 bucket. They must also be able to alter the padding scheme recorded in an object's metadata to force use of the legacy PKCS#1 v1.5 format.
What can an attacker obtain through exploitation?
The attacker can use a PKCS#1 v1.5 padding oracle attack to recover the RSA-wrapped envelope key. That key can then be used to decrypt the affected object.
Which versions should be used to remediate the issue?
Upgrade to Amazon S3 Encryption Client for .NET version 3.2.2 or 4.0.3. The 1.x and 2.x release lines will not be patched and require migration to one of those versions.