This vulnerability enables a remote attacker who can read encrypted objects in an S3 bucket to downgrade the padding scheme used for a wrapped envelope key to legacy PKCS#1 v1.5. The downgrade triggers a Bleichenbacher padding oracle, allowing the attacker to retrieve the RSA-wrapped key and consequently decrypt the file. The flaw represents a confidentiality compromise due to improper cryptographic key handling and key format validation.

Affected systems include Amazon’s .NET S3 Encryption Client (Amazon.Extensions.S3.Encryption). Versions 3.2.2 and 4.0.3 contain the fix; earlier 1.x and 2.x releases lack this patch and should be migrated. The advisory references NuGet packages for the patched releases.

The CVSS score of 6.0 classifies the vulnerability as moderate. The EPSS score is not available, and it is not listed in the CISA KEV catalog, indicating no documented active exploitation. The attack requires that the adversary already has read access to encrypted objects, which may be granted to many users in shared buckets, and relies on the ability to modify the object metadata. The potential impact is data confidentiality loss for all objects whose envelope keys are susceptible to the padding downgrade.