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.
Attack Parameters
Attack Vector
Network
Can be exploited remotely
Attack Complexity
Low
Easy to exploit
Attack Requirements
Present
Additional conditions required
Privileges Required
Low
Basic privileges needed
User Interaction
None
No user interaction needed
Impact Assessment
Confidentiality
High
Complete data leak
Integrity
None
No data modification
Availability
None
No disruption
CVSS Vector v4.0
References 4
https://aws.amazon.com/security/security-bulletins/2026-135-aws/
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https://github.com/aws/amazon-s3-encryption-client-dotnet/security/advisories/G…
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https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/3.2.2
ff89ba41-3aa1-4d27-914a-91399e9639e5
https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/4.0.3
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