CVE-2026-110310

RSA PKCS#1 v1.5 padding oracle via legacy envelope key format downgrade in Amazon S3 Encryption Client for .NET

State: Published
Severity: 6.0 Medium
Published: Oct 14, 2026
Updated: Oct 14, 2026
Reserved: Oct 13, 2026

Description

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.

Scores

CVSS	Score	Vector	Source
4.0	6.0 Medium	CVSS:4.0/AV:N/AC:L/AT:P/PR:L/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N	AMZN
3.1	5.3 Medium	CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N	AMZN

Weaknesses

CWE-204 Observable response discrepancy
CWE-757 Selection of Less-Secure algorithm during negotiation ('algorithm downgrade')

Affected Products

Product	Versions
Amazon.Extensions.S3.EncryptionAmazon	2.1.0 through 2.*Affected 3.0.0 through 3.2.1Affected 4.0.0 through 4.0.2Affected

References

https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/3.2.2release-notespatch
https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/4.0.3release-notespatch
https://aws.amazon.com/security/security-bulletins/2026-135-aws/vendor-advisory
https://github.com/aws/amazon-s3-encryption-client-dotnet/security/advisories/GHSA-322w-f6rw-r4fjthird-party-advisory

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