Medium 5.3Published Oct 14, 2026

CVE-2026-110310RSA PKCS#1 v1.5 padding oracle via legacy envelope key format downgrade in Amazon S3 Encryption Client for .NET

Updated under an hour ago (2026-10-19 10:30 UTC)

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No exploitation reported and no EPSS estimate yet.

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Exploited in the last 30 days

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.

CVSS: 5.3 MEDIUM (v3.1, CNA)CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
Weakness (CWE): CWE-204, CWE-757
Known exploited: Not in CISA's KEV catalog
EPSS (exploitation in the next 30 days): Not loaded for this CVE
Assigned by: AMZN
NVD analysis: Awaiting Analysis
Record last updated: Oct 14, 2026
First seen by InfoSec Signals: Oct 14, 2026

Affected products

Affected products

Amazon Amazon.Extensions.S3.Encryption amazon:amazon.extensions.s3.encryption (from the name)	2.1.0 builds before 2.*; from 3.0.0 through 3.2.1; from 4.0.0 through 4.0.2	CNA record

References

https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/3.2.2release-notes, patch
https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/4.0.3release-notes, patch
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

The CVE record on cve.org NVD's entry

The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.