---
id: "CVE-2026-110310"
kind: "vulnerability"
aliases: ["CVE-2026-110310"]
title: "AWS S3 Encryption Client for .NET: padding downgrade lets attackers decrypt objects"
severity: "medium"
cvss: 6.0
epss: 0.00508
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["aws", "dotnet", "encryption", "lab-disclosure", "package", "s3"]
not_affected_if: ["not-dotnet", "patched-version"]
published_at: "2026-10-14T00:00:00+00:00"
first_seen_at: "2026-10-24T20:05:35+00:00"
updated_at: "2026-10-25T12:38:49+00:00"
sources: [{"source": "lab", "url": "https://aws.amazon.com/security/security-bulletins/rss/2026-135-aws/", "first_seen_at": "2026-10-24T20:05:35+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-110310", "first_seen_at": "2026-10-24T20:15:27+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-110310", "first_seen_at": "2026-10-25T12:38:49+00:00"}]
---

# AWS S3 Encryption Client for .NET: padding downgrade lets attackers decrypt objects

The Amazon S3 Encryption Client for .NET can be tricked into using legacy RSA PKCS#1 v1.5 padding by editing an object's metadata, which enables a Bleichenbacher padding oracle attack. A remote user who can read the encrypted objects in the bucket can use this to recover the RSA-wrapped envelope key and decrypt the object.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed `AWS.Encryption.SDK` or `AmazonS3EncryptionClient` package version in the .csproj or packages.lock.json; affected: `>= 2.1.0`, `3.0.0` to `3.2.1`, `4.0.0` to `4.0.2`
- Search the codebase for S3 encryption client usage on the .NET side, including forked or derivative code

## Fix

- Upgrade the 3.x line to `3.2.2` or the 4.x line to `4.0.3`
- Migrate any 1.x or 2.x usage to `3.2.2` or `4.0.3`, since those lines will not receive patches
- Patch any forked or derivative copies of the client code

## Timeline

- 2026-10-24T20:05:35+00:00 lab https://aws.amazon.com/security/security-bulletins/rss/2026-135-aws/ — AWS: CVE-2026-110310 - RSA PKCS#1 v1.5 padding oracle via legacy envelope key format downgrade in Amazon S3 Encryption Client for .NET
- 2026-10-24T20:15:27+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-110310
- 2026-10-25T12:38:49+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-110310

## Documents

- [advisory] AWS: CVE-2026-110310 - RSA PKCS#1 v1.5 padding oracle via legacy envelope key format downgrade in Amazon S3 Encryption Client for .NET https://aws.amazon.com/security/security-bulletins/rss/2026-135-aws/

## From the sources

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

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

Bulletin ID: 2026-135-AWS Scope: AWS Content Type: Important (requires attention) Publication Date: 10/14/2026 12:30 PM PDT

Description:

We identified CVE-2026-110310, an issue in Amazon S3 encryption client for .NET that could allow 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 Bleichenbacher attack, the user can use a PKCS#1 v1.5 padding oracle to recover the RSA-wrapped envelope key and decrypt the object.

Impacted versions: >= 2.1.0; 3.0.0 to 3.2.1; 4.0.0 to 4.0.2

Resolution:

This issue has been addressed in Amazon S3 Encryption Client for .NET version 3.2.2 for customers on version 3.x and version 4.0.3 for customers on version 4.x. We recommend upgrading to the latest version and ensuring any forked or derivative code is patched to incorporate the new fixes. The 1.x and 2.x lines will not receive patches; users of those versions need to migrate to version 3.2.2 or 4.0.3.

Workarounds:

None.

References:

- CVE-2026-110310
- GHSA-322w-f6rw-r4fj

 Please email aws-security@amazon.com with any security questions or concerns.

---

_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_