CVE-2020-8911: CBC padding oracle issue in AWS S3 Crypto SDK for golang
Summary
CVE-2020-8911: CBC padding oracle issue in AWS S3 Crypto SDK for golang. Affects github.com/aws/aws-sdk-go versions < 1.34.0. Fixed in 1.34.0. How to fix ↓
Also tracked for
- github.com/aws/aws-sdk-go · ghsa
The same CVE-2020-8911 is tracked separately for each ecosystem and vendor. Its fix information is the same; the package and affected versions differ.
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Background
The golang AWS S3 Crypto SDK is a software development kit that provides encryption and decryption capabilities for files stored in Amazon S3 buckets. It allows users to encrypt files using different encryption algorithms, including AES-CBC. However, versions prior to 1.34.0 of the SDK had a vulnerability that allowed encryption with AES-CBC without computing a MAC on the data. This vulnerability exposes a padding oracle issue, which can result in loss of confidentiality and message forgery. The vulnerability requires write access to the S3 bucket and the ability to observe decryption failures without revealing the plaintext.
Vulnerability Detail
The vulnerability in the golang AWS S3 Crypto SDK allows an attacker with write access to an S3 bucket to reconstruct the plaintext of encrypted files that were encrypted using AES-CBC mode. By exploiting the padding oracle vulnerability, the attacker can manipulate the bytes of the next block and PKCS5 padding errors to gradually reveal the plaintext. This can lead to loss of confidentiality and message forgery. The vulnerability is fixed in version 2 of the SDK by disabling encryption with CBC mode for new files, but old files remain vulnerable until they are reencrypted with AES-GCM.
How To Fix
Latest published version: v1.55.8 as of 23 Oct 2026
go get github.com/aws/aws-sdk-go@v1.55.8Upgrading to it may include breaking changes; the version below is the minimum fixed release for this vulnerability.
If you need the minimum change
1.34.0
This is the first release that contains the fix.
go get github.com/aws/aws-sdk-go@v1.34.0Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
There is no specific workaround mentioned for this vulnerability.
Conclusion
The golang AWS S3 Crypto SDK is vulnerable to a CBC padding oracle issue that can result in loss of confidentiality and message forgery. Upgrading to version 2 of the SDK and reencrypting any existing files encrypted with CBC mode will mitigate the vulnerability. It is important to stay updated with security advisories and follow best practices for secure encryption and storage of sensitive data.
See every vulnerability recorded for github.com/aws/aws-sdk-go →
FAQ
The vulnerability can result in loss of confidentiality and message forgery. It allows an attacker with write access to an S3 bucket to gradually reconstruct the plaintext of encrypted files.
Using version 2 of the golang AWS S3 Crypto SDK will prevent the creation of vulnerable files. However, existing files encrypted with CBC mode remain vulnerable until they are reencrypted with AES-GCM.
Yes, a proof of concept exploit is available in a separate GitHub repository. It demonstrates how an attacker can exploit the padding oracle vulnerability to reconstruct the plaintext of encrypted files.
Yes, the CVSS score for this vulnerability is 5.6, indicating a moderate severity.
You can find more information about this vulnerability in the GitHub advisory database and the GitHub repository for the golang AWS S3 Crypto SDK.
References
Related vulnerabilities
Recent vulnerabilities recorded for github.com/aws/aws-sdk-go.
