CVE-2026-47775: Envoy OAuth2 Filter: Padding Oracle via AES-256-CBC Cookie Decryption

Severity: medium (CVSS 6.8 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.2% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2026-06-26 · updated 2026-10-25
Weakness: CWE-209

Description

Envoy is an open source edge and service proxy designed for cloud-native applications. Prior to 1.35.11, 1.36.7, 1.37.3, and 1.38.1, the OAuth2 HTTP filter's encrypt()/decrypt() functions use AES-256-CBC without an authentication tag (no HMAC, no AEAD). The /callback endpoint returns HTTP 302 on successful decryption and HTTP 401 on padding failure, creating a padding oracle. An attacker who obtains the encrypted CodeVerifier cookie can recover the plaintext PKCE code_verifier in ~6,200 requests (~100 seconds), then exchange it with a stolen authorization code to obtain the victim's access token. This vulnerability is fixed in 1.35.11, 1.36.7, 1.37.3, and 1.38.1.

Affected products

envoyproxy envoy: from 1.35.0 before 1.35.11; from 1.36.0 before 1.36.7; from 1.37.0 before 1.37.3; from 1.38.0 before 1.38.1 — fixed in 1.35.11, 1.36.7, 1.37.3, 1.38.1

How to fix

Upgrade to a fixed version: envoyproxy envoy 1.35.11, 1.36.7, 1.37.3, 1.38.1. Check the vendor advisory for mitigations.

References

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