envoyproxy/envoy

Stateful Session cookies incompatible between Envoy Processes.

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#44111 opened on Mar 25, 2026

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bughelp wanted

Description

Title: Stateful Session cookies incompatible between Envoy Processes.

Description: When using Envoy as an Edge Proxy with multiple instances (an NLB in front) AND using the StatefulSession with cookies doesn't work properly.

The cookie is encoded with the expiry time BUT it uses the monotonic clock:

https://github.com/envoyproxy/envoy/blob/21b70d126c0a5c16d844658ee2db68f94deb7548/source/extensions/http/stateful_session/cookie/cookie.cc#L21-L22

When HTTP requests are routed to different Envoy processes, they will have different values for their monotonic clock and the cookie can inadvertently expire sooner then its should, which will cause a new backend to get selected.

I believe the appropriate fix is for time_source_.systemTime() to be used so that two envoy processes can validate each other cookies.

cc @cpakulski

Contributor guide

Tech stack
cpp
Domain
backendinfrastructure
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
C++ knowledgeEnvoy internalsMonotonic vs system clock
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Examine the file source/extensions/http/stateful session/cookie/cookie.cc around line 21-22 where the monotonic clock is used. Consider replacing time source .monotonicTime() with time source .systemTime(). Verify that the change doesn't break existing tests. Check if any other parts of the codebase rely on the monotonic behavior for cookie expiry.