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OpenSIPS: Watcherinfo XML generation denial of service from oversized watcher URI

High 8.7

Published on May 21, 2026 in , , ,

Description

OpenSIPS published GHSA-gx83-2gh8-7v56 for a denial-of-service vulnerability in watcherinfo XML generation. The issue is caused by an oversized watcher URI being copied into a fixed-size stack buffer in modules/presence/notify.c:create_winfo_xml().

Enable Security reproduced the issue with a two-step SUBSCRIBE sequence on OpenSIPS 3.5.9. The vanilla build terminated with stack-smashing detection, and the ASan build reported a stack-buffer-overflow.

Technical details

The vulnerable function builds watcherinfo XML and copies watcher URI data into a fixed 200-byte stack buffer before passing it to XML generation. The code did not check whether the watcher URI length fit in that buffer before copying and adding the trailing NUL byte.

The verified attack flow was:

  1. Create a long watcher identity through an Event: presence SUBSCRIBE.
  2. Trigger presence.winfo watcherinfo generation for the same presentity.
  3. OpenSIPS enumerates the watcher list and copies the oversized watcher URI into the fixed stack buffer.

ASan confirmed an attacker-controlled stack write overflow in create_winfo_xml().

Configuration requirements

The verified crash path requires:

  • presence and presence_xml loaded
  • routing logic that exposes handle_subscribe() to attacker-controlled SUBSCRIBE traffic
  • ability to create or influence an oversized watcher URI
  • presence.winfo watcherinfo generation reachable for the same presentity

Real deployments may require subscription authorization depending on policy.

Impact

A remote attacker can crash an affected OpenSIPS worker in deployments that expose the relevant presence and watcherinfo functionality. The verified impact is denial of service. Because the issue is a stack-buffer-overflow, memory-corruption impact beyond denial of service cannot be ruled out, but no code execution was demonstrated.

Solutions and recommendations

Upgrade to a fixed OpenSIPS version. OpenSIPS lists fixes in the May 2026 release series, including:

  • OpenSIPS 3.6.6: c5970d3ee
  • OpenSIPS 4.0.0-rc1: dd86461b7
  • master: eeb331cd5

If immediate patching is not possible, restrict SUBSCRIBE access to trusted peers, disable presence.winfo if it is not required, and reject oversized watcher identities before calling handle_subscribe().

References

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Disclaimer

The information in the advisory is believed to be accurate at the time of publishing based on currently available information. Use of the information constitutes acceptance for use in an AS IS condition. There are no warranties with regard to this information. Neither the author nor the publisher accepts any liability for any direct, indirect, or consequential loss or damage arising from use of, or reliance on, this information.

Disclosure policy

This report is subject to Enable Security’s vulnerability disclosure policy which can be found at https://github.com/EnableSecurity/Vulnerability-Disclosure-Policy.