CVE-2026-84380 HTTPX2: Conflicting Content-Length and Transfer-Encoding headers can be auto-generated
HTTPX2 allows request smuggling via conflicting Content-Length and Transfer-Encoding headers, enabling desynchronization attacks against intermediaries.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-84380?
CVE-2026-84380 is a medium-severity vulnerability in pydantic httpx2, published on 2 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
HTTPX2 is a next generation HTTP client for Python. Prior to 2.11.0, Request._prepare() in src/httpx2/httpx2/_models.py can add a body-derived Content-Length header to a request that already contains a caller-supplied Transfer-Encoding header because its setdefault() processing checks each default header independently rather than treating the two framing headers as mutually exclusive. Fixed-size byte, JSON, form, and known-length multipart bodies can therefore be serialized over HTTP/1.1 with both headers, allowing request smuggling or connection desynchronization when downstream intermediaries disagree about which framing header takes precedence. This issue is fixed in version 2.11.0.
Weakness: CWE-444
Which versions are affected by CVE-2026-84380?
3 open source packages ship an affected version, including httpx2.
| Package | Affected | Fixed in |
|---|---|---|
httpx2 PyPI | All versions | 2.11.0 |
httpx2 PyPI | All versions | 2.11.0 |
httpx2 PyPI | All versions | 2.11.0 |
How do I fix CVE-2026-84380?
Upgrade httpx2 to 2.11.0 or later.
Is CVE-2026-84380 being exploited?
There is no known exploitation of CVE-2026-84380 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-84380, ECHO-6eae-343c-bdbc, GHSA-pf96-p4fj-6566, MINI-2chf-cvgf-w54r, MINI-qf3x-8j93-8pwv, PYSEC-2026-3849 and 1 more.