Summary
HTTPX2: Conflicting Content-Length and Transfer-Encoding headers can be auto-generated
HTTPX2 can automatically add a Content-Length header to a request that already contains a caller-supplied Transfer-Encoding header. The resulting HTTP/1.1 request contains both framing headers, which can create an ambiguous message boundary and enable request smuggling or connection desynchronization when processed by intermediaries that disagree about which header takes precedence.
Details
When a request body has a known size, HTTPX2's content encoder returns a default Content-Length. Request._prepare() applies each default header with setdefault(), which only checks whether that same header is already present. It does not check whether the mutually exclusive Transfer-Encoding header is present.
For example:
import httpx2
request = httpx2.Request(
"POST",
"http://example.com/",
headers={"Transfer-Encoding": "chunked"},
content=b"test 123",
)
print(request.headers)
The request contains both:
Transfer-Encoding: chunked
Content-Length: 8
On an HTTP/1.1 connection, the body is serialized using chunked transfer coding while both headers are sent on the wire. This violates HTTP message-framing requirements. Fixed-size byte, JSON, form, and known-length multipart bodies can reach the affected path.
Streaming bodies with an explicit Content-Length are not affected in current HTTPX2 releases because the automatically generated Transfer-Encoding is already suppressed in that direction.
Mitigation
Upgrade to HTTPX2 2.11.0 or later. Patched versions treat Content-Length and Transfer-Encoding as mutually exclusive when applying automatically generated request headers.
If upgrading is not immediately possible, remove Transfer-Encoding and other hop-by-hop framing headers from untrusted input before constructing outbound requests. Applications acting as proxies should derive outbound framing from the body rather than forwarding inbound Content-Length or Transfer-Encoding headers.
Impact
An attacker may be able to use the conflicting framing headers as a request-smuggling or desynchronization primitive. Exploitation requires an application to pass attacker-controlled request framing headers and associated body data to HTTPX2, use HTTP/1.1, and communicate through a proxy or origin that accepts conflicting headers and interprets them differently from another hop.
Depending on the downstream infrastructure, successful exploitation could interfere with requests sharing a persistent connection, bypass front-end routing or authorization decisions, or poison responses or caches. Applications that do not forward attacker-controlled Transfer-Encoding headers are not directly exposed.
CVE-2026-84380 has a CVSS score of 5.6 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.11.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-84380? CVE-2026-84380 is a medium-severity security vulnerability in httpx2 (pip), affecting versions < 2.11.0. It is fixed in 2.11.0.
- How severe is CVE-2026-84380? CVE-2026-84380 has a CVSS score of 5.6 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of httpx2 are affected by CVE-2026-84380? httpx2 (pip) versions < 2.11.0 is affected.
- Is there a fix for CVE-2026-84380? Yes. CVE-2026-84380 is fixed in 2.11.0. Upgrade to this version or later.
- Is CVE-2026-84380 exploitable, and should I be worried? Whether CVE-2026-84380 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-84380 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-84380? Upgrade
httpx2to 2.11.0 or later.