CVE-2026-103399 Libsoup: soupserver: http/1 request smuggling via undrained expect: 100-continue body
Remote attackers can smuggle HTTP requests via undrained bodies in SoupServer, causing unintended request processing on keep-alive connections.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-103399?
CVE-2026-103399 is a medium-severity vulnerability in Red Hat Enterprise Linux 10, published on 30 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
A flaw was found in SoupServer (libsoup). When an HTTP/1.x client sends a request with Expect: 100-continue and a request body, and SoupServer returns an early final (non-1xx) response before the body is read, the server neither drains the declared body bytes nor closes the connection. On a keep-alive connection, those leftover bytes are interpreted as a subsequent HTTP request. A remote, unauthenticated attacker can place a complete HTTP request in the body and cause SoupServer to process that smuggled request, leading to unintended request handling.
Weakness: CWE-444
Which versions are affected by CVE-2026-103399?
How do I fix CVE-2026-103399?
Apply the vendor's patch.
Is CVE-2026-103399 being exploited?
There is no known exploitation of CVE-2026-103399 in the wild and no public exploit code.
Sources
Also known as AZL-105543, DEBIAN-CVE-2026-103399, ECHO-0353-a267-f75e, GHSA-mfgq-q7gx-p678, UBUNTU-CVE-2026-103399.