CVE-2026-77802 HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic
A local attacker can exploit HTTP request smuggling in Fiddler Classic to poison responses and read data intended for other users sharing the proxy.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-77802?
CVE-2026-77802 is a medium-severity vulnerability in Progress® Telerik® Fiddler® Classic, published on 5 Oct 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them.
Weakness: CWE-444
Which versions are affected by CVE-2026-77802?
How do I fix CVE-2026-77802?
No fix has been published for CVE-2026-77802 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-77802 being exploited?
There is no known exploitation of CVE-2026-77802 in the wild and no public exploit code.
Sources
Also known as GHSA-p3gc-87rm-gmjp.