CVE-2026-77802HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic
- 6.3MediumCVSS base score
- 0.10%Exploitation in 30 daysEPSS · top 99% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
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.
Timeline
ID reserved
By ProgressSoftware
- 45 days later
· 45 days later
Published
- 1 day later
· 1 day later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| ProgressSoftwareCNA (assigner) | 3.1 | Medium6.3 | CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N |
| NVDNVD · Awaiting Analysis | 3.1 | Medium6.3 | CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N |
- EPSS (exploitation in 30 days)
- 0.10% · top 99% of all CVEs (12 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: no · Technical impact: total
The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 6.3.
Affected products
Progress Software · Progress® Telerik® Fiddler® Classic
- affected 1.0.0 – < 6.0.20262.10021
What to do
Workaround: Disable server connection reuse in Fiddler Classic: open Tools > Options > Connections and uncheck the "Reuse server connections" checkbox. Disabling server pipe reuse prevents a poisoned server connection from being handed to another session.
References
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-92425 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-77802)Record last updated .