# CVE-2026-77802: Progress Telerik Fiddler Classic HTTP request smuggling

Source: https://junglewise.ai/threats/cve-2026-77802-in-progress-telerik-fiddler-classic-for-windows-versions-prior-to  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-05. Updated: 2026-10-05  
License: CC BY 4.0 with a link back

CVE-2026-77802 · Severity: medium · CVSS 6.3 · Published 2026-10-05

Fiddler Classic is a web debugging proxy that helps developers monitor and modify HTTP traffic. A flaw in how it handles HTTP requests with conflicting headers allows an attacker sharing the same proxy instance to inject malicious requests and intercept other users' responses, potentially exposing sensitive data or manipulating the information users receive.

Technologies: [Progress Telerik Fiddler Classic](https://junglewise.ai/threats/technologies/telerik-fiddler-classic). Vendors: [Progress](https://junglewise.ai/threats/vendors/progress).

## Quick answers

**Is CVE-2026-77802 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-77802 being exploited in the wild.

**How severe is CVE-2026-77802?** Medium severity, with a CVSS score of 6.3, published 5 October 2026.

**Which products and versions are affected?** Progress Telerik Fiddler Classic prior to 6.0.20262.10021.

**Is there a fix for CVE-2026-77802?** Yes: fixed in 6.0.20262.10021. Until then: Disable server connection reuse in Fiddler Classic by opening Tools > Options > Connections and unchecking the "Reuse server connections" checkbox. This removes the possibility of request smuggling attacks via connection pool reuse. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-77802 attacked?** It is attacked with local access, in a web application.

**Am I affected by CVE-2026-77802?** Check whether you run Progress Telerik Fiddler Classic at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in 6.0.20262.10021

Disable server connection reuse in Fiddler Classic by opening Tools > Options > Connections and unchecking the "Reuse server connections" checkbox. This removes the possibility of request smuggling attacks via connection pool reuse.

## Technical details

HTTP request smuggling (CWE-444) in the proxy request forwarding component, where multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server while Fiddler frames using only the first value. A local low-privilege attacker with access to the same Fiddler proxy instance can exploit this desynchronization against non-RFC-9110-compliant servers that keep connections alive. The unread smuggled response remains buffered and is served to subsequent users reusing the connection, enabling response poisoning and data leakage.

## Affected products

- Progress Telerik Fiddler Classic prior to 6.0.20262.10021

## Timeline

- 2026-10-05: disclosed
- 2026-10-05: advisory

## References

- [https://docs.telerik.com/fiddler/knowledge-base/kb-security-http-request-smuggling-cve-2026-77802](https://docs.telerik.com/fiddler/knowledge-base/kb-security-http-request-smuggling-cve-2026-77802)

## Related threats

- [Progress Telerik Fiddler Classic weak signature verification](https://junglewise.ai/threats/cve-2026-77805-in-progress-telerik-fiddler-classic-for-windows-versions-prior-to)
- [Progress Telerik Fiddler Classic TOCTOU race condition in certificate installation](https://junglewise.ai/threats/cve-2026-77804-in-progress-telerik-fiddler-classic-for-windows-versions-prior-to)