# 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.

- **Severity:** Medium 6.3 (`CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 5 Oct 2026
- **Updated:** 6 Oct 2026
- **Also known as:** GHSA-p3gc-87rm-gmjp

## 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 (Inconsistent Interpretation of HTTP Requests)

## 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.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-77802 and fix it if it is affected.

What it is: 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.
Exploitation: There is no known exploitation of CVE-2026-77802 in the wild and no public exploit code.

Affected product: Progress® Telerik® Fiddler® Classic. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-77802/ (Markdown: https://insights.sbomify.com/CVE-2026-77802/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-77802
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77802
- OSV: https://osv.dev/vulnerability/CVE-2026-77802

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-77802/
Summaries are generated and may contain mistakes; the official description above is the record.