Progress Telerik Fiddler Classic prior 6.0.20262.10021 Proxy Request Forwarding encoding error

CVSS Meta Temp Score
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Current Exploit Price (≈)
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CTI Interest Score
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4.4$0-$5k2.23-

Summaryinfo

A vulnerability has been found in Progress Telerik Fiddler Classic and classified as problematic. Impacted is an unknown function of the component Proxy Request Forwarding. This manipulation causes encoding error. This vulnerability is registered as CVE-2026-77803. The attack needs to be launched locally. No exploit is available. The affected component should be upgraded.

Detailsinfo

A vulnerability classified as problematic was found in Progress Telerik Fiddler Classic. Affected by this vulnerability is an unknown part of the component Proxy Request Forwarding. The manipulation with an unknown input leads to a encoding error vulnerability. The CWE definition for the vulnerability is CWE-172. The product does not properly encode or decode the data, resulting in unexpected values. As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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.

The advisory is shared at docs.telerik.com. This vulnerability is known as CVE-2026-77803 since 08/21/2026. The exploitation appears to be easy. An attack has to be approached locally. Neither technical details nor an exploit are publicly available.

Upgrading to version 6.0.20262.10021 eliminates this vulnerability.

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

Productinfo

Vendor

Name

License

CPE 2.3info

CPE 2.2info

CVSSv4info

VulDB Vector: 🔒
VulDB Reliability: 🔍

CVSSv3info

VulDB Meta Base Score: 4.5
VulDB Meta Temp Score: 4.4

VulDB Base Score: 5.3
VulDB Temp Score: 5.1
VulDB Vector: 🔒
VulDB Reliability: 🔍

CNA Base Score: 3.6
CNA Vector (ProgressSoftware): 🔒

CVSSv2info

AVACAuCIA
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VectorComplexityAuthenticationConfidentialityIntegrityAvailability
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VulDB Base Score: 🔒
VulDB Temp Score: 🔒
VulDB Reliability: 🔍

Exploitinginfo

Class: Encoding error
CWE: CWE-172
CAPEC: 🔒
ATT&CK: 🔒

Physical: Partially
Local: Yes
Remote: No

Availability: 🔒
Status: Not defined
Price Prediction: 🔍
Current Price Estimation: 🔒

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Threat Intelligenceinfo

Interest: 🔍
Active Actors: 🔍
Active APT Groups: 🔍

Countermeasuresinfo

Recommended: Upgrade
Status: 🔍

0-Day Time: 🔒

Upgrade: Telerik Fiddler Classic 6.0.20262.10021

Timelineinfo

08/21/2026 CVE reserved
10/05/2026 +45 days Advisory disclosed
10/05/2026 +0 days VulDB entry created
10/05/2026 +0 days VulDB entry last update

Sourcesinfo

Advisory: docs.telerik.com
Status: Confirmed

CVE: CVE-2026-77803 (🔒)
GCVE (CVE): GCVE-0-2026-77803
GCVE (VulDB): GCVE-100-413605

Entryinfo

Created: 10/05/2026 15:16
Changes: 10/05/2026 15:16 (62)
Complete: 🔍
Cache ID: 216::103

If you want to get the best quality for vulnerability data then you always have to consider VulDB.

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