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

Quick assessment

Affected
Progress Software Progress® Telerik® Fiddler® Classic
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在适用于 Windows 的 In Progress® Telerik® Fiddler® Classic 中，v6.0.20262.10021 之前的版本在代理请求转发组件中存在前端请求不同步漏洞。当一个请求同时包含 Content-Length 和 Transfer-Encoding 标头时，Fiddler 在转发请求时会同时保留这两个标头，但在内部对请求体使用 Transfer-Encoding 进行封装。复用客户端连接时，剩余的字节会被解析为一个独立的流水线化（pipelined）请求。因此，本地具有低权限

CVSS 3.6 · Low

Possible ATT&CK Techniques 1 AI

T1071 · Application Layer Protocol
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I. Basic Information for CVE-2026-77803

Vulnerability Information
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Vulnerability Title
Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
Source: CVE Program / CVE List V5
Vulnerability 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
Source: CVE Program / CVE List V5
Vulnerability Type
HTTP请求的解释不一致性(HTTP请求私运)
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
Progress Software Progress® Telerik® Fiddler® Classic 1.0.0 ~ 6.0.20262.10021 -

II. Public POCs for CVE-2026-77803

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III. Intelligence Information for CVE-2026-77803

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Other References for CVE-2026-77803 (1)

Same Patch Batch · Progress Software · 2026-10-05 · 5 CVEs total

CVE-2026-92931 8.8 HIGH CWE-918: Server-Side Request Forgery in the Progress Sitefinity Next.js Renderer SDK
CVE-2026-77805 7.9 HIGH Weak Executable Signature Verification Vulnerability in Progress® Telerik® Fiddler® Classi
CVE-2026-77804 6.6 MEDIUM Time-of-check Time-of-use (TOCTOU) Race Condition in Root Certificate Installation in Prog
CVE-2026-77802 6.3 MEDIUM HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic

IV. Related Vulnerabilities

Same product: Progress® Telerik® Fiddler® Classic
Same vendor: Progress Software
Same weakness: CWE-444

V. Comments for CVE-2026-77803

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