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CVE-2026-77802— HTTP Request Smuggling 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.

In Progress® Telerik® Fiddler® Classic for Windows 版本早于 v6.0.20262.10021 时，在代理请求转发组件中存在 HTTP 请求走私漏洞。当请求包含多个值相互冲突的 Content-Length 头时，Fiddler 会将这些请求原封不动地转发给原始服务器，但其本身在构造请求体时仅使用第一个 Content-Length 的值。具备低权限的本地攻击者，若能通过另一个用户所使用的同一 Fiddler 代理实例发送请求，则可利用这种与原始服务器（该服务器不兼

CVSS 6.3 · Medium

Possible ATT&CK Techniques 2 AI

T1132 · Data Encoding T1090 · Proxy
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I. Basic Information for CVE-2026-77802

Vulnerability Information
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Vulnerability Title
HTTP Request Smuggling 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, 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.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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-2026-77802

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III. Intelligence Information for CVE-2026-77802

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Other References for CVE-2026-77802 (1)

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

CVE-2026-92931 10.0 CRITICAL 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-2026-77803 3.6 LOW Front-end Desynchronization 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-2026-77802

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