CVE-2026-73256Public exploitNetwork attack vector

Unauthorized requests via HTTP/1.0 desync

Published Aug 20, 2026 · Updated Aug 20, 2026

Request smuggling in Mongoose before 7.22 allows remote attackers to inject unauthorized backend requests via conflicting HTTP/1.0 framing. In http_cb() in src/http.c, the HTTP/1.0 check uses hm.proto.len > 8 even though mg_http_parse() accepts only eight-byte protocol strings, so is_http_1_0 is never set and chunked Transfer-Encoding is processed for HTTP/1.0 requests. Exposure requires Mongoose behind a reverse proxy that forwards requests in HTTP/1.0 mode; in that layout, proxy and backend parse the same request differently, enabling unauthorized access or state changes.

CVSS severity9.1
Critical
EPSS probability0.40%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Request smuggling in Mongoose before 7.22 allows remote attackers to inject unauthorized backend requests via conflicting HTTP/1.0 framing. In http_cb() in src/http.c, the HTTP/1.0 check uses hm.proto.len > 8 even though mg_http_parse() accepts only eight-byte protocol strings, so is_http_1_0 is never set and chunked Transfer-Encoding is processed for HTTP/1.0 requests. Exposure requires Mongoose behind a reverse proxy that forwards requests in HTTP/1.0 mode; in that layout, proxy and backend parse the same request differently, enabling unauthorized access or state changes.

The record

CVE
CVE-2026-73256
Published
Aug 20, 2026
Updated
Aug 20, 2026
Vendor
Cesanta
Product
Mongoose
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 20, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 20, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 7.22

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Cesanta · Mongooseversion=< 7.22

Published CVSS scores

9.1GitHub Advisory DatabaseCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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