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CVE-2026-73256

Cesanta Mongoose vulnerability

9.1 CRITICALPublic exploitEPSS 0.44%
  1. Published20 Aug 2026

Description

Mongoose is an embedded web server and network library. Prior to 7.22, a remote unauthenticated attacker can exploit an HTTP/1.0 reverse-proxy deployment by sending a request with Transfer-Encoding: chunked and conflicting framing. The http_cb() function in src/http.c tests hm.proto.len with an impossible greater-than-eight condition even though mg_http_parse() requires an eight-byte protocol string, so is_http_1_0 is never set. Mongoose consequently processes chunked encoding that an HTTP/1.0 proxy can ignore, enabling request smuggling and unauthorized access or state changes. This issue is fixed in version 7.22.

CVSS 3.1 breakdown

Attack vectorNetwork
Attack complexityLow
Privileges requiredNone
User interactionNone
ScopeUnchanged
ConfidentialityHigh
IntegrityHigh
AvailabilityNone
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Affected products

Cesanta Mongoose< 7.22

Remediation

Fixed in
Cesanta Mongoose≥ 7.22

References

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Frequently asked

Is CVE-2026-73256 being actively exploited?

CVE-2026-73256 is not in the CISA KEV catalog. A public exploit reference exists, so treat it as exploitable. Its EPSS exploit probability is 0.44%.

How severe is CVE-2026-73256?

CVE-2026-73256 has a CVSS 3.1 base score of 9.1, rated critical. EPSS estimates a 0.44% probability of exploitation.

Which products are affected by CVE-2026-73256?

Affected products include Cesanta Mongoose.

How do I fix CVE-2026-73256?

Upgrade affected software to a fixed version, for example Cesanta Mongoose 7.22 or later, then confirm no affected versions remain in your inventory.

How Secuno handles this

Secuno correlates vulnerabilities like CVE-2026-73256 against the real software inventory on every managed device, every day. Rather than a static severity, each match is scored in context: exploit availability, EPSS, the asset's exposure, and business criticality decide where it lands in the queue. If an affected version is present anywhere across your portfolio, it surfaces automatically with a prioritised remediation path.

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Data from the NVD, CISA KEV catalog and FIRST EPSS. CVSS 3.1 base score shown where available.