How severe is CVE-2026-73257?

CVE-2026-73257 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.

Is CVE-2026-73257 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (50th percentile), an estimate of the probability of exploitation in the next 30 days.

What products are affected by CVE-2026-73257?

CVE-2026-73257 affects Cesanta Mongoose. See the affected-products list for the exact vulnerable versions.

How do I fix CVE-2026-73257?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.

When was CVE-2026-73257 published?

CVE-2026-73257 was published on 2026-08-20 and last updated on 2026-09-29.

CVE-2026-73257

CVE-2026-73257 is a critical-severity vulnerability in Cesanta Mongoose with a CVSS 3.x base score of 9.1. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-444.

Key facts

Severity: Critical (CVSS 3.x base score 9.1)
EPSS exploit prediction: 1% (50th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-444
Affected product: Cesanta Mongoose
Published: 2026-08-20
Last modified: 2026-09-29

Description

Mongoose is an embedded web server and network library. Priro to version 7.22, a remote unauthenticated attacker can send an HTTP request containing both Content-Length and Transfer-Encoding: chunked. The cl_count and te_count checks in the mg_http_parse() and http_cb() paths in src/http.c accept both headers and prioritize chunked encoding, while a Content-Length-preferring reverse proxy can use a different request boundary. This CL.TE desynchronization can inject requests that access or modify resources in another user context. This issue is fixed in version 7.22.

Frequently asked questions

What is CVE-2026-73257?: Mongoose is an embedded web server and network library. Priro to version 7.22, a remote unauthenticated attacker can send an HTTP request containing both Content-Length and Transfer-Encoding: chunked. The cl_count and te_count checks in the mg_http_parse() and http_cb() paths in src/http.c accept both headers and prioritize chunked encoding, while a Content-Length-preferring reverse proxy can use a different request boundary. This CL.TE desynchronization can inject requests that access or modify resources in another user context. This issue is fixed in version 7.22.
How severe is CVE-2026-73257?: CVE-2026-73257 has a CVSS 3.x base score of 9.1, rated critical severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity high, and availability none.
Is CVE-2026-73257 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 1% (50th percentile), an estimate of the probability of exploitation in the next 30 days.
What products are affected by CVE-2026-73257?: CVE-2026-73257 affects Cesanta Mongoose. See the affected-products list for the exact vulnerable versions.
How do I fix CVE-2026-73257?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its critical severity, prioritise patching exposed systems.
When was CVE-2026-73257 published?: CVE-2026-73257 was published on 2026-08-20 and last updated on 2026-09-29.

References

Affected products (1)

cpe:2.3:a:cesanta:mongoose:*:*:*:*:*:*:*:*

More vulnerabilities in Cesanta Mongoose

CVE-2022-25299 — Critical (CVSS 9.8): This affects the package cesanta/mongoose before 7.6. The unsafe handling of file names during upload using…
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CVE-2019-19307 — Critical (CVSS 9.8): An integer overflow in parse_mqtt in mongoose.c in Cesanta Mongoose 6.16 allows an attacker to achieve remote DoS…
CVE-2019-12951 — Critical (CVSS 9.8): An issue was discovered in Mongoose before 6.15. The parse_mqtt() function in mg_mqtt.c has a critical heap-based…
CVE-2018-20356 — Critical (CVSS 9.8): An invalid read of 8 bytes due to a use-after-free vulnerability in the mg_http_free_proto_data_cgi function call in…
CVE-2018-20355 — Critical (CVSS 9.8): An invalid write of 8 bytes due to a use-after-free vulnerability in the mg_http_free_proto_data_cgi function call in…

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