Live Exposure Defense: From CVE to Confirmed Exposure in 12 Hours – See more

Back
New CVE Detected

CVE-2026-73257 – HTTP Request Smuggling – Cesanta Mongoose 7.21

Be the first to know when new zero-days emerge:

Subscribe to Email Alerts
Subscribe to RSS Feed

Summary

CVE-2026-73257 is an HTTP request smuggling vulnerability in Cesanta Mongoose, a lightweight embedded web server/networking library. Mongoose accepts HTTP requests that contain both a Content-Length and a Transfer-Encoding header at the same time, violating RFC 7230, which allows a remote unauthenticated attacker to desynchronize Mongoose from a front-end reverse proxy. The issue carries a CVSS v3.1 base score of 9.1 (Critical).

Technical details

  • Root cause: Mongoose does not reject HTTP requests where both Content-Length and Transfer-Encoding headers are present, and internally prioritizes Transfer-Encoding: chunked framing over Content-Length.
  • Trigger condition: A reverse proxy or load balancer placed in front of Mongoose that instead honors Content-Length will disagree with Mongoose about where one request ends and the next begins (a CL.TE desynchronization).
  • Attack vector: Network, no authentication or user interaction required — the attacker only needs to send a crafted HTTP request through the proxy to the Mongoose backend.
  • Impact: An attacker can smuggle a hidden request that Mongoose processes but the proxy attributes to a different, unrelated client connection, potentially allowing access to or modification of resources in another user’s context (high confidentiality and integrity impact; no availability impact).

Affected software

  • Cesanta Mongoose version 7.21 (fixed in version 7.22)

Severity

  • CVSS v3.1 Base Score: 9.1 (Critical)
  • Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Mitigation and recommended actions

  • Immediate: Upgrade Cesanta Mongoose to version 7.22 or later, which rejects requests containing both Content-Length and Transfer-Encoding headers.
  • If patching is not immediately possible: Configure any reverse proxy or load balancer in front of Mongoose to reject or normalize requests that contain both Content-Length and Transfer-Encoding headers, and ensure the proxy’s request-framing behavior matches Mongoose’s until the upgrade is applied.

References

Are you exposed?

Get a free report of your organization’s exposure to this CVE and threat

  • Mapping of all Assets with this Technology
  • Identification of potentially exposed assets to this CVE
  • Confirmation of verified exploitable assets
Get Exposure Report

How IONIX’s External Exposure Management Platform Detects and Validates
Zero-Days to Shrink MTTR

1

Map your entire attack surface (continously)

IONIX uses multi-factor discovery methods, including DNS analysis, certificate mapping, metadata inspection, and more, to automatically map every internet-facing asset across your environment. This includes cloud instances, third-party platforms, shadow IT, and even forgotten infrastructure that traditional tools miss.

2

Monitor for new CVEs

Dozens of threat intel feeds using agentic technology are continuously analyzed to detect the appearance of proof-of-concept code, exploit kits, and indicators of active targeting. IONIX goes further by applying AI to proactively evaluate whether emerging vulnerabilities are likely to be exploited, even before PoCs go public.

3

Identify Potential External Exposures

Not all CVEs matter. IONIX filters vulnerabilities by asking attacker-centric questions: Can it be reached from the internet? Does it require authentication? Is it being exploited in the wild? This dramatically reduces noise and focuses teams on threats that can actually be weaponized.

4

Create Safe, Scalable Exploit Validations

IONIX transforms real-world PoCs into safe, non-intrusive test payloads that can be run in production environments without disruption. These simulations are precisely targeted to the systems that are vulnerable, ensuring rapid validation without unnecessary load.

5

Execute Exploit Validations

By combining context about software stack, versioning, exposure status, and reachability, IONIX ensures that only the right payloads are executed against the right assets, maximizing efficiency and minimizing risk.

6

Drive Fast and Actionable Remediation

Results are routed through integrations with ticketing, SOAR, and SIEM tools. Issues are written in plain language, bundled into remediation clusters, and prioritized based on asset criticality, exploitability, and blast radius. This shortens mean time to remediation (MTTR) and empowers teams to act with confidence.

Are you exposed?

Get a free report of your organization’s exposure to this CVE and threat

  • Mapping of all Assets with this Technology
  • Identification of potentially exposed assets to this CVE
  • Confirmation of verified exploitable assets
Get Exposure Report
Close

Subscribe to Threat Center RSS

Copy/paste the link below into your preferred RSS reader or follow these instructions to subscribe to Slack alerts.

Open RSS feedSlack setup instructions

Close

Get Real-Time CVE Alerts to Your Email

Be the first to know when new zero-days emerge