CVE-2026-27703
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedRIOT is an open-source microcontroller operating system, designed to match the requirements of Internet of Things (IoT) devices and other embedded devices. In 2026.01 and earlier, the default handler for the well_known_core resource coap_well_known_core_default_handler writes user-provided option data and other data into a fixed size buffer without validating the buffer is large enough to contain the response. This vulnerability allows an attacker to corrupt neighboring stack location, including security-sensitive addresses like the return address, leading to denial of service or arbitrary code execution.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceRIOT OS versions 2026.01 and earlier contain a stack-based buffer overflow in the CoAP well-known core resource handler (coap_well_known_core_default_handler). The handler writes user-controlled CoAP option data into a fixed-size buffer without validating the buffer can accommodate the response, allowing an attacker to overwrite adjacent stack memory including return addresses and achieve arbitrary code execution.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data<= 2026.01CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- High
- Availability
- High
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify RIOT OS versionCheck the firmware build metadata, version.h, or git tag in the deployed RIOT OS image. Common locations include /usr/include/version.h, a version banner in the firmware, or check the build system's OS version identifier.Affected if The installed RIOT OS version is 2026.01 or earlier.
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Verify CoAP support is enabledInspect the board or application's Makefile or Kconfig for CONFIG_SOCK_COAP,riot Applications with CoAP support typically include 'coap' in their module dependencies or have CONFIG_NETWORKING enabled along with coap-related modules.Affected if CoAP/CoAP server support is compiled into the firmware.
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Confirm CoAP well-known core handler is presentExamine the compiled firmware binary for the symbol coap_well_known_core_default_handler using nm, objdump, or a disassembler. This symbol should be present if the vulnerable code path is included.Affected if The coap_well_known_core_default_handler function symbol exists in the firmware.
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Check if CoAP UDP service is exposedVerify the device is running a CoAP server listening on UDP port 5683 (the default CoAP port). This can be done via netstat, ss, or by observing network traffic for CoAP requests.Affected if The device accepts CoAP requests on UDP port 5683 or alternative CoAP ports.
A system is affected if it runs RIOT OS version 2026.01 or earlier with CoAP and the well-known core resource handler enabled and exposed to network requests.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scopedImplement bounds checking in coap_well_known_core_default_handler to validate buffer capacity before writing response data, or use dynamic buffer allocation with proper size calculations.
RIOT OS version 2026.02 or later (the next release after 2026.01)
- Identify the specific RIOT OS version in use by checking the VERSION file or git describe output in the project repository
- Navigate to the official RIOT OS GitHub repository at github.com/RIOT-OS/RIOT
- Locate the coap_well_known_core_default_handler function in the source code (likely in pkg/lwip or sys/include/net/gcoap.h)
- Review commit history and pull requests for CVE-2026-27703 or the fix for the buffer overflow vulnerability in well_known_core resource handling
- Obtain the patched version (2026.02 or later) from the releases page
- Update the RIOT repository to the fixed version using 'git fetch origin' followed by 'git checkout <fixed-tag>'
- Rebuild and flash the firmware to all affected IoT devices
- Verify the fix by testing CoAP responses with large option data to ensure no buffer overflow occurs
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing8.0 h
- Review / QA4.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-27703 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-27703 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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