CVE-2026-9267 Overview
CVE-2026-9267 is an out-of-bounds read vulnerability in Eclipse tinydtls, a lightweight Datagram Transport Layer Security (DTLS) implementation used in constrained Internet of Things (IoT) environments. The flaw resides in the check_server_certificate() function and stems from missing buffer length validation before uint24 reads, memcmp, and memcpy operations. Unauthenticated attackers can craft a Certificate handshake message with a specific fragment_length value to trigger reads beyond valid buffer boundaries. Exploitation occurs during DTLS epoch 0 on both client and server code paths. Successful exploitation causes denial of service on memory-constrained devices. The issue is classified under [CWE-125] Out-of-Bounds Read.
Critical Impact
Remote unauthenticated attackers can crash DTLS-enabled IoT devices by sending a single crafted Certificate handshake message during epoch 0.
Affected Products
- Eclipse tinydtls before commit b3efd41ad111a4920f599f51ffa4f5e9f1e72221
- DTLS client implementations built on affected tinydtls versions
- DTLS server implementations built on affected tinydtls versions
Discovery Timeline
- 2026-06-29 - CVE-2026-9267 published to the National Vulnerability Database (NVD)
- 2026-06-29 - Last updated in NVD database
Technical Details for CVE-2026-9267
Vulnerability Analysis
The vulnerability exists in the check_server_certificate() function within Eclipse tinydtls. The function processes incoming DTLS Certificate handshake messages during epoch 0, before any encrypted session state is established. An attacker controls the fragment_length field of the handshake record. The parser uses this value to read subsequent structures without first validating that the declared length fits within the actual received buffer. As a result, attacker-influenced offsets drive uint24 field reads, memcmp comparisons, and memcpy copies that extend past the buffer boundary. The read primitive is reachable on both client-side and server-side code paths, expanding the attack surface across any tinydtls-based endpoint that participates in DTLS handshakes.
Root Cause
The root cause is missing bounds validation on attacker-supplied length fields inside check_server_certificate(). The function trusts the fragment_length value in the handshake message and computes read offsets from it. Because tinydtls targets constrained devices, error handling for malformed handshake records is minimal, and the invalid access leads to memory faults rather than a graceful abort. The upstream fix, applied in commit b3efd41ad111a4920f599f51ffa4f5e9f1e72221, introduces length checks before each uint24, memcmp, and memcpy operation.
Attack Vector
The attack vector is network-based and requires no authentication or user interaction. An attacker sends a crafted DTLS Certificate handshake message over User Datagram Protocol (UDP) to a listening tinydtls endpoint. The malformed fragment_length value causes the receiver to read outside allocated memory, terminating the process or corrupting adjacent state on memory-constrained targets. Because DTLS operates over UDP, source addresses can be spoofed, and no prior session establishment is required to reach the vulnerable code path.
No verified public exploit code is available. Technical details are documented in the Eclipse GitLab CVE Assignment Work Item.
Detection Methods for CVE-2026-9267
Indicators of Compromise
- Unexpected restarts, watchdog resets, or crash loops on IoT devices running DTLS services
- Inbound DTLS handshake records containing anomalous fragment_length values that do not match observed payload sizes
- Repeated malformed Certificate handshake messages from a single source targeting UDP DTLS ports
Detection Strategies
- Inspect DTLS traffic at the network boundary for Certificate handshake messages with inconsistent length fields relative to record size
- Correlate device crash telemetry with inbound UDP DTLS traffic during epoch 0 handshakes
- Perform software composition analysis on firmware images to identify embedded tinydtls versions prior to commit b3efd41ad111a4920f599f51ffa4f5e9f1e72221
Monitoring Recommendations
- Alert on abnormal volumes of DTLS handshake failures or truncated Certificate messages on IoT network segments
- Log and review UDP flows to devices exposing DTLS ports such as 5684 (CoAP over DTLS)
- Track device availability metrics for embedded endpoints to catch DoS patterns early
How to Mitigate CVE-2026-9267
Immediate Actions Required
- Identify all firmware and applications that link against Eclipse tinydtls and inventory their commit versions
- Update tinydtls to a build that includes commit b3efd41ad111a4920f599f51ffa4f5e9f1e72221 or later
- Restrict inbound DTLS traffic to trusted peers using network ACLs or firewall rules until patched firmware is deployed
Patch Information
The upstream fix is available in Eclipse tinydtls at commit b3efd41ad111a4920f599f51ffa4f5e9f1e72221. The patch adds explicit buffer length validation before each uint24 read, memcmp, and memcpy operation inside check_server_certificate(). Vendors shipping firmware based on tinydtls must rebuild and redistribute images incorporating this commit. Refer to the Eclipse GitLab CVE Assignment Work Item for authoritative fix references.
Workarounds
- Segment IoT devices onto isolated networks and block untrusted sources from reaching DTLS UDP ports
- Disable DTLS on affected endpoints where the protocol is not required for operation
- Enforce peer allow-lists at gateways to accept DTLS handshakes only from known client addresses
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
