CVE-2026-18416
LowCVSS 3.7Exploitation Probability (EPSS)
Low risk13th percentile - higher than 13% of all known CVEs
Summary
In the CoAP subsystem (subsys/net/lib/coap/coap_link_format.c), the match_path_uri() function compares a registered resource path against the URI in the Uri-Query href option. The URI is not NUL-terminated, and the inner loop advances index k without checking it against the option length len, causing an out-of-bounds read when the resource path is longer than the URI and the URI is a prefix. The vulnerability is reachable via an unauthenticated GET /.well-known/core?href=/<prefix> request to a device with CONFIG_COAP_SERVER_WELL_KNOWN_CORE enabled (default y) and at least one resource with struct coap_core_metadata attributes.
Risk Assessment
The risk is bounded — the over-read is limited to one byte in practice and does not lead to information disclosure or matching errors. The consequence is undefined behavior, which in extreme cases may cause crashes or unpredictable behavior, but not remote code execution.
Recommendation
Apply the fix that adds a k >= len guard at the top of the inner loop to ensure every uri[k] dereference stays within the option value. Update the system to a patched version or manually patch the source code.
Related vulnerabilities
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- CVE-2026-112917Critical
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- CVE-2026-44267Critical
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Original NVD description (English source)
The CoAP link-format helper match_path_uri() in subsys/net/lib/coap/coap_link_format.c compares a registered resource path against the URI carried in a Uri-Query href= option. That URI is not NUL terminated, but the inner character loop advanced its index k once per path character without ever testing it against the option length len. When a registered path segment is longer than the supplied URI and the URI is a prefix of it, the loop reads uri[len] and beyond, past the end of the option value. The path is reached from coap_well_known_core_get_len() and coap_well_known_core_get() via match_queries_resource(), i.e. by any unauthenticated GET /.well-known/core?href=/<prefix> request to a device that serves /.well-known/core (for the CoAP server subsystem, CONFIG_COAP_SERVER_WELL_KNOWN_CORE, default y) and has at least one resource that declares struct coap_core_metadata attributes. The over-read does not reach the receive buffer. The well-known-core builders parse the query into a stack-local struct coap_option, whose value is a fixed array (value[12], or CONFIG_COAP_EXTENDED_OPTIONS_LEN_VALUE bytes) that the option bytes are copied into, so uri points into that copy. Reading past len therefore reads the unused, uninitialized tail of the array and, when the option fills it, the bytes just past it in the same stack frame. (In the ZoAP library of v1.8.0 to v1.9.x the option value was instead a pointer into the received packet, and the over-read ran past the option inside the packet buffer.) The impact is bounded. The number of bytes read past the end is limited by the length of the resource path segment, and each additional byte is only read if it happens to equal the next path character, so in practice the over-read is one byte. It also cannot influence the response: returning a match requires the final compared index to be len - 1 or len, both in bounds, so out-of-bounds bytes only ever steer the loop to the next candidate resource. The consequence is undefined behaviour, not information disclosure and not a matching error. The fix adds a k >= len guard at the top of the inner loop, so every uri[k] dereference is within the option value while still allowing a trailing * wildcard to match a longer path.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

