Description

The HTTP/2 server in subsys/net/lib/http/http_server_http2.c clears the connection's stream binding at the start of every frame. handle_http_frame_header() executed client->current_stream = NULL unconditionally, but a CONTINUATION frame does not open a new stream and enter_http_frame_continuation_state() never re-bound one, so the rest of the header block was processed with client->current_stream unset. Two paths then write through the NULL pointer. A request for an unregistered resource, or a method the matched resource does not support, is answered by send_http2_404() or send_http2_405(), and both call send_headers_frame(), which unconditionally sets client->current_stream->headers_sent = true. A POST, PUT or PATCH to a registered dynamic resource reaches handle_http2_dynamic_resource(), which unlike its siblings (handle_http2_static_resource(), dynamic_get_del_opts_req_v2(), dynamic_post_put_req_v2()) had no client->current_stream == NULL guard, and stores through the pointer with client->current_stream->current_detail = dynamic_detail. Any peer that can reach the server can trigger this without authentication or prior state. A single stray CONTINUATION frame with END_HEADERS set, sent right after the client preface and requesting any unregistered path, is enough, because nothing rejected a CONTINUATION that no HEADERS frame had opened. A legitimate split header block (a HEADERS frame with END_HEADERS clear whose payload ends on a header-field boundary, then a CONTINUATION with END_HEADERS set) reaches the same state. The in-tree test test_http2_post_headers_with_continuation() did not catch this because it splits the header block mid field, which takes the handle_incomplete_http_header() shortcut that keeps the stream bound. The result is a write to a fixed low address (a small offset into struct http2_stream_ctx), which faults on mainstream targets (flash/vector-table region on Cortex-M, or a trap under MPU/MMU null-pointer detection), and the default fatal-error handler halts the system. Impact is an unauthenticated, remotely triggerable denial of service against any device running the HTTP/2 server: the 404 path needs no particular resource to be registered, and HTTP/2 is built into CONFIG_HTTP_SERVER with cleartext h2c accepted, so TLS does not prevent it. Neither the target address nor the written value is attacker-controlled, so there is no confidentiality or integrity impact. Applications built without CONFIG_HTTP_SERVER are not affected.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-11-06T15:17:26.850Z",
  "pubdate": "2026-11-06T15:17:26.850Z",
  "executiveSummary": "A critical null pointer dereference vulnerability exists in the HTTP/2 server implementation within the Zephyr Project network stack. The flaw originates from the improper management of stream bindings during the processing of HTTP/2 CONTINUATION frames.\nAn unauthenticated, remote attacker can trigger this vulnerability by sending a maliciously crafted sequence of HTTP/2 frames, specifically utilizing CONTINUATION frames to circumvent stream state binding.\nSuccessful exploitation results in a forced system halt, leading to a complete Denial of Service (DoS). The vulnerability affects systems configured with the integrated HTTP/2 server (CONFIG_HTTP_SERVER) regardless of TLS enablement.\nThere is no impact on data confidentiality or integrity, as the exploit does not allow for arbitrary code execution or attacker-controlled data injection; it is strictly a crash-inducing vulnerability.\nThe risk is elevated due to the requirement for zero authentication and the ability to trigger the crash from any network peer capable of reaching the HTTP/2 server.",
  "technicalDetails": "The root cause is a logic error in subsys/net/lib/http/http_server_http2.c where the HTTP/2 frame handler, handle_http_frame_header(), unconditionally clears the connection's stream binding (client->current_stream = NULL) at the start of every frame.\nThe CONTINUATION frame handler, enter_http_frame_continuation_state(), fails to re-bind the current stream context. Consequently, subsequent processing of the header block occurs with a NULL stream pointer.\nTwo primary execution paths lead to a NULL pointer dereference: 1) Error handling paths (send_http2_404() and send_http2_405()) attempt to access client->current_stream->headers_sent when returning status codes for invalid resources. 2) Valid requests for dynamic resources routed to handle_http2_dynamic_resource() attempt to write to client->current_stream->current_detail without prior validation.\nAn attacker can exploit this by sending a stray CONTINUATION frame with the END_HEADERS flag set immediately following the client preface. Because the server does not enforce that a CONTINUATION frame must follow an active HEADERS frame in the same stream context, the server proceeds to process the frame using an unset stream context.\nAlternatively, a legitimate split header block that satisfies the protocol requirements can still lead to the fault if the state machine transitions into a path that does not properly validate the stream pointer. The existing test case, test_http2_post_headers_with_continuation(), fails to detect this because it splits header fields such that the internal handle_incomplete_http_header() logic is triggered, which inadvertently maintains the correct stream binding.\nThe resulting dereference attempts to write to or read from a small offset into a NULL-based struct http2_stream_ctx. On common embedded architectures such as Cortex-M, this memory region typically corresponds to the vector table or restricted flash, triggering a hard fault. On systems with an MPU or MMU, this results in a memory access violation. The system's default fatal-error handler subsequently halts the system to prevent undefined behavior, effectively creating a Denial of Service."
}