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The vulnerability exists in SvelteKit’s experimental Remote Form functions. When form data is submitted, the client encodes it in a custom binary format. The request body begins with an 8-byte header that specifies the `data_length` of the expected payload. Upon receiving a request with the `application/x-sveltekit-formdata` content type, the server reads this header first. If the actual request body stream is not yet fully available (e.g., due to network latency or an attacker deliberately stalling transmission), the server code path eagerly allocates an ArrayBuffer of the size declared in the `data_length` field to hold the anticipated data. An attacker can exploit this by sending a request containing only the 8-byte header with an extremely large `data_length` value (like 0xFFFFFFFF) and then never sending the rest of the body. This causes the server to allocate a massive, contiguous memory block for each malicious connection, leading to memory exhaustion and a Denial-of-Service condition.
Platform: SvelteKit
Version: <2.0.0-update.2
Vulnerability : Memory Amplification DoS
Severity: Critical
date: May 2024
Prediction: Patch by June 2024
What Undercode Say:
curl -X POST http://target/app/remote-form-endpoint \ -H "Content-Type: application/x-sveltekit-formdata" \ --data-binary "@malicious_header.bin" --limit-rate 1
malicious_header.bin contents (8 bytes): 0xFFFFFFFF 0x00000000 (big-endian length, then zeros)
How Exploit:
1. Enable `experimental.remoteFunctions`.
2. Identify exposed remote form endpoint.
3. Craft request with huge `data_length` header.
4. Stall TCP connection after sending header.
5. Repeat to exhaust server memory.
Protection from this CVE
Disable `experimental.remoteFunctions`.
Upgrade to SvelteKit >=2.0.0-update.2.
Implement request size limiting.
Use a reverse proxy/load balancer.
Impact:
Memory exhaustion leads to DoS.
Affects availability only.
Requires remote forms enabled.
Unauthenticated attack vector.
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Sources:
Reported By: github.com
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