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Mitigate fragment decompression DoS by rejecting lz codec #1

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Mitigate fragment decompression DoS by rejecting lz codec #1

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26 changes: 16 additions & 10 deletions src/lib/payload/fragment.ts
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Open in desktop
Original file line number Diff line number Diff line change
@@ -1,4 +1,3 @@
import { compressToEncodedURIComponent, decompressFromEncodedURIComponent } from "lz-string";
import { normalizeEnvelope } from "@/lib/payload/envelope";
import {
MAX_DECODED_PAYLOAD_LENGTH,
Expand Down Expand Up @@ -30,17 +29,13 @@ function fromBase64Url(input: string): string {
return new TextDecoder().decode(bytes);
}

function encodePayload(json: string, codec: PayloadCodec): string {
if (codec === "lz") {
return compressToEncodedURIComponent(json);
}

function encodePayload(json: string): string {
return toBase64Url(json);
}

function decodePayload(encoded: string, codec: PayloadCodec): string | null {
if (codec === "lz") {
return decompressFromEncodedURIComponent(encoded);
return null;
}

return fromBase64Url(encoded);
Expand All @@ -49,10 +44,14 @@ function decodePayload(encoded: string, codec: PayloadCodec): string | null {
function buildFragment(envelope: PayloadEnvelope, codec: PayloadCodec): string {
const payloadEnvelope = { ...envelope, codec };
const json = JSON.stringify(payloadEnvelope);
return `${PAYLOAD_FRAGMENT_KEY}=v1.${codec}.${encodePayload(json, codec)}`;
return `${PAYLOAD_FRAGMENT_KEY}=v1.${codec}.${encodePayload(json)}`;
}

export function encodeEnvelope(envelope: PayloadEnvelope, options: EncodeOptions = {}): string {
if (options.codec === "lz") {
return buildFragment(envelope, "plain");
}

if (options.codec) {
return buildFragment(envelope, options.codec);
}
Expand All @@ -62,8 +61,7 @@ export function encodeEnvelope(envelope: PayloadEnvelope, options: EncodeOptions
return plainFragment;
}

const compressedFragment = buildFragment(envelope, "lz");
return compressedFragment.length < plainFragment.length ? compressedFragment : plainFragment;
return plainFragment;
}

export function decodeFragment(hash: string): ParsedPayload {
Expand Down Expand Up @@ -113,6 +111,14 @@ export function decodeFragment(hash: string): ParsedPayload {
};
}

if (codec === "lz") {
return {
ok: false,
code: "invalid-format",
message: 'Unsupported codec "lz". Please re-share using codec "plain".',
};
}
Comment on lines +114 to +120
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@coderabbitai coderabbitai Bot Mar 9, 2026

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⚠️ Potential issue | 🟠 Major

Don’t report rejected lz links as generic format errors.

These are legacy fragments generated by older clients, not malformed input. Returning invalid-format makes old #agent-render=v1.lz... bookmarks indistinguishable from genuinely corrupt hashes, so the UI has no reliable way to show a targeted recovery message. Please give this branch its own result code.

🤖 Prompt for AI Agents 
Verify each finding against the current code and only fix it if needed.

In `@src/lib/payload/fragment.ts` around lines 114 - 120, The branch that handles
codec === "lz" should return a distinct error code instead of "invalid-format"
so legacy lz fragments are distinguishable; update the return value in the codec
=== "lz" branch in src/lib/payload/fragment.ts to use a new, specific code
(e.g., "legacy-codec" or "unsupported-legacy-fragment") while keeping a clear
message, and update any tests or callers that rely on the old "invalid-format"
code to expect the new code.


let parsed: unknown;

try {
Expand Down
18 changes: 16 additions & 2 deletions tests/fragment.test.ts
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Original file line number Diff line number Diff line change
Expand Up @@ -53,7 +53,7 @@ describe("fragment payload transport", () => {
expect(parsed.code).toBe("invalid-json");
});

it("uses compressed transport when it is smaller", () => {
it("always emits plain transport", () => {
const repetitiveEnvelope: PayloadEnvelope = {
...envelope,
artifacts: [
Expand All @@ -68,11 +68,25 @@ describe("fragment payload transport", () => {

const hash = encodeEnvelope(repetitiveEnvelope);

expect(hash.startsWith("agent-render=v1.lz.")).toBe(true);
expect(hash.startsWith("agent-render=v1.plain.")).toBe(true);
const parsed = decodeFragment(`#${hash}`);
expect(parsed.ok).toBe(true);
});

it("rejects lz transport fragments", () => {
const hash = encodeEnvelope(envelope, { codec: "lz" });

expect(hash.startsWith("agent-render=v1.plain.")).toBe(true);

const parsed = decodeFragment("#agent-render=v1.lz.abcd");
expect(parsed.ok).toBe(false);
if (parsed.ok) {
return;
}

expect(parsed.code).toBe("invalid-format");
});

it("normalizes an invalid active artifact id to the first artifact", () => {
const hash = `#${encodeEnvelope(
{
Expand Down