CVE-2026-105752: vLLM Harmony Tool Continuations Drop `cache_salt` Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-935w-9g4m-p28p",
  "modified" : "2026-10-06T00:02:03Z",
  "published" : "2026-10-06T00:02:02Z",
  "aliases" : [ "CVE-2026-105752" ],
  "summary" : "vLLM: Harmony tool continuations drop `cache_salt` — restoring a cross-tenant prefix-cache membership oracle",
  "details" : "## Affected\n\n- **Ecosystem / package:** pip / `vllm`\n- **Affected versions:** vLLM ≤ 0.25.1 (confirmed on 0.25.1, commit [`752a3a504485`](https://github.com/vllm-project/vllm/tree/752a3a504485790a2e8491cacbb35c137339ad34)). The lower bound predates 0.25.1; maintainers can confirm how far back the tool-continuation re-submission has omitted the salt.\n\n## Summary\n\nOn the GPT-OSS \"Harmony\" path (`POST /v1/responses`), a request that uses a built-in or MCP tool runs as a multi-turn loop: after each tool call vLLM re-renders the full next-turn Harmony prompt and re-submits it to the engine. Turn 1 correctly carries `request.cache_salt`, but the tool-continuation re-submission rebuilds the engine input via `tokens_input(token_ids)` with **no** `cache_salt`. The continuation prefix is therefore cached in the global *unsalted* namespace even though the caller opted into salting. A second tenant who can guess the low-entropy post-tool history submits the reconstructed continuation (unsalted) and reads exact per-turn cached-token counts from the Responses usage — restoring the prompt-membership oracle that `cache_salt` is documented to prevent.\n\nSilently dropping a preserved salt *after* the supported tool workflow is enabled is a broken isolation control: the caller enabled salting and every turn should stay isolated, but continuation turns leak into the shared cache.\n\nThis is distinct from [GHSA-4qjh-9fv9-r85r](https://github.com/vllm-project/vllm/security/advisories/GHSA-4qjh-9fv9-r85r) ([CVE-2025-46570](https://nvd.nist.gov/vuln/detail/CVE-2025-46570)): that advisory is the prefix-cache membership oracle for which `cache_salt` is the documented mitigation, and its PR-17045 fix does not close this site — the Harmony tool continuation silently drops the preserved salt, caching in the unsalted namespace and leaking exact `cached_tokens_per_turn` counts from a different sink (the Responses serving continuation, not general TTFT timing).\n\n## Affected code\n\nLinks pinned to the confirmed commit [`752a3a504485`](https://github.com/vllm-project/vllm/tree/752a3a504485790a2e8491cacbb35c137339ad34) (v0.25.1):\n\n- **The drop (sink):** [`vllm/entrypoints/openai/responses/serving.py#L712-L713`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/entrypoints/openai/responses/serving.py#L712-L713) — `token_ids = context.render_for_completion()` then `engine_input = tokens_input(token_ids)`, with no `cache_salt`.\n- **Correct turn-1 call for contrast:** [`vllm/entrypoints/openai/responses/serving.py#L755`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/entrypoints/openai/responses/serving.py#L755) — `tokens_input(prompt_token_ids, cache_salt=request.cache_salt)`.\n- **`tokens_input` stores the salt only if passed:** [`vllm/inputs/engine.py#L51-L66`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/inputs/engine.py#L51-L66) (`if cache_salt is not None: inputs[\"cache_salt\"] = cache_salt`).\n- **The engine request copies only the current input's salt:** [`vllm/v1/engine/input_processor.py#L380`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/v1/engine/input_processor.py#L380) (`cache_salt=decoder_inputs.get(\"cache_salt\")` → `None` for the continuation).\n- **Prefix-cache hashing keys on the salt only when present:** [`vllm/v1/core/kv_cache_utils.py#L560-L561`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/v1/core/kv_cache_utils.py#L560-L561) (`[request.cache_salt] if (start_token_idx == 0 and request.cache_salt) else []`).\n- **The oracle the attacker reads:** [`vllm/entrypoints/openai/responses/serving.py#L909`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/entrypoints/openai/responses/serving.py#L909) (`cached_tokens_per_turn`).\n- **The documented control being defeated:** [`vllm/entrypoints/openai/responses/protocol.py#L235`](https://github.com/vllm-project/vllm/blob/752a3a504485790a2e8491cacbb35c137339ad34/vllm/entrypoints/openai/responses/protocol.py#L235) (`cache_salt` field).\n\nThe tool-continuation re-submission rebuilds the engine input with no `cache_salt`:\n\n```python\n# vllm/entrypoints/openai/responses/serving.py Lines 711-715\n            if isinstance(context, HarmonyContext):\n                token_ids = context.render_for_completion()\n                engine_input = tokens_input(token_ids)\n\n                sampling_params.max_tokens = max_model_len - len(token_ids)\n```\n\nContrast with the correct turn-1 call, which does preserve the caller's salt:\n\n```python\n# vllm/entrypoints/openai/responses/serving.py Lines 754-755\n        prompt_token_ids = render_for_completion(messages)\n        engine_input = tokens_input(prompt_token_ids, cache_salt=request.cache_salt)\n```\n\n`tokens_input` stores the salt on the engine input only when it is passed, so the continuation input carries none and lands in the unsalted namespace:\n\n```python\n# vllm/inputs/engine.py Lines 51-66\ndef tokens_input(\n    prompt_token_ids: list[int],\n    *,\n    prompt: str | None = None,\n    cache_salt: str | None = None,\n) -> TokensInput:\n    \"\"\"\n    Construct [`TokensInput`][vllm.inputs.engine.TokensInput]\n    from optional values.\n    \"\"\"\n    inputs = TokensInput(type=\"token\", prompt_token_ids=prompt_token_ids)\n\n    if prompt is not None:\n        inputs[\"prompt\"] = prompt\n    if cache_salt is not None:\n        inputs[\"cache_salt\"] = cache_salt\n```\n\n## Impact\n\nAn authenticated tenant of a shared deployment can recover whether a guessed post-tool prompt or history was processed by another tenant, with exact cached-token counts rather than noisy latency — the exact prompt-membership oracle `cache_salt` is documented to prevent. It defeats the multi-user prefix-cache isolation guarantee for salted Harmony tool sessions.\n\nPreconditions: a GPT-OSS Harmony model on `/v1/responses`; prefix caching enabled (default); an operator-enabled built-in or MCP tool server; the victim sets `cache_salt` and triggers at least one tool continuation; and the attacker can reconstruct the post-tool history closely enough to match the token prefix. The `AC:H` metric reflects that guessable-history precondition.\n\n\n## Suggested Fix\n\nPropagate `request.cache_salt` into every Harmony (and Parsable) tool-continuation re-submission — at the continuation call site call `tokens_input(token_ids, cache_salt=request.cache_salt)`, mirroring the correct turn-1 call. Carry the salt on the `HarmonyContext` (thread the originating `request` into the context) so no continuation path can omit it:\n\n```diff\n# vllm/entrypoints/openai/responses/serving.py\n             if isinstance(context, HarmonyContext):\n                 token_ids = context.render_for_completion()\n-                engine_input = tokens_input(token_ids)\n+                engine_input = tokens_input(\n+                    token_ids,\n+                    cache_salt=(\n+                        context.request.cache_salt\n+                        if context.request is not None\n+                        else None\n+                    ),\n+                )\n```\n\nwith `HarmonyContext.__init__` gaining a `request: ResponsesRequest | None = None` parameter (stored as `self.request`) that `_create_responses` passes when constructing the context. The continuation prefix is then cached in the victim's salted namespace, mirroring turn 1.\n\nSuggested regression test: assert `cached_tokens_per_turn == 0` for a different-salt probe against a salted victim continuation (the four-way control from the proof of concept).\n\n## Credit\n\n**Reported by:** Patch the Planet (Trail of Bits + OpenAI collaboration)\n\nThis vulnerability was discovered using GPT-5.5-Cyber as part of the Patch the Planet security initiative.\n\n---\n\n**Proposed fix:** a fix for this issue is proposed in a public pull request: https://github.com/vllm-project/vllm/pull/51818",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "vllm"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.30.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/security/advisories/GHSA-935w-9g4m-p28p"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/pull/50195"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/pull/51818"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/commit/6a2a2bb02b563b83f946012959fd3927984d072a"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/vllm-project/vllm"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/vllm-project/vllm/releases/tag/v0.30.0"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-200", "CWE-524" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-10-06T00:02:02Z",
    "nvd_published_at" : null
  }
}