Potential Timing Side-Channel Vulnerability in vLLM’s Chunk-Based Prefix Caching

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

2.6 · CVSS:3.1/AV:N/AC:H/PR:L/UI:R/S:U/C:L/I:N/A:N

EPSS

• CVE-2025-46570: 0.43% estimated exploitation activity in the next 30 days; percentile 37.28 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

This issue arises from the prefix caching mechanism, which may expose the system to a timing side-channel attack. Description When a new prompt is processed, if the PageAttention mechanism finds a matching prefix chunk, the prefill process speeds up, which is reflected in the TTFT (Time to First Token). Our tests revealed that the timing differences caused by matching chunks are significant enough to be recognized and exploited. For instance, if the victim has submitted a sensitive prompt or if a valuable system prompt has been cached, an attacker sharing the same backend could attempt to guess the victim's input. By measuring the TTFT based on prefix matches, the attacker could verify if their guess is correct, leading to potential leakage of private information. Unlike token-by-token sharing mechanisms, vLLM’s chunk-based approach (PageAttention) processes tokens in larger units (chunks). In our tests, with chunksize=2, the timing differences became noticeable enough to allow…

Published by the source

28 May 2025, 18:02 UTC

Last modified by the source

7 Aug 2026, 08:12 UTC

Source last checked

26 Oct 2026, 13:32 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-4qjh-9fv9-r85r

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-4qjh-9fv9-r85r

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# Patch Next advisory brief: CVE-2025-46570

Advisory: Potential Timing Side-Channel Vulnerability in vLLM’s Chunk-Based Prefix Caching
IDs: CVE-2025-46570, GHSA-4qjh-9fv9-r85r
Record: https://patchnext.com/advisories/cve-2025-46570 (revision 2, source checked 26 Oct 2026, 13:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: vllm 0.9.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI vllm: affected < 0.9.0; fixed in 0.9.0

## Suggested inspection (read-only)
- `python -m pip show vllm`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on vllm.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-4qjh-9fv9-r85r (OSV record GHSA-4qjh-9fv9-r85r lists affected ranges: PyPI vllm >=0 <0.9.0 (fixed))
- Verified: https://github.com/advisories/GHSA-4qjh-9fv9-r85r (GitHub-reviewed advisory GHSA-4qjh-9fv9-r85r lists affected ranges: PyPI vllm >=0 <0.9.0)

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