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GHSA-4qjh-9fv9-r85r CVE-2025-46570 PYSEC-2025-53

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

Low PyPI
Severity
2.6 / 10
Low · CVSS 3.1
Exploit chance (EPSS)
0.43%
Chance of exploitation in the next 30 days. Higher than 37.26% of scored vulnerabilities.
Known exploited
No
Not in CISA's catalog
Weakness
CWE-208 Observable Timing Discrepancy
Fix
0.9.0
Upgrade vllm to this version or later

Description

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 chunk_size=2, the timing differences became noticeable enough to allow attackers to infer whether portions of their input match the victim's prompt at the chunk level.

Environment

  • GPU: NVIDIA A100 (40G)
  • CUDA: 11.8
  • PyTorch: 2.3.1
  • OS: Ubuntu 18.04
  • vLLM: v0.5.1 Configuration: We launched vLLM using the default settings and adjusted chunk_size=2 to evaluate the TTFT.

Leakage

We conducted our tests using LLaMA2-70B-GPTQ on a single device. We analyzed the timing differences when prompts shared prefixes of 2 chunks, and plotted the corresponding ROC curves. Our results suggest that timing differences can be reliably used to distinguish prefix matches, demonstrating a potential side-channel vulnerability. <img src="https://github.com/user-attachments/assets/db3491e9-02b7-424c-9b6d-56f553b39f2f" alt="roc_curves_combined_block_2" width="400"/>

Results

In our experiment, we analyzed the response time differences between cache hits and misses in vLLM's PageAttention mechanism. Using ROC curve analysis to assess the distinguishability of these timing differences, we observed the following results:

  • With a 1-token prefix, the ROC curve yielded an AUC value of 0.571, indicating that even with a short prefix, an attacker can reasonably distinguish between cache hits and misses based on response times.
  • When the prefix length increases to 8 tokens, the AUC value rises significantly to 0.99, showing that the attacker can almost perfectly identify cache hits with a longer prefix.

Fixes

Affected versions

PyPI

PackageVulnerableFixed in
vllm
< 0.9.0
0.9.0
vllm
< 0.9.0
0.9.0

How to fix

Move to the fixed version or later with your package manager, update the lockfile, and run your tests. If the package only arrives through another dependency, upgrade that dependency first. If none of its releases pulls a fixed version yet, force the version with your package manager's override.

vllm PyPI → 0.9.0

pip
pip install --upgrade "vllm>=0.9.0"
poetry
poetry add "vllm>=0.9.0"
uv
uv add "vllm>=0.9.0"

Severity breakdown

CVSS 3.1 2.6

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

Attack vector
Network
Attack complexity
High
Privileges required
Low
User interaction
Required
Scope
Unchanged
Confidentiality
Low
Integrity
None
Availability
None

Weakness

CWE-208: Observable Timing Discrepancy

Two separate operations in a product require different amounts of time to complete, in a way that is observable to an actor and reveals security-relevant information about the state of the product, such as whether a particular operation was successful or not.

References

Advisories

Packages

Other links

Timeline

  1. Published
  2. Last updated by the source
  3. Latest EPSS score
AdvisorySeverityWhy it's related
GHSA-8397-8qf6-f2gm
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GHSA-x567-49g5-p4cf
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GHSA-vvpg-rrww-wpmh
vLLM: Permissive default CORS origins allow cross-origin requests to the vLLM HTTP server
Medium Also affects vllm
GHSA-q97c-p368-75c4
vLLM: Out-of-vocabulary prompt token id passes input validation and triggers a CUDA device-side assert (engine crash / DoS)
Medium Also affects vllm
GHSA-h3rc-j82p-2c3h
vLLM: Untrusted EPS inputs can trigger denial of service or Ghostscript RCE
High Also affects vllm
GHSA-gq5q-89c7-vhvx
vLLM DeepSeek-OCR image processing vulnerable to denial of service
Medium Also affects vllm
GHSA-6757-hggm-hp2w
vLLM: Public multimodal SHM handle forgery reaches worker-side `pickle.loads()` with insecure serialization disabled
High Also affects vllm
GHSA-542m-9234-8gvf
vLLM: DoS via unbounded chat_template Jinja2 rendering exhausts the renderer thread pool
Medium Also affects vllm
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