ReDoS Vulnerability in vLLM Prior to 0.24.0

Executive Summary

The vulnerability exists in vLLM versions prior to 0.24.0 in the structured_outputs.regex API parameter. This parameter accepts a user-supplied regular expression string and passes it directly to the grammar compiler backends without any compilation timeout or complexity checks. Specifically, in the xgrammar backend, the regex string is compiled with no guard, and in the outlines backend, although some structural issues are blocked, complex patterns with nested quantifiers are not analyzed for complexity. This allows an adversarial regex pattern to cause exponential state-space expansion, which can hang an inference worker indefinitely and result in a denial of service.

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Detection Guidance

This vulnerability can be detected by monitoring for inference workers hanging or becoming unresponsive when processing requests containing adversarial regular expressions. Since the issue involves regex compilation causing indefinite hangs, detection involves identifying requests with complex or nested quantifier regex patterns that trigger exponential state-space expansion.

To detect exploitation attempts or the vulnerability on your system, you can look for processes or threads stuck during regex compilation or unusually high CPU usage related to regex processing.

There is no direct command provided in the resources, but you can use system monitoring tools to identify hung processes or high CPU usage, for example:

• Use `top` or `htop` to monitor CPU usage and identify stuck inference worker processes.
• Use `strace -p <pid>` on suspected hung processes to see if they are stuck in regex compilation calls.
• Check application logs for errors or timeouts related to regex compilation if the fix with timeout is applied.

Additionally, enabling the environment variable `VLLM_REGEX_COMPILATION_TIMEOUT_S` (default 5 seconds) can help detect and prevent hangs by raising errors on regex compilation timeout, which can be logged and monitored.

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Impact Analysis

This vulnerability can cause a denial of service by allowing a specially crafted regular expression to hang an inference worker indefinitely. This means that a single malicious request can exhaust system resources, making the service unavailable to legitimate users and potentially disrupting operations that rely on the vLLM inference engine.

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Compliance Impact

The provided information does not include any details about the impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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Mitigation Strategies

To mitigate this vulnerability, upgrade vLLM to version 0.24.0 or later, where the issue with the structured_outputs.regex API parameter has been fixed.

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