CVE-2025-46560: vLLM phi4mm: Quadratic Time Complexity in Input Token Processing​ leads to denial of service

First published: Tue Apr 29 2025(Updated: )

### Summary A critical performance vulnerability has been identified in the input preprocessing logic of the multimodal tokenizer. The code dynamically replaces placeholder tokens (e.g., <|audio_*|>, <|image_*|>) with repeated tokens based on precomputed lengths. Due to ​​inefficient list concatenation operations​​, the algorithm exhibits ​​quadratic time complexity (O(n²))​​, allowing malicious actors to trigger resource exhaustion via specially crafted inputs. ### Details ​​Affected Component​​: input_processor_for_phi4mm function. https://github.com/vllm-project/vllm/blob/8cac35ba435906fb7eb07e44fe1a8c26e8744f4e/vllm/model_executor/models/phi4mm.py#L1182-L1197 The code modifies the input_ids list in-place using input_ids = input_ids[:i] + tokens + input_ids[i+1:]. Each concatenation operation copies the entire list, leading to O(n) operations per replacement. For k placeholders expanding to m tokens, total time becomes O(kmn), approximating O(n²) in worst-case scenarios. ### PoC Test data demonstrates exponential time growth: ```python test_cases = [100, 200, 400, 800, 1600, 3200, 6400] run_times = [0.002, 0.007, 0.028, 0.136, 0.616, 2.707, 11.854] # seconds ``` Doubling input size increases runtime by ~4x (consistent with O(n²)). ### Impact ​​Denial-of-Service (DoS):​​ An attacker could submit inputs with many placeholders (e.g., 10,000 <|audio_1|> tokens), causing CPU/memory exhaustion. Example: 10,000 placeholders → ~100 million operations. ### Remediation Recommendations​ Precompute all placeholder positions and expansion lengths upfront. Replace dynamic list concatenation with a single preallocated array. ```python # Pseudocode for O(n) solution new_input_ids = [] for token in input_ids: if token is placeholder: new_input_ids.extend([token] * precomputed_length) else: new_input_ids.append(token) ```

Credit: security-advisories@github.com

Never miss a vulnerability like this again

Sign up to SecAlerts for real-time vulnerability data matched to your software, aggregated from hundreds of sources.

Read MoreStart Free Trial

• agent/weakness
• agent/softwarecombine
• agent/first-publish-date
• agent/references
• collector/mitre-cve
• source/MITRE
• agent/title
• agent/type
• agent/description
• agent/author
• agent/severity
• agent/event
• collector/epss-latest
• source/FIRST
• agent/source
• agent/epss
• agent/tags
• collector/nvd-api
• source/NVD
• agent/last-modified-date
• collector/nvd-cve
• collector/github-advisory-latest
• source/GitHub
• alias/GHSA-vc6m-hm49-g9qg
• alias/CVE-2025-46560
• agent/software-canonical-lookup
• package-manager/pip