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​​: inputprocessorforphi4mm function. https://github.com/vllm-project/vllm/blob/8cac35ba435906fb7eb07e44fe1a8c26e8744f4e/vllm/modelexecutor/models/phi4mm.py#L1182-L1197

The code modifies the inputids list in-place using inputids = inputids[:i] + tokens + inputids[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 testcases = [100, 200, 400, 800, 1600, 3200, 6400] runtimes = [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 <|audio1|> 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 newinputids = [] for token in inputids: if token is placeholder: newinputids.extend([token] precomputedlength) else: newinputids.append(token)