CVE-2026-73555

vLLM is an inference and serving engine for large language models. Prior to 0.26.0, the validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts FastAPI RequestValidationError objects with str(exc), and sanitize_message in vllm/entrypoints/utils.py does not remove traceback-style file paths, allowing unauthenticated malformed JSON requests to /v1/chat/completions, /v1/completions, /tokenize, and /detokenize to disclose the OS username, home and virtual-environment paths, Python version, internal package structure, line numbers, and endpoint handler names. This issue is fixed in version 0.26.0.

vLLM is an inference and serving engine for large language models. Prior to 0.26.0, the validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts FastAPI RequestValidationError objects with str(exc), and sanitize_message in vllm/entrypoints/utils.py does not remove traceback-style file paths, allowing unauthenticated malformed JSON requests to /v1/chat/completions, /v1/completions, /tokenize, and /detokenize to disclose the OS username, home and virtual-environment paths, Python version, internal package structure, line numbers, and endpoint handler names. This issue is fixed in version 0.26.0.

Summary

When the vLLM API receives a malformed request (e.g., invalid JSON or missing required fields), FastAPI raises a Pydantic RequestValidationError. The validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts this exception to a string via str(exc), which includes the internal file path and line number of the handler function. The existing sanitize_message() function in vllm/entrypoints/utils.py strips memory addresses (e.g., 0x7f...) but does not strip File "...", line X patterns. The result is a user-facing HTTP response that leaks internal system information.

Impact

An unauthenticated attacker can extract the following with a single malformed request:

• OS username running the vLLM process (e.g., ubuntu)
• Home directory path (e.g., /home/ubuntu/)
• Virtual environment path (e.g., vllm-env/)
• Python version (e.g., 3.12)
• Internal package structure and line numbers (e.g., vllm/entrypoints/openai/chat_completion/api_router.py)
• Handler function names per endpoint, enabling precise version fingerprinting

This information aids attackers in constructing targeted exploits: environment paths narrow the attack surface, and handler function names + line numbers enable exact version identification even when the /version endpoint is disabled.

All POST endpoints that accept JSON bodies are affected, including /v1/chat/completions, /v1/completions, /tokenize, and /detokenize.

Workarounds

Deploying vLLM behind a reverse proxy that rewrites error response bodies to strip file paths would mitigate this, though it is fragile.

Remediation Recommendation

Two possible fixes (either suffices):

Option A — Fix validation_exception_handler: Construct the error message from exc.errors() (the structured Pydantic error list) rather than str(exc). This avoids the traceback-style string entirely.

Option B — Fix sanitize_message: Add a regex to strip File "...", line \d+ patterns, similar to how memory addresses are already stripped:

import re
msg = re.sub(r'File ".*?", line \d+, in \w+', '[internal]', msg)

Option A is preferred as it addresses the root cause rather than filtering symptoms.

Environment Tested

• vLLM 0.20.1 (pip install, latest stable as of May 2026)
• Python 3.12
• Ubuntu 22.04
• Model: Qwen/Qwen2-0.5B (text-only; bug is model-independent)

This was fixed here: https://github.com/vllm-project/vllm/commit/e87521626f

vLLM is an inference and serving engine for large language models. Prior to 0.26.0, the validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts FastAPI RequestValidationError objects with str(exc), and sanitize_message in vllm/entrypoints/utils.py does not remove traceback-style file paths, allowing unauthenticated malformed JSON requests to /v1/chat/completions, /v1/completions, /tokenize, and /detokenize to disclose the OS username, home and virtual-environment paths, Python version, internal package structure, line numbers, and endpoint handler names. This issue is fixed in version 0.26.0.

vLLM is an inference and serving engine for large language models. Prior to 0.26.0, the validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts FastAPI RequestValidationError objects with str(exc), and sanitize_message in vllm/entrypoints/utils.py does not remove traceback-style file paths, allowing unauthenticated malformed JSON requests to /v1/chat/completions, /v1/completions, /tokenize, and /detokenize to disclose the OS username, home and virtual-environment paths, Python version, internal package structure, line numbers, and endpoint handler names. This issue is fixed in version 0.26.0.

Summary

When the vLLM API receives a malformed request (e.g., invalid JSON or missing required fields), FastAPI raises a Pydantic RequestValidationError. The validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts this exception to a string via str(exc), which includes the internal file path and line number of the handler function. The existing sanitize_message() function in vllm/entrypoints/utils.py strips memory addresses (e.g., 0x7f...) but does not strip File "...", line X patterns. The result is a user-facing HTTP response that leaks internal system information.

Impact

An unauthenticated attacker can extract the following with a single malformed request:

• OS username running the vLLM process (e.g., ubuntu)
• Home directory path (e.g., /home/ubuntu/)
• Virtual environment path (e.g., vllm-env/)
• Python version (e.g., 3.12)
• Internal package structure and line numbers (e.g., vllm/entrypoints/openai/chat_completion/api_router.py)
• Handler function names per endpoint, enabling precise version fingerprinting

This information aids attackers in constructing targeted exploits: environment paths narrow the attack surface, and handler function names + line numbers enable exact version identification even when the /version endpoint is disabled.

All POST endpoints that accept JSON bodies are affected, including /v1/chat/completions, /v1/completions, /tokenize, and /detokenize.

Workarounds

Deploying vLLM behind a reverse proxy that rewrites error response bodies to strip file paths would mitigate this, though it is fragile.

Remediation Recommendation

Two possible fixes (either suffices):

Option A — Fix validation_exception_handler: Construct the error message from exc.errors() (the structured Pydantic error list) rather than str(exc). This avoids the traceback-style string entirely.

Option B — Fix sanitize_message: Add a regex to strip File "...", line \d+ patterns, similar to how memory addresses are already stripped:

import re
msg = re.sub(r'File ".*?", line \d+, in \w+', '[internal]', msg)

Option A is preferred as it addresses the root cause rather than filtering symptoms.

Environment Tested

• vLLM 0.20.1 (pip install, latest stable as of May 2026)
• Python 3.12
• Ubuntu 22.04
• Model: Qwen/Qwen2-0.5B (text-only; bug is model-independent)

This was fixed here: https://github.com/vllm-project/vllm/commit/e87521626f

Summary

When the vLLM API receives a malformed request (e.g., invalid JSON or missing required fields), FastAPI raises a Pydantic RequestValidationError. The validation_exception_handler in vllm/entrypoints/openai/server_utils.py converts this exception to a string via str(exc), which includes the internal file path and line number of the handler function. The existing sanitize_message() function in vllm/entrypoints/utils.py strips memory addresses (e.g., 0x7f...) but does not strip File "...", line X patterns. The result is a user-facing HTTP response that leaks internal system information.

Impact

An unauthenticated attacker can extract the following with a single malformed request:

• OS username running the vLLM process (e.g., ubuntu)
• Home directory path (e.g., /home/ubuntu/)
• Virtual environment path (e.g., vllm-env/)
• Python version (e.g., 3.12)
• Internal package structure and line numbers (e.g., vllm/entrypoints/openai/chat_completion/api_router.py)
• Handler function names per endpoint, enabling precise version fingerprinting

This information aids attackers in constructing targeted exploits: environment paths narrow the attack surface, and handler function names + line numbers enable exact version identification even when the /version endpoint is disabled.

All POST endpoints that accept JSON bodies are affected, including /v1/chat/completions, /v1/completions, /tokenize, and /detokenize.

Workarounds

Deploying vLLM behind a reverse proxy that rewrites error response bodies to strip file paths would mitigate this, though it is fragile.

Remediation Recommendation

Two possible fixes (either suffices):

Option A — Fix validation_exception_handler: Construct the error message from exc.errors() (the structured Pydantic error list) rather than str(exc). This avoids the traceback-style string entirely.

Option B — Fix sanitize_message: Add a regex to strip File "...", line \d+ patterns, similar to how memory addresses are already stripped:

import re
msg = re.sub(r'File ".*?", line \d+, in \w+', '[internal]', msg)

Option A is preferred as it addresses the root cause rather than filtering symptoms.

Environment Tested

• vLLM 0.20.1 (pip install, latest stable as of May 2026)
• Python 3.12
• Ubuntu 22.04
• Model: Qwen/Qwen2-0.5B (text-only; bug is model-independent)

This was fixed here: https://github.com/vllm-project/vllm/commit/e87521626f