Summary
vLLM: Unauthenticated Internal Path and Username Disclosure via Validation Error Messages
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.
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
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.
CVE-2026-73555 has a CVSS score of 5.3 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.26.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2026-73555? CVE-2026-73555 is a medium-severity security vulnerability in vllm (pip), affecting versions < 0.26.0. It is fixed in 0.26.0.
- How severe is CVE-2026-73555? CVE-2026-73555 has a CVSS score of 5.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of vllm are affected by CVE-2026-73555? vllm (pip) versions < 0.26.0 is affected.
- Is there a fix for CVE-2026-73555? Yes. CVE-2026-73555 is fixed in 0.26.0. Upgrade to this version or later.
- Is CVE-2026-73555 exploitable, and should I be worried? Whether CVE-2026-73555 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-73555 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-73555? Upgrade
vllmto 0.26.0 or later.