Is CVE-2026-73556 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-73556 being exploited in the wild.

How severe is CVE-2026-73556?

Medium severity, with a CVSS score of 5.3, published 13 August 2026.

Which products and versions are affected?

vllm (PyPI) ECOSYSTEM:introduced=0,fixed=0.26.0.

Is there a fix for CVE-2026-73556?

Yes. vllm (PyPI): upgrade to 0.26.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-73556 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-73556?

Check whether you run vllm (PyPI) or Vllm at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-73556: vLLM ReDoS in lm-format-enforcer backend regex parsing

CVE-2026-73556 · Severity: medium · CVSS 5.3 · Published 2026-08-13

vLLM is an open-source large language model inference engine. The lm-format-enforcer backend, used to constrain generated output to match user-supplied regex patterns, fails to validate or time-limit regex compilation. An attacker can submit a catastrophic regex (such as `(a{1,300}){300}`) that causes the regex compiler to hang indefinitely, consuming a CPU core and blocking all concurrent structured-output requests on that worker.

Technologies: vllm (PyPI), Vllm. Vendors: PyPI, Vllm.

Quick answers

Is CVE-2026-73556 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-73556 being exploited in the wild.
How severe is CVE-2026-73556?: Medium severity, with a CVSS score of 5.3, published 13 August 2026.
Which products and versions are affected?: vllm (PyPI) ECOSYSTEM:introduced=0,fixed=0.26.0.
Is there a fix for CVE-2026-73556?: Yes. vllm (PyPI): upgrade to 0.26.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-73556 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-73556?: Check whether you run vllm (PyPI) or Vllm at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

vllm (PyPI): upgrade to 0.26.0 or later
```
pip install "vllm>=0.26.0"
```

Technical details

The vulnerability is a Regular Expression Denial of Service (ReDoS) in vLLM's lm-format-enforcer structured-output backend (vllm/v1/structured_output/backend_lm_format_enforcer.py, line 110). The backend calls `lmformatenforcer.RegexParser(grammar_spec)` directly on user-supplied regex strings without timeout or buildability checks. The underlying interegular FSM construction exhibits catastrophic complexity on exponential patterns like `(a{1,300}){300}`, which does not complete in 20+ seconds and pegs a CPU core at 100%. The sibling xgrammar and outlines backends were patched for the same issue (GHSA-rwxx-mrjm-wc2m) by wrapping regex compile in `compile_regex_with_timeout`, but lm-format-enforcer was overlooked. The attack is unauthenticated (vLLM has no auth by default) and requires only network access and selection of the lm-format-enforcer backend via `--structured-outputs-config` (not the default). Patched in version 0.26.0.

Affected products

vLLM Project vLLM < 0.26.0

Identifiers

CVE-2026-73556
GHSA-48jh-3gj7-fg8v
PYSEC-2026-3933

Timeline

2026-09-04: disclosed: GitHub Security Advisory published
2026-09-04: patched: Fix released in v0.26.0

References

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