CWE-1322: Use of Blocking Code in Single-threaded, Non-blocking Context

Extended Description

When an attacker can directly invoke the blocking code, or the blocking code can be affected by environmental conditions that can be influenced by an attacker, then this can lead to a denial of service by causing unexpected hang or freeze of the code. Examples of blocking code might be an expensive computation or calling blocking library calls, such as those that perform exclusive file operations or require a successful network operation. Due to limitations in multi-thread models, single-threaded models are used to overcome the resource constraints that are caused by using many threads. In such a model, all code should generally be non-blocking. If blocking code is called, then the event loop will effectively be stopped, which can be undesirable or dangerous. Such models are used in Python asyncio, Vert.x, and Node.js, or other custom event loop code.

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-1322 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-1322, Use of Blocking Code in Single-threaded, Non-blocking Context, throughout the product lifecycle. Derive review questions, static or dynamic checks, and negative tests from the CWE's causal behavior; define the components and lifecycle stages each check covers; and retain findings with enough evidence to distinguish the root cause from symptoms and impacts. Track escapes and false negatives, then improve the verification plan after every confirmed occurrence.

NIST SP 800-218 Secure Software Development Framework — National Institute of Standards and Technology

Address Use of Blocking Code in Single-threaded, Non-blocking Context during Implementation

MITRE associates mitigation with Implementation; recorded impacts include DoS: Resource Consumption (CPU); the entry maps to CAPEC-25. Use these source-defined anchors to turn CWE-1322 into implementation, review, and verification checks for the affected component.

CWE-1322: Use of Blocking Code in Single-threaded, Non-blocking Context — MITRE CWE

Triage CWE-1322 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-1322, Use of Blocking Code in Single-threaded, Non-blocking Context, has evidence of exploitation in the wild. Confirm the CVE-to-CWE root-cause mapping independently before attaching the example, then capture the affected product, required action, and remediation deadline. A missing KEV match is not evidence that the weakness is unexploited, and a KEV entry must not be generalized to every occurrence of this CWE.

Known Exploited Vulnerabilities Catalog — Cybersecurity and Infrastructure Security Agency

Apply precise root-cause mapping to CWE-1322

Apply MITRE's full root-cause mapping guidance when using CWE-1322, Use of Blocking Code in Single-threaded, Non-blocking Context. Separate weakness language from attacker prerequisites and technical impact, check the entry's abstraction and vulnerability-mapping notes, and prefer the most specific Base or Variant supported by the evidence. Record the rejected alternatives and require an independent review before the mapping is used for remediation trends or program metrics.

CVE to CWE Root Cause Mapping Guidance — MITRE CWE

Validate CWE-1322 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-1322, Use of Blocking Code in Single-threaded, Non-blocking Context. Confirm the finding describes the causal weakness rather than an impact or attack pattern, compare the abstraction and mapping notes with plausible alternatives, and have a second reviewer challenge the selection. Preserve the evidence and reasoning so recurring defects can be measured against one consistent identifier.

CVE to CWE Root Cause Mapping Quick Tips — MITRE CWE

Frequently Asked Questions

What is CWE-1322: Use of Blocking Code in Single-threaded, Non-blocking Context?+

CWE-1322: Use of Blocking Code in Single-threaded, Non-blocking Context is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product uses a non-blocking model that relies on a single threaded process for features such as scalability, but it contains code that can block when it is invoked. When an attacker can directly invoke the blocking code, or the blocking code can be affected by environmental conditions that can be influenced by an attacker, then this can lead to a denial of service by causing unexpected hang or freeze of the code. Examples of blocking code might be an expensive computation or calling blocking library calls, such as those that perform exclusive file operations or require a successful network operation. Due to limitations in multi-thread models, single-threaded models are used to overcome the resource constraints that are caused by using many threads. In such a model, all code should generally be non-blocking. If blocking code is called, then the event loop will effectively be stopped, which can be undesirable or dangerous. Such models are used in Python asyncio, Vert.x, and Node.js, or other custom event loop code.

What are the security consequences of Use of Blocking Code in Single-threaded, Non-blocking Context?+

If exploited, CWE-1322 (Use of Blocking Code in Single-threaded, Non-blocking Context) it can compromise Availability, leading to outcomes such as DoS: Resource Consumption (CPU).

How do you prevent or mitigate Use of Blocking Code in Single-threaded, Non-blocking Context?+

Recommended mitigations for CWE-1322 include: Generally speaking, blocking calls should be replaced with non-blocking alternatives that can be used asynchronously. Expensive computations should be passed off to worker threads, although the correct approach depends on the framework being used. For expensive computations, consider breaking them up into multiple smaller computations. Refer to the documentation of the framework being used for guidance.

Which programming languages are affected by Use of Blocking Code in Single-threaded, Non-blocking Context?+

CWE-1322 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What is the difference between a CWE and a CVE?+

A CWE (Common Weakness Enumeration) like CWE-1322 describes a category of software weakness — the underlying flaw type. A CVE (Common Vulnerabilities and Exposures) identifies a specific, real-world vulnerability in a particular product. In short, a CWE is the kind of mistake, and a CVE is an instance of that mistake being found in software.

CWE-1322: Use of Blocking Code in Single-threaded, Non-blocking Context

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