What CVEs are caused by CWE-86?

10 recorded CVEs are attributed to CWE-86, including CVE-2023-31126, CVE-2026-28417, CVE-2024-21864.

What are the consequences of CWE-86?

Exploiting CWE-86 can lead to: Read Application Data, Execute Unauthorized Code or Commands.

Is CWE-86 actively exploited?

10 recorded CVEs are caused by CWE-86; none are currently in CISA's KEV catalog of actively exploited flaws.

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Variant weakness

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CWE-86: Improper Neutralization of Invalid Characters in Identifiers in Web Pages

The product does not neutralize or incorrectly neutralizes invalid characters or byte sequences in the middle of tag names, URI schemes, and other identifiers.

Last updated May 1, 2026

10

Recorded CVEs

With this primary weakness

0

KEVs

In the CISA KEV catalog

Not rated

Likelihood of exploit

Per MITRE

Variant

Abstraction

MITRE classification

Overview

Some web browsers may remove these sequences, resulting in output that may have unintended control implications. For example, the product may attempt to remove a "javascript:" URI scheme, but a "java%00script:" URI may bypass this check and still be rendered as active javascript by some browsers, allowing XSS or other attacks.

Real-world CVEs

10 recorded CVEs are caused by CWE-86 (Improper Neutralization of Invalid Characters in Identifiers in Web Pages). The highest-severity and most recent are shown first. 2 new CWE-86 CVEs have been recorded so far in 2026 (3 in 2025).

Common consequences

What can happen when CWE-86 is exploited.

Read Application Data, Execute Unauthorized Code or Commands

Affects: Confidentiality, Integrity, Availability

How it happens

When it is introduced

Typically introduced during these phases of the software lifecycle.

Implementation

Applies to

Technologies

Web Based (Often)

Web Server

How to prevent it

Practical mitigations for CWE-86, grouped by where in the lifecycle they apply.

Implementation

Output Encoding

Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.

The problem of inconsistent output encodings often arises in web pages. If an encoding is not specified in an HTTP header, web browsers often guess about which encoding is being used. This can open up the browser to subtle XSS attacks.

Implementation

Attack Surface Reduction

To help mitigate XSS attacks against the user's session cookie, set the session cookie to be HttpOnly. In browsers that support the HttpOnly feature (such as more recent versions of Internet Explorer and Firefox), this attribute can prevent the user's session cookie from being accessible to malicious client-side scripts that use document.cookie. This is not a complete solution, since HttpOnly is not supported by all browsers. More importantly, XmlHttpRequest and other powerful browser technologies provide read access to HTTP headers, including the Set-Cookie header in which the HttpOnly flag is set.

Effectiveness: Defense in Depth

How to detect it

Automated Static Analysis

Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)

Effectiveness: High

Illustrative examples

Real CVEs that MITRE cites as examples of this weakness.

CVE-2004-0595 — XSS filter doesn't filter null characters before looking for dangerous tags, which are ignored by web browsers. Multiple Interpretation Error (MIE) and validate-before-cleanse.

Terminology & mappings

Mapped taxonomies

PLOVER: Invalid Characters in Identifiers
Software Fault Patterns: Tainted input to command (SFP24)

Attack patterns

CAPEC attack patterns that exploit this weakness.

Frequently asked questions

Common questions about CWE-86.

What is CWE-86?: The product does not neutralize or incorrectly neutralizes invalid characters or byte sequences in the middle of tag names, URI schemes, and other identifiers.
What CVEs are caused by CWE-86?: 10 recorded CVEs are attributed to CWE-86, including CVE-2023-31126, CVE-2026-28417, CVE-2024-21864.
How do you prevent CWE-86?: Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.
How is CWE-86 detected?: Automated Static Analysis: Automated static analysis, commonly referred to as Static Application Security Testing (SAST), can find some instances of this weakness by analyzing source code (or binary/compiled code) without having to execute it. Typically, this is done by building a model of data flow and control flow, then searching for potentially-vulnerable patterns that connect "sources" (origins of input) with "sinks" (destinations where the data interacts with external components, a lower layer such as the OS, etc.)
What are the consequences of CWE-86?: Exploiting CWE-86 can lead to: Read Application Data, Execute Unauthorized Code or Commands.
Is CWE-86 actively exploited?: 10 recorded CVEs are caused by CWE-86; none are currently in CISA's KEV catalog of actively exploited flaws.

References

Weakness data is sourced from the MITRE CWE catalog (v4.20). CVE associations are aggregated and kept current by RadicalNotion.AI.

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Overview

Real-world CVEs

Common consequences

How it happens

When it is introduced

Applies to

How to prevent it

How to detect it

Automated Static Analysis

Illustrative examples

Related weaknesses

Parent

Peer

Terminology & mappings

Mapped taxonomies

Attack patterns

Frequently asked questions

References

Stay ahead of CWE-86