CWE-138: Improper Neutralization of Special Elements

Q: What is CWE-138: Improper Neutralization of Special Elements?

CWE-138: Improper Neutralization of Special Elements is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as control elements or syntactic markers when they are sent to a downstream component. Most languages and protocols have their own special elements such as characters and reserved words. These special elements can carry control implications. If product does not prevent external control or influence over the inclusion of such special elements, the control flow of the program may be altered from what was intended. For example, both Unix and Windows interpret the symbol < ("less than") as meaning "read input from a file".

Q: What are the security consequences of Improper Neutralization of Special Elements?

If exploited, CWE-138 (Improper Neutralization of Special Elements) it can compromise Confidentiality, Integrity, Availability and Other, leading to outcomes such as Execute Unauthorized Code or Commands, Alter Execution Logic and DoS: Crash, Exit, or Restart.

Q: How do you prevent or mitigate Improper Neutralization of Special Elements?

Recommended mitigations for CWE-138 include: Developers should anticipate that special elements (e.g. delimiters, symbols) will be injected into input vectors of their product. One defense is to create an allowlist (e.g. a regular expression) that defines valid input according to the requirements specifications. Strictly filter any input that does not match against the allowlist. Properly encode your output, and quote any elements that have special meaning to the component with which you are communicating. Use and specify an appropriate output encoding to ensure that the special elements are well-defined. A normal byte sequence in one encoding could be a special element in another.

Q: Which programming languages are affected by Improper Neutralization of Special Elements?

CWE-138 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

Q: What are real-world examples of Improper Neutralization of Special Elements?

MITRE documents real CVEs mapped to CWE-138, including CVE-2001-0677, CVE-2000-0703, CVE-2003-0020 and CVE-2003-0083. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

Q: What is the difference between a CWE and a CVE?

A CWE (Common Weakness Enumeration) like CWE-138 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.

Extended Description

Most languages and protocols have their own special elements such as characters and reserved words. These special elements can carry control implications. If product does not prevent external control or influence over the inclusion of such special elements, the control flow of the program may be altered from what was intended. For example, both Unix and Windows interpret the symbol < ("less than") as meaning "read input from a file".

Code Examples & CVEs

Demonstrative Examples

The following code takes untrusted input and uses a regular expression to filter "../" from the input. It then appends this result to the /home/user/ directory and attempts to read the file in the final resulting path.

Since the regular expression does not have the /g global match modifier, it only removes the first instance of "../" it comes across. So an input value such as:

BadPerl

my $Username = GetUntrustedInput();$Username =~ s/\.\.\///;my $filename = "/home/user/" . $Username;ReadAndSendFile($filename);

The following example assigns some character values to a list of characters and prints them each individually, and then as a string. The third character value is intended to be an integer taken from user input and converted to an int. The first print statement will print each character separated by a space.

However, if a NULL byte is read from stdin by fgetc, then it will return 0. When foo is printed as a string, the 0 at character foo[2] will act as a NULL terminator, and the second printf() statement will not print foo[3].

BadC

char *foo; 				   foo=malloc(sizeof(char)*5); 				   foo[0]='a'; 				   foo[1]='a'; 				   foo[2]=fgetc(stdin); 				   foo[3]='c'; 				   foo[4]='\0'; 				   printf("%c %c %c %c %c \n",foo[0],foo[1],foo[2],foo[3],foo[4]); 				   printf("%s\n",foo);

Observed CVE Examples (4)

CVE-2001-0677

Read arbitrary files from mail client by providing a special MIME header that is internally used to store pathnames for attachments.

View Details

CVE-2000-0703

Setuid program does not cleanse special escape sequence before sending data to a mail program, causing the mail program to process those sequences.

View Details

CVE-2003-0020

Multi-channel issue. Terminal escape sequences not filtered from log files.

View Details

CVE-2003-0083

Multi-channel issue. Terminal escape sequences not filtered from log files.

View Details

Source-backed guidance

Additional facts reviewed against primary or authoritative security sources.

Verify controls for CWE-138 with SSDF evidence

Use NIST SSDF verification and vulnerability-response practices to detect CWE-138, Improper Neutralization of Special Elements, 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

Apply Input Validation controls for Improper Neutralization of Special Elements

MITRE associates mitigation with Implementation; the listed strategies include Input Validation, and Output Encoding; recorded impacts include Execute Unauthorized Code or Commands, Alter Execution Logic, and DoS: Crash, Exit, or Restart. Use these source-defined anchors to turn CWE-138 into implementation, review, and verification checks for the affected component.

CWE-138: Improper Neutralization of Special Elements — MITRE CWE

Triage CWE-138 against known exploitation evidence

Use CISA's Known Exploited Vulnerabilities catalog to test whether a vulnerability mapped to CWE-138, Improper Neutralization of Special Elements, 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-138

Apply MITRE's full root-cause mapping guidance when using CWE-138, Improper Neutralization of Special Elements. 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-138 with root-cause mapping checks

Apply MITRE's root-cause mapping quick tips to CWE-138, Improper Neutralization of Special Elements. 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-138: Improper Neutralization of Special Elements?+

CWE-138: Improper Neutralization of Special Elements is a Common Weakness Enumeration (CWE) entry maintained by MITRE. The product receives input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could be interpreted as control elements or syntactic markers when they are sent to a downstream component. Most languages and protocols have their own special elements such as characters and reserved words. These special elements can carry control implications. If product does not prevent external control or influence over the inclusion of such special elements, the control flow of the program may be altered from what was intended. For example, both Unix and Windows interpret the symbol < ("less than") as meaning "read input from a file".

What are the security consequences of Improper Neutralization of Special Elements?+

If exploited, CWE-138 (Improper Neutralization of Special Elements) it can compromise Confidentiality, Integrity, Availability and Other, leading to outcomes such as Execute Unauthorized Code or Commands, Alter Execution Logic and DoS: Crash, Exit, or Restart.

How do you prevent or mitigate Improper Neutralization of Special Elements?+

Recommended mitigations for CWE-138 include: Developers should anticipate that special elements (e.g. delimiters, symbols) will be injected into input vectors of their product. One defense is to create an allowlist (e.g. a regular expression) that defines valid input according to the requirements specifications. Strictly filter any input that does not match against the allowlist. Properly encode your output, and quote any elements that have special meaning to the component with which you are communicating. Use and specify an appropriate output encoding to ensure that the special elements are well-defined. A normal byte sequence in one encoding could be a special element in another.

Which programming languages are affected by Improper Neutralization of Special Elements?+

CWE-138 commonly affects Not Language-Specific. Note that weaknesses are often language-agnostic patterns, so secure coding practices apply broadly.

What are real-world examples of Improper Neutralization of Special Elements?+

MITRE documents real CVEs mapped to CWE-138, including CVE-2001-0677, CVE-2000-0703, CVE-2003-0020 and CVE-2003-0083. You can look up the full details of each CVE, including CVSS scores and remediation guidance, on our CVE Lookup tool.

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

A CWE (Common Weakness Enumeration) like CWE-138 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-138: Improper Neutralization of Special Elements

Extended Description

Technical Details

Applicable To

Security Consequences

Scope

Impact

Mitigation Strategies

Phase

Description

Phase

Description

Detection Methods

Code Examples & CVEs

Demonstrative Examples

Observed CVE Examples (4)

CWE Relationships

Parent Weaknesses

Source-backed guidance

Verify controls for CWE-138 with SSDF evidence

Apply Input Validation controls for Improper Neutralization of Special Elements

Triage CWE-138 against known exploitation evidence

Apply precise root-cause mapping to CWE-138

Validate CWE-138 with root-cause mapping checks

Frequently Asked Questions

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