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.)
CWE-91 Base Draft
XML Injection (aka Blind XPath Injection)
XML Injection occurs when an application fails to properly validate or escape user-controlled input before including it in XML documents or queries. This allows attackers to inject malicious XML…
Definition
What is CWE-91?
XML Injection occurs when an application fails to properly validate or escape user-controlled input before including it in XML documents or queries. This allows attackers to inject malicious XML elements or syntax, potentially altering the document's structure, extracting sensitive data, or disrupting processing logic.
XML documents use special characters like <, >, &, and " to define elements and attributes. If user input containing these characters is inserted without neutralization, an attacker can break out of intended data fields and inject new XML tags, modify queries (like XPath), or even reference external entities. This can lead to data theft, logic bypasses, or denial of service.
Preventing XML injection requires strict input validation, context-aware output encoding, and the use of parameterized XPath interfaces. Managing this at scale across numerous codebases and APIs is difficult; an ASPM platform like Plexicus can help you track and remediate these flaws across your entire stack by correlating SAST findings with runtime behavior and prioritizing the most critical exposures.
Real-world impact
Real-world CVEs caused by CWE-91
No public CVE references are linked to this CWE in MITRE's catalog yet.
How attackers exploit it
Step-by-step attacker path
- 1
Identify a code path that handles untrusted input without validation.
- 2
Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.
- 3
Deliver the payload through a normal request and observe the application's reaction.
- 4
Iterate until the response leaks data, executes attacker code, or escalates privileges.
Vulnerable code example
Vulnerable pseudo
MITRE has not published a code example for this CWE. The pattern below is illustrative — see Resources for canonical references.
// Example pattern — see MITRE for the canonical references.
function handleRequest(input) {
// Untrusted input flows directly into the sensitive sink.
return executeUnsafe(input);
} Secure code example
Secure pseudo
// Validate, sanitize, or use a safe API before reaching the sink.
function handleRequest(input) {
const safe = validateAndEscape(input);
return executeWithGuards(safe);
} What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.
Prevention checklist
How to prevent CWE-91
- Implementation Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
Detection signals
How to detect CWE-91
Plexicus auto-detects CWE-91 and opens a fix PR in under 60 seconds.
Codex Remedium scans every commit, identifies this exact weakness, and ships a reviewer-ready pull request with the patch. No tickets. No hand-offs.
Frequently asked questions What is CWE-91?
How serious is CWE-91?
What languages or platforms are affected by CWE-91?
How can I prevent CWE-91?
How does Plexicus detect and fix CWE-91?
Where can I learn more about CWE-91?
Frequently asked questions
What is CWE-91?
XML Injection occurs when an application fails to properly validate or escape user-controlled input before including it in XML documents or queries. This allows attackers to inject malicious XML elements or syntax, potentially altering the document's structure, extracting sensitive data, or disrupting processing logic.
How serious is CWE-91?
MITRE has not published a likelihood-of-exploit rating for this weakness. Treat it as medium-impact until your threat model proves otherwise.
What languages or platforms are affected by CWE-91?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-91?
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and…
How does Plexicus detect and fix CWE-91?
Plexicus's SAST engine matches the data-flow signature for CWE-91 on every commit. When a match is found, our Codex Remedium agent opens a fix PR with the corrected code, tests, and a one-line summary for the reviewer.
Where can I learn more about CWE-91?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/91.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-91
CWE-74 Parent
Improper Neutralization of Special Elements in Output Used by a Downstream Component ('Injection')
This vulnerability occurs when an application uses untrusted external input to build a command, query, or data structure for another…
CWE-1236 Sibling
Improper Neutralization of Formula Elements in a CSV File
This vulnerability occurs when an application writes user-supplied data into a CSV file without properly sanitizing special characters.…
CWE-75 Sibling
Failure to Sanitize Special Elements into a Different Plane (Special Element Injection)
This vulnerability occurs when an application fails to properly filter or encode user-supplied data containing special characters or…
CWE-77 Sibling
Improper Neutralization of Special Elements used in a Command ('Command Injection')
This vulnerability occurs when an application builds a system command using untrusted user input without properly sanitizing it. An…
CWE-78 Sibling
Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection')
OS Command Injection occurs when an application builds a system command using untrusted, external input without properly sanitizing it.…
CWE-79 Sibling
Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
This vulnerability occurs when a web application fails to properly sanitize or encode user-supplied input before displaying it on a…
CWE-88 Sibling
Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')
This vulnerability occurs when an application builds a command string for execution by another component, but fails to properly separate…
CWE-89 Sibling
Improper Neutralization of Special Elements used in an SQL Command ('SQL Injection')
SQL Injection occurs when an application builds a database query using untrusted user input without properly sanitizing it. This allows an…
CWE-917 Sibling
Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')
Expression Language Injection occurs when an application uses untrusted, external input to build an expression language statement—common…
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