Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-827 Variant Incomplete
Improper Control of Document Type Definition
This vulnerability occurs when an application fails to properly restrict which Document Type Definitions (DTDs) can be referenced during XML parsing. Attackers can exploit this by injecting…
Definition
What is CWE-827?
This vulnerability occurs when an application fails to properly restrict which Document Type Definitions (DTDs) can be referenced during XML parsing. Attackers can exploit this by injecting references to malicious DTDs, potentially leading to unauthorized file access, server-side request forgery (SSRF), or denial-of-service through resource exhaustion.
When an XML parser processes a DTD, it may automatically retrieve and execute external resources or read local files specified within that DTD. If an attacker controls the DTD reference, they can point the parser to sensitive system files, internal network endpoints, or malicious payloads, effectively using the application's parsing functionality as an attack vector.
This is why many modern web standards explicitly forbid DTDs in certain contexts—for instance, the SOAP specification prohibits DTDs within messages. To prevent this, developers must either disable external DTD processing entirely in their XML parser configuration or implement strict allowlists that restrict DTD sources to trusted, intended locations only.
Real-world impact
Real-world CVEs caused by CWE-827
-
Product does not properly reject DTDs in SOAP messages, which allows remote attackers to read arbitrary files, send HTTP requests to intranet servers, or cause a denial of service.
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-827
- Architecture Use safe-by-default frameworks and APIs that prevent the unsafe pattern from being expressible.
- Implementation Validate input at trust boundaries; use allowlists, not denylists.
- Implementation Apply the principle of least privilege to credentials, file paths, and runtime permissions.
- Testing Cover this weakness in CI: SAST rules + targeted unit tests for the data flow.
- Operation Monitor logs for the runtime signals listed in the next section.
Detection signals
How to detect CWE-827
Run dynamic application security testing against the live endpoint.
Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.
Code review: flag any new code that handles input from this surface without using the validated framework helpers.
CWE-827
Don't catalog this weakness. Prove it's reachable.
Plexicus turns CWE catalogs into evidence: every CWE-pattern is matched against your real code graph, reach is proven on a sandbox clone, and verified findings ship as reviewed PRs.
Frequently asked questions What is CWE-827?
How serious is CWE-827?
What languages or platforms are affected by CWE-827?
How can I prevent CWE-827?
How does Plexicus detect and fix CWE-827?
Where can I learn more about CWE-827?
Frequently asked questions
What is CWE-827?
This vulnerability occurs when an application fails to properly restrict which Document Type Definitions (DTDs) can be referenced during XML parsing. Attackers can exploit this by injecting references to malicious DTDs, potentially leading to unauthorized file access, server-side request forgery (SSRF), or denial-of-service through resource exhaustion.
How serious is CWE-827?
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-827?
MITRE lists the following affected platforms: XML.
How can I prevent CWE-827?
Use safe-by-default frameworks, validate untrusted input at trust boundaries, and apply the principle of least privilege. Cover the data-flow signature in CI with SAST.
How does Plexicus detect and fix CWE-827?
Plexicus's SAST engine matches the data-flow signature for CWE-827 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-827?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/827.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-827
CWE-706 Parent
Use of Incorrectly-Resolved Name or Reference
This vulnerability occurs when software uses a name, path, or reference to access a resource, but that identifier points to something…
CWE-178 Sibling
Improper Handling of Case Sensitivity
This vulnerability occurs when software fails to consistently handle uppercase and lowercase letters when checking or accessing resources,…
CWE-22 Sibling
Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
This vulnerability occurs when an application builds a file path using user input but fails to properly validate it, allowing an attacker…
CWE-386 Sibling
Symbolic Name not Mapping to Correct Object
This vulnerability occurs when a program uses a fixed symbolic name (like a constant or identifier) to refer to an object, but that name…
CWE-41 Sibling
Improper Resolution of Path Equivalence
This vulnerability occurs when an application fails to properly handle different text representations that refer to the same file or…
CWE-59 Sibling
Improper Link Resolution Before File Access ('Link Following')
This vulnerability occurs when an application uses a filename to access a file but fails to properly check if that name points to a…
CWE-66 Sibling
Improper Handling of File Names that Identify Virtual Resources
This vulnerability occurs when software incorrectly processes a filename that points to a 'virtual' resource—like a device, pipe, or…
CWE-98 Sibling
Improper Control of Filename for Include/Require Statement in PHP Program ('PHP Remote File Inclusion')
This vulnerability occurs when a PHP application uses unvalidated or insufficiently restricted user input directly within file inclusion…
CWE-776 Can precede
Improper Restriction of Recursive Entity References in DTDs ('XML Entity Expansion')
This vulnerability occurs when an XML parser allows Document Type Definitions (DTDs) to contain recursively defined entities without…
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