Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.
CWE-1265 Base Draft
Unintended Reentrant Invocation of Non-reentrant Code Via Nested Calls
This vulnerability occurs when a non-reentrant function is called, and during its execution, another call is triggered that unexpectedly re-enters the same non-reentrant code path, corrupting its…
Definition
What is CWE-1265?
This vulnerability occurs when a non-reentrant function is called, and during its execution, another call is triggered that unexpectedly re-enters the same non-reentrant code path, corrupting its internal state.
In complex software, a single function call can branch into many unexpected execution paths, especially when processing external inputs. Attackers can often manipulate these inputs—like scripts in a web browser or embedded code in a PDF—to force the program into a state where a nested call re-enters code that wasn't designed to handle re-invocation. This is dangerous because the non-reentrant code typically relies on global or static data that gets corrupted when called again before the first invocation finishes.
For developers, the core issue is that a function assumes its own internal state or global resources remain stable for the duration of its execution. When a nested call—perhaps via a callback, event handler, or virtual method—breaks that assumption, it leads to race conditions, data corruption, or crashes. To prevent this, you must audit code paths triggered during non-reentrant operations and isolate or protect any state that shouldn't be accessed concurrently, even from within the same thread.
Real-world impact
Real-world CVEs caused by CWE-1265
-
In this vulnerability, by registering a malicious onerror handler, an adversary can produce unexpected re-entrance of a CDOMRange object. [REF-1098]
-
This CVE covers several vulnerable scenarios enabled by abuse of the Class_Terminate feature in Microsoft VBScript. In one scenario, Class_Terminate is used to produce an undesirable re-entrance of ScriptingDictionary during execution of that object's destructor. In another scenario, a vulnerable condition results from a recursive entrance of a property setter method. This recursive invocation produces a second, spurious call to the Release method of a reference-counted object, causing a UAF when that object is freed prematurely. This vulnerability pattern has been popularized as "Double Kill". [REF-1099]
How attackers exploit it
Step-by-step attacker path
- 1
The implementation of the Widget class in the following C++ code is an example of code that is not designed to be reentrant. If an invocation of a method of Widget inadvertently produces a second nested invocation of a method of Widget, then data member backgroundImage may unexpectedly change during execution of the outer call.
- 2
Looking closer at this example, Widget::click() calls backgroundImage->click(), which in turn calls scriptEngine->fireOnImageClick(). The code within fireOnImageClick() invokes the appropriate script handler routine as defined by the document being rendered. In this scenario this script routine is supplied by an adversary and this malicious script makes a call to Widget::changeBackgroundImage(), deleting the Image object pointed to by backgroundImage. When control returns to Image::click, the function's backgroundImage "this" pointer (which is the former value of backgroundImage) is a dangling pointer. The root of this weakness is that while one operation on Widget (click) is in the midst of executing, a second operation on the Widget object may be invoked (in this case, the second invocation is a call to different method, namely changeBackgroundImage) that modifies the non-local variable.
- 3
This is another example of C++ code that is not designed to be reentrant.
- 4
The expected order of operations is a call to Request::setup(), followed by a call to Request::send(). Request::send() calls scriptEngine->coerceToString(_data) to coerce a script-provided parameter into a string. This operation may produce script execution. For example, if the script language is ECMAScript, arbitrary script execution may result if _data is an adversary-supplied ECMAScript object having a custom toString method. If the adversary's script makes a new call to Request::setup, then when control returns to Request::send, the field uri and the local variable credentials will no longer be consistent with one another. As a result, credentials for one resource will be shared improperly with a different resource. The root of this weakness is that while one operation on Request (send) is in the midst of executing, a second operation may be invoked (setup).
Vulnerable code example
Vulnerable C++
The implementation of the Widget class in the following C++ code is an example of code that is not designed to be reentrant. If an invocation of a method of Widget inadvertently produces a second nested invocation of a method of Widget, then data member backgroundImage may unexpectedly change during execution of the outer call.
class Widget
{
private:
Image* backgroundImage;
public:
void click()
{
if (backgroundImage)
{
backgroundImage->click();
}
}
void changeBackgroundImage(Image* newImage)
{
if (backgroundImage)
{
delete backgroundImage;
}
backgroundImage = newImage;
}
}
class Image
{
public:
void click()
{
scriptEngine->fireOnImageClick();
/* perform some operations using "this" pointer */
}
} 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-1265
- Architecture and Design When architecting a system that will execute untrusted code in response to events, consider executing the untrusted event handlers asynchronously (asynchronous message passing) as opposed to executing them synchronously at the time each event fires. The untrusted code should execute at the start of the next iteration of the thread's message loop. In this way, calls into non-reentrant code are strictly serialized, so that each operation completes fully before the next operation begins. Special attention must be paid to all places where type coercion may result in script execution. Performing all needed coercions at the very beginning of an operation can help reduce the chance of operations executing at unexpected junctures.
- Implementation Make sure the code (e.g., function or class) in question is reentrant by not leveraging non-local data, not modifying its own code, and not calling other non-reentrant code.
Detection signals
How to detect CWE-1265
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.
Plexicus auto-detects CWE-1265 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-1265?
How serious is CWE-1265?
What languages or platforms are affected by CWE-1265?
How can I prevent CWE-1265?
How does Plexicus detect and fix CWE-1265?
Where can I learn more about CWE-1265?
Frequently asked questions
What is CWE-1265?
This vulnerability occurs when a non-reentrant function is called, and during its execution, another call is triggered that unexpectedly re-enters the same non-reentrant code path, corrupting its internal state.
How serious is CWE-1265?
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-1265?
MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.
How can I prevent CWE-1265?
When architecting a system that will execute untrusted code in response to events, consider executing the untrusted event handlers asynchronously (asynchronous message passing) as opposed to executing them synchronously at the time each event fires. The untrusted code should execute at the start of the next iteration of the thread's message loop. In this way, calls into non-reentrant code are strictly serialized, so that each operation completes fully before the next operation begins. Special…
How does Plexicus detect and fix CWE-1265?
Plexicus's SAST engine matches the data-flow signature for CWE-1265 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-1265?
MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/1265.html. You can also reference OWASP and NIST documentation for adjacent guidance.
Related weaknesses
Weaknesses related to CWE-1265
CWE-691 Parent
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CWE-1281 Sibling
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CWE-362 Sibling
Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition')
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CWE-430 Sibling
Deployment of Wrong Handler
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CWE-431 Sibling
Missing Handler
This vulnerability occurs when a software component lacks the necessary code to properly handle an error or unexpected event.
CWE-662 Sibling
Improper Synchronization
This vulnerability occurs when a multi-threaded or multi-process application allows shared resources to be accessed by multiple threads or…
CWE-670 Sibling
Always-Incorrect Control Flow Implementation
This weakness occurs when a section of code is structured in a way that always executes incorrectly, regardless of input or conditions.…
CWE-696 Sibling
Incorrect Behavior Order
This weakness occurs when a system executes multiple dependent actions in the wrong sequence, leading to unexpected and potentially…
CWE-705 Sibling
Incorrect Control Flow Scoping
This vulnerability occurs when a program fails to return execution to the correct point in the code after finishing a specific operation…
Resources
Further reading
- MITRE — official CWE-1265 https://cwe.mitre.org/data/definitions/1265.html
- Root Cause Analysis of CVE-2014-1772 - An Internet Explorer Use After Free Vulnerability https://www.trendmicro.com/en_us/research.html
- It's Time To Terminate The Terminator https://www.zerodayinitiative.com/blog/2018/5/15/its-time-to-terminate-the-terminator
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