CWE-294 Base Incomplete High likelihood

Authentication Bypass by Capture-replay

This vulnerability occurs when an attacker can intercept and record legitimate authentication traffic, then replay it later to gain unauthorized access. The system accepts the replayed data as…

Definition

What is CWE-294?

This vulnerability occurs when an attacker can intercept and record legitimate authentication traffic, then replay it later to gain unauthorized access. The system accepts the replayed data as valid, effectively bypassing normal authentication checks.
Capture-replay attacks are a prevalent network threat where an adversary eavesdrops on a valid communication session, saves the transmitted data (like login requests or session tokens), and later resends that identical or slightly modified data to the server. Because the replayed data appears legitimate, the system grants access without requiring the attacker to know the actual credentials or break any encryption. These attacks are a specific type of network injection that can be challenging to prevent without implementing proper cryptographic defenses. To mitigate this risk, developers need to ensure that each authentication request is unique and cannot be reused, typically by incorporating timestamps, nonces, or sequence numbers that the server can validate.
Real-world impact

Real-world CVEs caused by CWE-294

  • CVE-2005-3435

    product authentication succeeds if user-provided MD5 hash matches the hash in its database; this can be subjected to replay attacks.

  • CVE-2007-4961

    Chain: cleartext transmission of the MD5 hash of password (CWE-319) enables attacks against a server that is susceptible to replay (CWE-294).

How attackers exploit it

Step-by-step attacker path

  1. 1

    Identify a code path that handles untrusted input without validation.

  2. 2

    Craft a payload that exercises the unsafe behavior — injection, traversal, overflow, or logic abuse.

  3. 3

    Deliver the payload through a normal request and observe the application's reaction.

  4. 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.

Vulnerable pseudo 
// 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

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-294

  • Architecture and Design Utilize some sequence or time stamping functionality along with a checksum which takes this into account in order to ensure that messages can be parsed only once.
  • Architecture and Design Since any attacker who can listen to traffic can see sequence numbers, it is necessary to sign messages with some kind of cryptography to ensure that sequence numbers are not simply doctored along with content.
Detection signals

How to detect CWE-294

SAST High

Run static analysis (SAST) on the codebase looking for the unsafe pattern in the data flow.

DAST Moderate

Run dynamic application security testing against the live endpoint.

Runtime Moderate

Watch runtime logs for unusual exception traces, malformed input, or authorization bypass attempts.

Code review Moderate

Code review: flag any new code that handles input from this surface without using the validated framework helpers.

Plexicus auto-fix

Plexicus auto-detects CWE-294 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.

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Frequently asked questions

Frequently asked questions

What is CWE-294?

This vulnerability occurs when an attacker can intercept and record legitimate authentication traffic, then replay it later to gain unauthorized access. The system accepts the replayed data as valid, effectively bypassing normal authentication checks.

How serious is CWE-294?

MITRE rates the likelihood of exploit as High — this weakness is actively exploited in the wild and should be prioritized for remediation.

What languages or platforms are affected by CWE-294?

MITRE has not specified affected platforms for this CWE — it can apply across most application stacks.

How can I prevent CWE-294?

Utilize some sequence or time stamping functionality along with a checksum which takes this into account in order to ensure that messages can be parsed only once. Since any attacker who can listen to traffic can see sequence numbers, it is necessary to sign messages with some kind of cryptography to ensure that sequence numbers are not simply doctored along with content.

How does Plexicus detect and fix CWE-294?

Plexicus's SAST engine matches the data-flow signature for CWE-294 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-294?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/294.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-294

CWE-1390 Parent

Weak Authentication

This vulnerability occurs when a system's login or identity verification process is too easy to bypass or fool. While it attempts to check…

CWE-1391 Sibling

Use of Weak Credentials

This vulnerability occurs when a system relies on weak authentication credentials—like default passwords, hard-coded keys, or easily…

CWE-262 Sibling

Not Using Password Aging

This vulnerability occurs when a system lacks password expiration policies, allowing users to keep the same password indefinitely.

CWE-263 Sibling

Password Aging with Long Expiration

The system enforces password changes, but the time allowed between changes is excessively long, weakening security.

CWE-289 Sibling

Authentication Bypass by Alternate Name

This vulnerability occurs when a system checks access based on a resource or user name, but fails to account for all the different names…

CWE-290 Sibling

Authentication Bypass by Spoofing

This weakness occurs when an application's authentication system can be tricked into accepting forged or manipulated credentials, allowing…

CWE-301 Sibling

Reflection Attack in an Authentication Protocol

A reflection attack is a flaw in mutual authentication protocols that allows an attacker to impersonate a legitimate user without knowing…

CWE-302 Sibling

Authentication Bypass by Assumed-Immutable Data

This vulnerability occurs when an authentication system incorrectly treats certain data as unchangeable, when in fact an attacker can…

CWE-303 Sibling

Incorrect Implementation of Authentication Algorithm

This weakness occurs when a developer implements a standard authentication algorithm, but makes critical mistakes in the code that cause…

Resources

Further reading

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