CWE-836 Base Incomplete

Use of Password Hash Instead of Password for Authentication

This vulnerability occurs when an application's authentication system accepts a password hash directly from the client for verification, instead of receiving and hashing the plaintext password on…

Definition

What is CWE-836?

This vulnerability occurs when an application's authentication system accepts a password hash directly from the client for verification, instead of receiving and hashing the plaintext password on the server.
Some systems are designed to have the client (like a web browser or mobile app) send a pre-computed password hash to the server, often with the intention of reducing server load or avoiding transmission of the raw password. However, this design fundamentally breaks the security model. Once an attacker steals a hash from the server's database—through methods like SQL injection or data exposure—they can simply submit that same hash using a modified client to impersonate the user. The server cannot distinguish between a legitimate client's hash and an attacker's replayed one. Consequently, authenticating with a client-supplied hash provides no more security than using plaintext passwords. The server loses the ability to apply its own cryptographic salts and work factors, and the hash effectively becomes the password itself. To be secure, the server must always receive the plaintext password, perform the hashing operation itself using a strong, salted algorithm, and compare the result to its stored hash.
Real-world impact

Real-world CVEs caused by CWE-836

  • CVE-2009-1283

    Product performs authentication with user-supplied password hashes that can be obtained from a separate SQL injection vulnerability (CVE-2009-1282).

  • CVE-2005-3435

    Product allows attackers to bypass authentication by obtaining the password hash for another user and specifying the hash in the pwd argument.

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-836

  • 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-836

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-836 and opens a fix PR in under 60 seconds.

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Frequently asked questions

Frequently asked questions

What is CWE-836?

This vulnerability occurs when an application's authentication system accepts a password hash directly from the client for verification, instead of receiving and hashing the plaintext password on the server.

How serious is CWE-836?

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-836?

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

How can I prevent CWE-836?

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-836?

Plexicus's SAST engine matches the data-flow signature for CWE-836 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-836?

MITRE publishes the canonical definition at https://cwe.mitre.org/data/definitions/836.html. You can also reference OWASP and NIST documentation for adjacent guidance.

Related weaknesses

Weaknesses related to CWE-836

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-294 Sibling

Authentication Bypass by Capture-replay

This vulnerability occurs when an attacker can intercept and record legitimate authentication traffic, then replay it later to gain…

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…

Resources

Further reading

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