CWE-208 Base Incomplet

Observable Timing Discrepancy

Définition

What is CWE-208?

This vulnerability occurs when an application takes measurably different amounts of time to perform different operations, such as checking a password or processing a request. An attacker can observe these timing differences to learn sensitive information, like whether a username is valid or a cryptographic key guess is correct.

Timing discrepancies act as a side channel, leaking information through the back door of performance. Even tiny, millisecond differences in response times can be statistically analyzed by an attacker to map out internal application logic, bypassing intended security controls. This is especially dangerous in authentication, authorization, and cryptographic functions where a 'fast fail' for an incorrect input can reveal its validity. To exploit this, attackers don't need direct access to error messages or data—they simply measure how long operations take. For example, a string comparison that stops at the first mismatched character will return faster for a wrong password starting with an incorrect letter than for one starting with the correct letter. Over many requests, this allows an attacker to gradually infer secrets, piece by piece, by observing which operations take longer to complete.

Impact réel

Real-world CVEs caused by CWE-208

CVE-2019-10071

Java-oriented framework compares HMAC signatures using String.equals() instead of a constant-time algorithm, causing timing discrepancies
CVE-2019-10482

Smartphone OS uses comparison functions that are not in constant time, allowing side channels
CVE-2014-0984

Password-checking function in router terminates validation of a password entry when it encounters the first incorrect character, which allows remote attackers to obtain passwords via a brute-force attack that relies on timing differences in responses to incorrect password guesses, aka a timing side-channel attack.
CVE-2003-0078

SSL implementation does not perform a MAC computation if an incorrect block cipher padding is used, which causes an information leak (timing discrepancy) that may make it easier to launch cryptographic attacks that rely on distinguishing between padding and MAC verification errors, possibly leading to extraction of the original plaintext, aka the "Vaudenay timing attack."
CVE-2000-1117

Virtual machine allows malicious web site operators to determine the existence of files on the client by measuring delays in the execution of the getSystemResource method.
CVE-2003-0637

Product uses a shorter timeout for a non-existent user than a valid user, which makes it easier for remote attackers to guess usernames and conduct brute force password guessing.
CVE-2003-0190

Product immediately sends an error message when a user does not exist, which allows remote attackers to determine valid usernames via a timing attack.
CVE-2004-1602

FTP server responds in a different amount of time when a given username exists, which allows remote attackers to identify valid usernames by timing the server response.

Comment les attaquants l'exploitent

Parcours de l'attaquant étape par étape

1
Consider an example hardware module that checks a user-provided password to grant access to a user. The user-provided password is compared against a golden value in a byte-by-byte manner.
2
Since the code breaks on an incorrect entry of password, an attacker can guess the correct password for that byte-check iteration with few repeat attempts.
3
To fix this weakness, either the comparison of the entire string should be done all at once, or the attacker is not given an indication whether pass or fail happened by allowing the comparison to run through all bits before the grant_access signal is set.
4
In this example, the attacker observes how long an authentication takes when the user types in the correct password.
5
When the attacker tries their own values, they can first try strings of various length. When they find a string of the right length, the computation will take a bit longer, because the for loop will run at least once. Additionally, with this code, the attacker can possibly learn one character of the password at a time, because when they guess the first character right, the computation will take longer than a wrong guesses. Such an attack can break even the most sophisticated password with a few hundred guesses.

Exemple de code vulnérable

Vulnerable Verilog

Consider an example hardware module that checks a user-provided password to grant access to a user. The user-provided password is compared against a golden value in a byte-by-byte manner.

Vulnérable Verilog

always_comb @ (posedge clk)

 begin

```
   assign check_pass[3:0] = 4'b0;
   for (i = 0; i < 4; i++) begin
  	 if (entered_pass[(i*8 - 1) : i] eq golden_pass([i*8 - 1) : i])
  		 assign check_pass[i] = 1;
  		 continue;
  	 else
  		 assign check_pass[i] = 0;
  		 break;
  	 end
   assign grant_access = (check_pass == 4'b1111) ? 1'b1: 1'b0;
 end

Exemple de code sécurisé

Secure Verilog

To fix this weakness, either the comparison of the entire string should be done all at once, or the attacker is not given an indication whether pass or fail happened by allowing the comparison to run through all bits before the grant_access signal is set.

Sécurisé Verilog

always_comb @ (posedge clk)
 begin

```
   assign check_pass[3:0] = 4'b0;
   for (i = 0; i < 4; i++) begin
  	 if (entered_pass[(i*8 - 1) : i] eq golden_pass([i*8 -1) : i])
  		 assign check_pass[i] = 1;
  		 continue;
  	 else
  		 assign check_pass[i] = 0;
  		 continue;
  	 end
   assign grant_access = (check_pass == 4'b1111) ? 1'b1: 1'b0;
 end

What changed: the unsafe sink is replaced (or the input is validated/escaped) so the same payload no longer triggers the weakness.

Liste de contrôle de prévention

How to prevent CWE-208

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.

Signaux de détection

How to detect CWE-208

SAST High

Exécuter une analyse statique (SAST) sur le code source à la recherche du motif non sécurisé dans le flux de données.

DAST Moderate

Exécuter des tests de sécurité applicative dynamique (DAST) contre le point de terminaison en ligne.

Runtime Moderate

Surveiller les journaux runtime pour détecter des traces d'exception inhabituelles, des entrées malformées ou des tentatives de contournement d'autorisation.

Code review Moderate

Revue de code : signaler tout nouveau code qui traite les entrées de cette surface sans utiliser les helpers du framework validés.

Correction automatique Plexicus

Plexicus détecte automatiquement CWE-208 et ouvre une PR de correction en moins de 60 secondes.

Codex Remedium analyse chaque commit, identifie cette faiblesse précise et livre une pull request prête à être relue avec le correctif. Pas de tickets. Pas de transferts.

Obtenir une démo Essayer Plexicus gratuitement

Questions fréquentes

Frequently asked questions

Qu'est-ce que CWE-208 ?

Quelle est la gravité de CWE-208 ?

MITRE n'a pas publié de note de probabilité d'exploitation pour cette faiblesse. Traitez-la comme un impact moyen jusqu'à ce que votre modèle de menace prouve le contraire.

Quels langages ou plateformes sont affectés par CWE-208 ?

MITRE n'a pas spécifié les plateformes affectées pour ce CWE — il peut s'appliquer à la plupart des stacks applicatives.

Comment puis-je prévenir CWE-208 ?

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.

Comment Plexicus détecte et corrige CWE-208 ?

Le moteur SAST de Plexicus reconnaît la signature de flux de données de CWE-208 à chaque commit. Lorsqu'une correspondance est trouvée, notre agent Codex Remedium ouvre une PR de correction avec le code corrigé, les tests et un résumé d'une ligne pour le relecteur.

Où puis-je en savoir plus sur CWE-208 ?

MITRE publie la définition canonique à https://cwe.mitre.org/data/definitions/208.html. Vous pouvez également consulter la documentation OWASP et NIST pour des conseils adjacents.

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