Threaded Race Condition in Authentication Allows Bypass of Authentication Attempt Restrictions in zammad/zammad


Reported on

Jun 27th 2022


A threaded race condition exists in how the application handles authentication attempts in the application. The application recognizes and protects against single-threaded attempts with a five-attempt lockout function. By increasing threads in an authentication brute force attack it is possible to exponentially increase the number of allowed attempts without a lockout. Tester was able to increase attempts by a factor of 40 (5 attempts times 40 threads). Additional thread counts were inconsistent, but in one testing scenario the tester was able to find a valid authentication attempt at number 499 (at the highest).


Proof of Concept

Authentication Brute Force PoC - This is the Burp Intruder payload

POST /api/v1/signin HTTP/1.1
Host: localhost:8080
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:101.0) Gecko/20100101 Firefox/101.0
Accept: application/json, text/javascript, */*; q=0.01
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: http://localhost:8080/
Content-Type: application/json
X-Requested-With: XMLHttpRequest
X-CSRF-Token: jA18mk7hOGTpj+qgmFfs0z7Mx/EtYoeliFT9nphfSQ44r7+ETthl4fJMvNnp7HQeMYYYXd5Ws34mSdltjpvRtg==
Content-Length: 88
Origin: http://localhost:8080
DNT: 1
Connection: close
Cookie: _zammad_session_a138cfd0f37=d7a577940ea805561fe5f8ad93998bd3
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: cors
Sec-Fetch-Site: same-origin



The immediate impact is the possibility for an attacker to make up to 200 consistent attempts at authenticating to a user's account with an unknown password. A skilled attacker with a known user list and common password list would be able to make hundreds of attempts against the application. If an attacker can identify an account password in those 200 attempts per account, they would gain full access to the user account.

We are processing your report and will contact the zammad team within 24 hours. a month ago
Joe Helle modified the report
a month ago
Joe Helle modified the report
a month ago
Joe Helle modified the report
a month ago
Joe Helle
a month ago


@admin FYI on the second report. Can we consider CVE status for both please? Thank you.

zammad/zammad maintainer has acknowledged this report a month ago
Jamie Slome
a month ago


Of course :) Just for reference, @maintainer, you are able to opt-in to a CVE by adjusting the CVE to "Yes" at the top right of the report.

Joe Helle modified the report
a month ago
zammad/zammad maintainer
a month ago


@dievus we decided to try and fix the brute force prevention issue by proper DB synchronization rather than rack_attack.

To verify, would you be able to test the changes? Just exchange the valid? method in lib/auth.rb with the following code:

  def valid?
    validated = auth_user&.user&.with_lock do
      if auth_user.can_login? && backends.valid?
        auth_user.increase_login_failed if increase_login_failed_attempts

    avoid_brute_force_attack if !validated
    validated == true

That should prevent the issue (after server restart, of course) no matter how much concurrency is involved. I would highly appreciate your feedback on this.

Joe Helle
a month ago


Of course, @maintainer. I'll test it sometime today/tonight. In the meantime, can you validate both reports and apply CVEs to them? Thanks!

Joe Helle
a month ago


@maintainer, I've updated the code in the container appropriately. Anything after request #9, regardless of the threads, appears to be locked out. Anything up to request #9 is still making it through. This is consistent.

zammad/zammad maintainer validated this vulnerability a month ago
Joe Helle has been awarded the disclosure bounty
The fix bounty is now up for grabs
The researcher's credibility has increased: +7
We have sent a fix follow up to the zammad team. We will try again in 7 days. a month ago
zammad/zammad maintainer confirmed that a fix has been merged on 22cd7c a month ago
The fix bounty has been dropped
zammad/zammad maintainer gave praise a month ago
Dear Joe @researcher, today we released Zammad 5.2.1 with a fix for this issue. You can find our advisory with your credits at, CVE assignment is still pending. Thank you for your efforts and for handling this security issue in a responsible way! Best regards, Martin
The researcher's credibility has slightly increased as a result of the maintainer's thanks: +1
Joe Helle
a month ago


Thank you @maintainer. Please keep me updated with posting of the CVE.

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