Ghost in the Machine: Exploiting TOCTOU in Mattermost

Concurrency is the double-edged sword of modern software engineering. In Go—the language powering Mattermost—it's practically a religion. We love goroutines because they make our servers blazingly fast, handling thousands of concurrent connections with the grace of a ballet dancer. But where there is concurrency, there is chaos. And in that chaos, we find race conditions.

Mattermost is the go-to open-source alternative to Slack. It holds the secrets of organizations: engineering plans, HR complaints, and the lunch menu. Ideally, when an administrator hits the "Deactivate User" button, that user is nuked from orbit immediately. Their session should die, their sockets should close, and their API calls should return a harsh 403 Forbidden.

But CVE-2026-20796 reveals a crack in that logic. It turns out that if you ask the server for information at the exact right millisecond—specifically the /common_teams endpoint—the server might just nod, check your ID badge, watch the admin rip the badge off your chest, and then proceed to hand you the classified documents anyway. It's a classic Time-of-Check Time-of-Use (TOCTOU) vulnerability, and while the CVSS score is a lowly 3.1, the implications for logic flaws in high-concurrency systems are fascinating.

To understand this vulnerability, you have to think like a bureaucrat. Imagine you want to enter a secure vault. There is a guard at the entrance (The Check) and the vault door itself (The Use).

In a secure system, the guard walks you to the vault and holds your hand while you grab the gold. In a vulnerable TOCTOU system, the guard checks your ID at the front desk, says "You're good to go," and then goes back to reading his newspaper. You then have a 30-second walk to the vault. If your employment is terminated during that 30-second walk, the vault doesn't know. You already passed the check.

In Mattermost's case, the vulnerable endpoint is /api/v4/channels/{channel_id}/common_teams. This endpoint is designed to show which teams are common between a user and a channel. The logic flaw is a temporal gap:

1. Time of Check: The generic middleware or the handler entry point verifies, "Is User X active? Yes."
2. The Race Window: The runtime scheduler switches context, database latency occurs, or complex logic processes the request parameters.
3. State Change: An admin commits a transaction setting User.DeleteAt = NOW().
4. Time of Use: The handler proceeds to query the database for team names and returns them to the user, oblivious to the fact that the user is now a digital ghost.

This isn't a buffer overflow; it's a logic error born from the assumption that the state of the world remains constant for the duration of a request. In high-load systems, that assumption is fatal.

While the exact source code diff is often protected by vendor silence, we can reconstruct the vulnerability based on standard Go patterns and the advisory details. The vulnerability lies in the separation of the authorization from the query.

Here is a reconstruction of the vulnerable pattern:

func (a *API) getCommonTeams(c *Context, w http.ResponseWriter, r *http.Request) {
    // [1] CHECK: The Authorization Middleware has likely already passed.
    // Or, a manual check is performed here:
    user, err := a.App.GetUser(c.Session.UserId)
    if err != nil || user.DeleteAt != 0 {
        http.Error(w, "Unauthorized", http.StatusForbidden)
        return
    }
 
 
    // --- THE RACE WINDOW OPENS HERE ---
    // In a complex application, we might do some heavy lifting here.
    // Parsing params, loading channel metadata, or just Go runtime scheduling pauses.
    // If the database is under load, this gap widens.
 
 
    channelId := r.URL.Query().Get("channel_id")
    
    // [2] USE: The data retrieval happens here.
    // Notice we don't re-check the user's status inside the transaction
    // that fetches the teams.
    teams, err := a.App.GetCommonTeams(user.Id, channelId)
    
    if err != nil {
        // handle error
    }
 
 
    w.Write(json.Marshal(teams))
}

The Fix involves atomicity. You cannot trust a check performed at line 5 when you are executing line 15. The patch implemented in versions 11.3.0 and 10.11.10 likely pushes the validation into the query or wraps the entire operation in a transaction that locks the user state, ensuring that the GetCommonTeams query joins against the Users table to verify DeleteAt = 0 at the exact moment of retrieval.

Exploiting a race condition is like trying to run through a spinning fan without getting hit. You need precision, volume, and a bit of luck. Since the Attack Complexity is rated High (AC:H), we know this isn't trivial to reproduce manually. We need automation.

To weaponize this, we need to widen the race window or just fire enough bullets to statistically guarantee a hit. The scenario requires two actors: the Victim (the user being deactivated) and the Attacker (who controls the Victim's session token).

The Attack Chain:

1. Setup: The attacker logs in and obtains a valid session token (MMAUTHTOKEN).
2. The Trigger: The attacker knows (or guesses) that an admin acts on a schedule, or perhaps the attacker triggers a violation that causes an automated deactivation bot to kick in.
3. The Flood: Using a tool like Turbo Intruder (a Burp Suite extension) or a custom multi-threaded Python script, the attacker floods the /common_teams endpoint.

# Concept Exploit Script
def race_condition_request(session):
    # We want to hit the gap between Check and Use
    url = "https://mattermost.target.local/api/v4/channels/xyz/common_teams"
    headers = {"Authorization": "Bearer <token>"}
    resp = session.get(url, headers=headers)
    if resp.status_code == 200 and "confidential_team" in resp.text:
        print("[+] RACE WON: Retrieved data after deactivation!")

The goal is to have a request in flight where the authorization check passes at T+0, the deactivation happens at T+1, and the data fetch happens at T+2. If successful, the server returns the JSON containing team names even though the user is technically dead in the database.

Let's be realistic: CVSS 3.1 is not going to wake up the CISO at 3 AM. The impact here is strictly Information Disclosure. We aren't getting Remote Code Execution (RCE) or dropping the database tables.

However, in the world of corporate espionage and red teaming, "Low" severity is just a challenge, not a dead end. What is the actual data leaked? Team Names.

Imagine a scenario where a contractor is fired because they were suspected of being a mole for a competitor. As their access is cut, they run this race condition script. They might see a new team pop up in the common list named Project_Acquisition_CompetitorX or Layoffs_Q3_2026.

Sometimes, the existence of a project is as sensitive as the data inside it. This vulnerability allows a user to peek behind the curtain one last time as they are being thrown out the door. It undermines the fundamental security promise that "Revoke means Revoke Immediately."

The remediation is straightforward: Update. Mattermost has patched this in versions 11.3.0 and 10.11.10.

For developers reading this, the lesson is clear: Never trust state across async boundaries. If your authorization check happens in middleware, and your data fetch happens in a controller, and there is any blocking call in between, you have a race condition.

The robust fix usually involves:

1. Transactional Integrity: verifying the user's status within the same database transaction that fetches the data.
2. Re-validation: explicitly re-checking !user.IsDeactivated() immediately before writing the sensitive response to the wire.

If you are a Mattermost admin and cannot upgrade immediately, there are no clean configuration workarounds for a code-level race condition. Your best bet is to rely on WAF rules to detect API flooding, which would likely accompany any attempt to exploit this race window.