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How CVE-2026-62641 Works
Roundcube Webmail incorporates a TNEF (Transport Neutral Encapsulation Format) decoder to process `winmail.dat` attachments commonly generated by Microsoft Outlook. This decoder includes a routine named `_decompressRTF` responsible for expanding compressed Rich Text Format (RTF) data embedded within TNEF structures. Prior to versions 1.6.17 and 1.7.2, this function suffers from a critical boundary-check omission that allows a remote attacker to mount a denial‑of‑service attack.
The vulnerable code resides in program/lib/Roundcube/rcube_tnef_decoder.php. The `_decompressRTF` method reads a compressed RTF stream and repeatedly copies data from the input buffer into an output buffer ($uncomp) based on control flags and length values extracted from the stream. The central loop advances an input pointer ($in) as it processes each token, but the loop’s termination condition is governed solely by a preloaded length field ($length_preload) derived from the compressed data. Critically, the code never compares the current `$in` offset against the actual byte length of the input string (strlen($data)).
By crafting a `winmail.dat` file with a deliberately forged or truncated compressed‑RTF size field, an attacker can cause the loop to continue reading far beyond the legitimate end of the input buffer. Because no bounds check exists, the loop will keep reading from the now‑invalid memory region and appending garbage data to the output buffer, leading to an infinite loop that consumes 100% CPU and continuously allocates memory. This resource exhaustion renders the Roundcube process unresponsive, and if the mail rendering thread is shared, the entire webmail service may become unavailable to all users.
The vulnerability is exploitable remotely simply by sending an email with the malicious TNEF attachment to any Roundcube user. When the recipient views the email (or when the system automatically processes the attachment for preview), the decoder is triggered, initiating the denial‑of‑service condition. No authentication or special privileges are required beyond the ability to deliver an email to the target mailbox. The CVSSv3.1 base score is 4.3 (Medium) with a vector string of CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:L, reflecting the low complexity, network attack vector, and requirement for user interaction (the victim must view the email). The fix, applied in commits `6d1004fd3764a9606c53130b322c0f295c38be64` (1.7.x) and `bf253c72d4293c93fda511b8464fe9cb34b522c1` (1.6.x), introduces a `$max_len = strlen($data)` check at the start of `_decompressRTF` and adds a break condition `if ($in >= $max_len) { break; }` after each decompression branch, ensuring the loop halts as soon as the input is exhausted.
DailyCVE Form
Platform: Roundcube Webmail
Version: <1.6.17, 1.7.x<1.7.2
Vulnerability: DoS via crafted RTF
Severity: Medium (4.3)
date: 2026-07-14
Prediction: 2026-07-14
Analytics
What Undercode Say
The following commands and code snippets help administrators identify vulnerable installations, test for the flaw, and understand the patched logic.
Check Roundcube version from the command line:
grep -E "^\$config['version']" /path/to/roundcube/config/config.inc.php
Locate the vulnerable `_decompressRTF` function in the source tree:
grep -n "_decompressRTF" /path/to/roundcube/program/lib/Roundcube/rcube_tnef_decoder.php
Extract the vulnerable loop condition (pre‑patch):
sed -n '/function _decompressRTF/,/^}/p' /path/to/roundcube/program/lib/Roundcube/rcube_tnef_decoder.php | grep -A5 -B5 "while"
Python script to generate a minimal malicious `winmail.dat` (proof‑of‑concept trigger):
import struct
Craft a TNEF attachment with a compressed RTF stream
that has a forged length field causing an infinite loop.
The actual data is minimal; the length field is set to 0xFFFFFFFF.
tnef_header = b"TNEF\0" + b"\x01" 10
rtf_length = 0xFFFFFFFF
compressed_rtf = struct.pack("<I", rtf_length) + b"\x00" 4 fake stream
winmail = tnef_header + compressed_rtf
with open("winmail.dat", "wb") as f:
f.write(winmail)
print("[+] Malicious winmail.dat created. Send this as an attachment to a Roundcube user.")
Verify the fix is applied by checking for the `$max_len` variable:
grep -n "\$max_len" /path/to/roundcube/program/lib/Roundcube/rcube_tnef_decoder.php
Exploit
An attacker exploits CVE‑2026‑62641 by sending an email to a Roundcube user with a `winmail.dat` attachment that contains a compressed RTF block whose declared size is artificially inflated or malformed. When the recipient opens the email (or when the mail client automatically renders a preview), Roundcube invokes the TNEF decoder. The `_decompressRTF` function begins processing the stream and, because it trusts the forged length field without validating it against the actual buffer size, enters an infinite loop. The loop continuously reads from the input pointer beyond the buffer limits, appending garbage to the output string and consuming CPU and memory until the PHP process is killed or the server runs out of resources. This results in a persistent denial of service that can only be mitigated by restarting the web server or the PHP‑FPM pool. The attack requires no authentication and can be launched from any email sender, making it a low‑effort, high‑impact vector for disrupting mail services.
Protection
– Upgrade Roundcube Webmail to version 1.6.17 or 1.7.2 (or later). These releases include the bounds‑checking patch in `_decompressRTF` that stops the loop when the input buffer is exhausted.
– If an immediate upgrade is not possible, apply the official patches manually by backporting the changes from commits `6d1004fd3764a9606c53130b322c0f295c38be64` (1.7.x) and `bf253c72d4293c93fda511b8464fe9cb34b522c1` (1.6.x) to the affected `rcube_tnef_decoder.php` file.
– As a temporary workaround, disable TNEF attachment processing by removing or renaming the `rcube_tnef_decoder.php` file (though this may break legitimate winmail.dat attachments).
– Use a Web Application Firewall (WAF) or email gateway to filter or block messages containing suspiciously large or malformed `winmail.dat` attachments.
– Monitor server resource usage (CPU/memory) for unusual spikes that could indicate an ongoing exploitation attempt.
Impact
Successful exploitation leads to a denial‑of‑service condition where the Roundcube process becomes stuck in an infinite loop, consuming 100% CPU and continuously allocating memory until the operating system kills the process or the server becomes unresponsive. This renders the webmail interface inaccessible to all users, disrupting business communication and potentially causing productivity loss. Because the attack can be triggered remotely by any sender without authentication, it poses a significant risk to public‑facing Roundcube installations. The CVSS score of 4.3 (Medium) reflects the moderate severity, but the ease of exploitation and the potential for service‑wide outages make this vulnerability a critical concern for system administrators.
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Sources:
Reported By: nvd.nist.gov
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