CVE-2026-107580 (CVSS 6.5)

Description

Inefficient algorithmic complexity in the decoding of message header fields in Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the IMAP, SMTP and POP3 services, or the webmail, unavailable by sending a message. The server unfolded a decoded header value by finding each line break from the end of the value and removing it, moving the rest of the value each time, so its work grew with the square of the number of line breaks, and an RFC 2047 encoded word may decode to any number of them. A received message whose Subject or other header field holds such a value keeps a worker thread busy for minutes or longer each time the value is read: when the recipient's IMAP client searches, sorts or threads the folder by a header, when the webmail lists the folder, and, where configured, when a rule tests a header, a spam tag is added to the Subject or an abuse report is read during delivery. The IMAP worker threads are shared with SMTP and POP3, so a few such messages stop those services responding. The server's reading of a message header from its file also searched everything read so far after each 4,000 bytes, costing seconds for a header of tens of megabytes.

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CVSS v3.1 Score

6.5

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:N/A:H

EPSS — Exploit Prediction

0.0026

Probability of exploitation

0.16%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-407 CWE-407

References

Other References

https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.6 https://gitlab.com/hmailserver/hmailserver/-/work_items/69

Frequently Asked Questions

What is CVE-2026-107580? +

Inefficient algorithmic complexity in the decoding of message header fields in Progressive Robot hMailServer 6.0.0 through 6.3.5 allows a remote unauthenticated attacker to make the IMAP, SMTP and POP3 services, or the webmail, unavailable by sending a message. The server unfolded a decoded header value by finding each line break from the end of the value and removing it, moving the rest of the value each time, so its work grew with the square of the number of line breaks, and an RFC 2047 encoded word may decode to any number of them. A received message whose Subject or other header field holds such a value keeps a worker thread busy for minutes or longer each time the value is read: when the recipient's IMAP client searches, sorts or threads the folder by a header, when the webmail lists the folder, and, where configured, when a rule tests a header, a spam tag is added to the Subject or an abuse report is read during delivery. The IMAP worker threads are shared with SMTP and POP3, so a few such messages stop those services responding. The server's reading of a message header from its file also searched everything read so far after each 4,000 bytes, costing seconds for a header of tens of megabytes. It has a CVSS v3.1 base score of 6.5 (MEDIUM).

How severe is CVE-2026-107580? +

CVE-2026-107580 has a CVSS v3.1 score of 6.5 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0026, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-107580? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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