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CVE-2026-109786

Quick assessment

Affected
fail2ban fail2ban
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

在 fail2ban 0.10.5 至 1.1.1（不含 ddd879d 提交之前版本）中，sshd 过滤器（config/filter.d/sshd.conf）允许在主机地址后附加“on <token>”后缀。由于 OpenSSH 会原样记录所提供的用户名，远程未认证攻击者可以构造如下形式的用户名：“x VICTIM-SOURCE-IP-ADDRESS on”，从而使 fail2ban 将失败的登录尝试关联到任意源 IP 地址，并对该 IP 实施封禁（例如，用于阻止合法用户的登录）。此漏洞影响默认的过滤器模式。

CVSS 5.3 · Medium EPSS 0.31% · P23

Affected Version Matrix 1

VendorProduct Version RangeStatus
fail2ban fail2ban 0.10.5≤ 1.1.1 affected
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I. Basic Information for CVE-2026-109786

Vulnerability Information
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Vulnerability Title
N/A
Source: CVE Program / CVE List V5
Vulnerability Description
In fail2ban 0.10.5 through 1.1.1 before ddd879d, the sshd filter (config/filter.d/sshd.conf) accepts an "on <token>" suffix after the host address. Because OpenSSH logs the offered username verbatim, a remote unauthenticated attacker can use a username such as "x VICTIM-SOURCE-IP-ADDRESS on" to make fail2ban associate failed login attempts with an arbitrary source IP address and ban it (e.g., to block logins from a legitimate user). The default filter mode is affected.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Source: CVE Program / CVE List V5
Vulnerability Type
日志输出的转义处理不恰当
Source: CVE Program / CVE List V5

Affected Products

Vendor Product Affected Versions CPE Subscribe
fail2ban fail2ban 0.10.5 ~ 1.1.1 -

II. Public POCs for CVE-2026-109786

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III. Intelligence Information for CVE-2026-109786

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Patches & Fixes for CVE-2026-109786 (1)

Other References for CVE-2026-109786 (1)

IV. Related Vulnerabilities

Same product: fail2ban
Same vendor: fail2ban
Same weakness: CWE-117

V. Comments for CVE-2026-109786

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