CVE-2026-107569 (GCVE-0-2026-107569)

Vulnerability from cvelistv5 – Published: 2026-10-19 15:34 – Updated: 2026-10-19 15:34
VLAI
Title
Unchecked Input for Loop Condition in hMailServer
Summary
Unchecked input for a loop condition in Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to stop the server carrying mail. The server bounds the mail it generates in response to a message - a rule forward, reply or copy, an account forward, a Sieve redirect, vacation or notify, a bounce, an out-of-office reply - by a counter it carries in the message's X-hMailServer-LoopCount header, and makes no more once the counter reaches the configured rule loop limit (5 by default); that is what ends a forwarding or auto-reply cycle in a bounded number of hops. X-hMailServer-LoopCount is an ordinary header that is not stripped from mail arriving from outside, and the server read it with a signed conversion and no validation, so a count below zero passed every test against the limit: a sender who sets X-hMailServer-LoopCount to a negative value, or on the Linux builds to a value above the 32-bit signed range that the conversion wraps to a negative one, lifts the bound, and a message that enters a generated-mail cycle then loops up to about two thousand million times, each hop a queued message and a delivery, filling the queue and disk and stopping mail service. The Sieve notify action (from 6.2.22) consulted no limit at all, and its increment of the count could overflow a signed integer; and an account could reset the counter on its own mail through a Sieve editheader of the header (from 6.2.22) or a rule Set header value action (all versions) ahead of a forward or redirect.
Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CWE
  • CWE-606 - Unchecked Input for Loop Condition
Assigner
GitLab CNA under the mitre root
CNA scorecard C 75/100 over 311 records in the last 180 days details
References
Impacted products
Vendor Product Version CPE status
Progressive Robot Ltd hMailServer Affected: 6.0.0 , ≤ 6.3.6 (semver)
guessed Create a notification for this product.
Credits
Found in the hMailServer project's own security review (Progressive Robot Ltd)
Show details on NVD website
To clipboard 

{
  "containers": {
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        {
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          "product": "hMailServer",
          "repo": "https://gitlab.com/hmailserver/hmailserver",
          "vendor": "Progressive Robot Ltd",
          "versions": [
            {
              "lessThanOrEqual": "6.3.6",
              "status": "affected",
              "version": "6.0.0",
              "versionType": "semver"
            }
          ]
        }
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          "value": "Unchecked input for a loop condition in Progressive Robot hMailServer 6.0.0 through 6.3.6 allows a remote unauthenticated attacker to stop the server carrying mail. The server bounds the mail it generates in response to a message - a rule forward, reply or copy, an account forward, a Sieve redirect, vacation or notify, a bounce, an out-of-office reply - by a counter it carries in the message\u0027s X-hMailServer-LoopCount header, and makes no more once the counter reaches the configured rule loop limit (5 by default); that is what ends a forwarding or auto-reply cycle in a bounded number of hops. X-hMailServer-LoopCount is an ordinary header that is not stripped from mail arriving from outside, and the server read it with a signed conversion and no validation, so a count below zero passed every test against the limit: a sender who sets X-hMailServer-LoopCount to a negative value, or on the Linux builds to a value above the 32-bit signed range that the conversion wraps to a negative one, lifts the bound, and a message that enters a generated-mail cycle then loops up to about two thousand million times, each hop a queued message and a delivery, filling the queue and disk and stopping mail service. The Sieve notify action (from 6.2.22) consulted no limit at all, and its increment of the count could overflow a signed integer; and an account could reset the counter on its own mail through a Sieve editheader of the header (from 6.2.22) or a rule Set header value action (all versions) ahead of a forward or redirect."
        }
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            "baseScore": 7.5,
            "baseSeverity": "HIGH",
            "confidentialityImpact": "NONE",
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            "privilegesRequired": "NONE",
            "scope": "UNCHANGED",
            "userInteraction": "NONE",
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              "description": "CWE-606: Unchecked Input for Loop Condition",
              "lang": "en",
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        "shortName": "GitLab"
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          "url": "https://gitlab.com/hmailserver/hmailserver/-/work_items/81"
        },
        {
          "url": "https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.7"
        }
      ],
      "solutions": [
        {
          "lang": "en",
          "value": "Upgrade to hMailServer 6.3.7, which reads X-hMailServer-LoopCount in one validated place (a count only when it is digits and never above the limit; anything else is read as the limit so no generated mail is made from it), holds a Sieve notify to the same limit, and refuses an account Sieve script or rule that edits the counter, and writes a line break in a header value as spaces unless it folds the line, so that no value a generated message is built from writes a field of its own. Until then there is no setting that stops the header being read or that turns Sieve off: reduce forwarding and auto-reply arrangements that can feed one another, keep the Sieve script surfaces closed to accounts (ManageSieveServerPort=0, the default, and the REST API off where it is not needed, RestApiPort=0, the default), review active Sieve scripts and account rules for a notify of the account\u0027s own address and for an edit of X-hMailServer-LoopCount, and watch the queue for a message generated far past the loop limit."
        }
      ],
      "title": "Unchecked Input for Loop Condition in hMailServer"
    }
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  "cveMetadata": {
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    "datePublished": "2026-10-19T15:34:37.492Z",
    "dateReserved": "2026-10-08T10:51:10.642Z",
    "dateUpdated": "2026-10-19T15:34:37.492Z",
    "state": "PUBLISHED"
  },
  "dataType": "CVE_RECORD",
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                "vendor": "Progressive Robot Ltd",
                "versions": [
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                    "status": "affected",
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          }
        ],
        "id": "CVE-2026-107569",
        "lastModified": "2026-10-19T21:14:14.150",
        "metrics": {
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                "userInteraction": "NONE",
                "vectorString": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
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        "published": "2026-10-19T16:17:01.830",
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            "url": "https://gitlab.com/hmailserver/hmailserver/-/releases/v6.3.7"
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            "source": "cve@gitlab.com",
            "url": "https://gitlab.com/hmailserver/hmailserver/-/work_items/81"
          }
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              }
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          }
        ]
      }
    }
  }
}



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