---
id: CVE-2026-55556
title: Rsyslog is a rocket-fast system for log processing
summary: >-
  Rsyslog is a rocket-fast system for log processing. From 8.2110.0 until
  8.2604.0, the optional imhttp module's parse_auth_header function in
  contrib/imhttp/imhttp.c allocates a zero-byte heap buffer with calloc(0, len)
  when an HTTP Basic…
severity: high
cvss: 8.2
cvssVector: 'CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N'
cwe:
  - CWE-122
  - CWE-131
vendor: rsyslog
product: rsyslog
affected:
  - 'rsyslog >= 8.2110.0, < 8.2604.0'
published: '2026-09-18'
updated: '2026-09-24'
sourceUpdated: '2026-09-24T21:22:19.873'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2026-55556'
references:
  - url: >-
      https://github.com/rsyslog/rsyslog/commit/acde2ba25ea33816694b787859f4a727a247b6d6
    label: security-advisories@github.com
  - url: 'https://github.com/rsyslog/rsyslog/pull/6722'
    label: security-advisories@github.com
  - url: 'https://github.com/rsyslog/rsyslog/releases/tag/v8.2604.0'
    label: security-advisories@github.com
  - url: 'https://github.com/rsyslog/rsyslog/security/advisories/GHSA-947w-69ph-mc2r'
    label: security-advisories@github.com
  - url: 'http://www.openwall.com/lists/oss-security/2026/06/23/4'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://github.com/rsyslog/rsyslog/security/advisories/GHSA-947w-69ph-mc2r'
    label: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  - url: >-
      https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-55556.json
  - url: 'https://access.redhat.com/security/cve/CVE-2026-55556'
  - url: 'https://bugzilla.redhat.com/show_bug.cgi?id=2538463'
  - url: 'https://www.cve.org/CVERecord?id=CVE-2026-55556'
  - url: 'https://nvd.nist.gov/vuln/detail/CVE-2026-55556'
tags:
  - nvd
  - cve.org
  - exploit-available
  - csaf
  - vex
  - red-hat
epss: 0.00853
epssPercentile: 0.56503
exploitAvailable: true
ssvc:
  exploitation: poc
  automatable: 'yes'
  technicalImpact: partial
  timestamp: '2026-09-22T15:04:09.398563Z'
cvssSource: cna
scores:
  cna: 8.2
  vendor: 7.5
ingestedAt: '2026-09-18T17:46:41.520Z'
---

## Overview

Rsyslog is a rocket-fast system for log processing. From 8.2110.0 until 8.2604.0, the optional imhttp module's parse_auth_header function in contrib/imhttp/imhttp.c allocates a zero-byte heap buffer with calloc(0, len) when an HTTP Basic Authorization value exceeds its fixed work buffer, then passes that pointer to apr_base64_decode. An unauthenticated remote attacker can send an oversized encoded credential to an imhttp endpoint configured for Basic Authentication, causing decoded data to overwrite adjacent heap memory before credential validation. Deployments that do not install, load, and use imhttp with Basic Authentication are not affected. The demonstrated impact is a process crash that interrupts log collection, and code execution has not been demonstrated. This issue is fixed in version 8.2604.0.

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.

## Vendor advisories

- **Red Hat VEX** · Important · affected: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9 · no fix planned: Red Hat Enterprise Linux 10, Red Hat Enterprise Linux 9 · updated 2026-09-24 · [vex](https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-55556.json)