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Vulnerability record · CVE-2026-55556 · published 18 September 2026

CVE-2026-55556: Heap-based buffer overflow vulnerability

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.

8.2 CVSS 4.0 High EPSS 0.85% · top 43.4% CWE-122 · Heap-based buffer overflow Awaiting Analysis
8.2CVSS 4.0 base score
0.85%EPSS exploitation probability, 30 days
NoNot in CISA KEV
0Affected product versions listed by NVD
6References
24 Sep 2026Last modified by NVD

Description

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.

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:

References

LinkTags
https://github.com/rsyslog/rsyslog/commit/acde2ba25ea33816694b787859f4a727a247b6d6
https://github.com/rsyslog/rsyslog/pull/6722
https://github.com/rsyslog/rsyslog/releases/tag/v8.2604.0
https://github.com/rsyslog/rsyslog/security/advisories/GHSA-947w-69ph-mc2r
http://www.openwall.com/lists/oss-security/2026/06/23/4
https://github.com/rsyslog/rsyslog/security/advisories/GHSA-947w-69ph-mc2r

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Source: NIST National Vulnerability Database (record CVE-2026-55556), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.