Unrated

Heap overflow in rsyslog imhttp Basic Authentication parser

IdentifiersCVE-2026-55556CWE-122

CVE-2026-55556 is a heap overflow vulnerability in rsyslog’s optional contributed imhttp input module. The flaw is in the HTTP Basic Authentication parsing logic used by imhttp when decoding an oversized Basic Authentication header. The vulnerable code path used auth->pworkbuf = calloc(0, len); instead of allocating a buffer of len bytes, leading to incorrect heap allocation and subsequent heap memory corruption during processing of attacker-controlled input. The issue affects only deployments where rsyslog is built with the imhttp module, the module is installed, loaded, and configured, and Basic Authentication is enabled on the affected imhttp endpoint. The vulnerable code path was removed upstream in rsyslog 8.2604.0 as part of an authentication refactor in commit acde2ba25ea33816694b787859f4a727a247b6d6.

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ANALYST BRIEF

Impact, mitigation & remediation

What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.

Impact

What an attacker gets, and what they’ve been doing with it.

A remote attacker able to reach the affected imhttp listener can trigger heap memory corruption by sending a crafted oversized HTTP Basic Authentication header to an endpoint with Basic Authentication enabled. The expected practical impact is denial of service through process crash or instability. More severe impact, including potential code execution, cannot be ruled out categorically, but available information states that stronger outcomes depend on allocator behavior, compiler options, platform characteristics, and process hardening, and are not confirmed.

Mitigation

If you can’t patch tonight, do this now.

If immediate patching is not possible, disable or unload the imhttp module where it is not strictly required. If imhttp must remain enabled, disable HTTP Basic Authentication on affected endpoints if operationally feasible, and restrict network access to the imhttp listener to trusted sources only using firewalling, ACLs, reverse proxies, or bind-address restrictions. Because imhttp is non-default and uncommon, removing the module from builds or packages is also an effective mitigation where the feature is unnecessary.

Remediation

Patch, then assume compromise.

Upgrade to rsyslog 8.2604.0 or later, where the vulnerable authentication code path was removed as part of the imhttp authentication refactor. If backporting is required, apply the targeted fix in contrib/imhttp/imhttp.c that replaces calloc(0, len) with calloc(1, len), or otherwise incorporate the upstream changes associated with commit acde2ba25ea33816694b787859f4a727a247b6d6. Validate that any packaged or downstream builds including imhttp receive the fix.

PUBLIC EXPLOITS

Exploits

No public exploits tracked yet. Mallory keeps watching.

VALID 0 / 0 TOTALView more in app

No public exploit code observed for this vulnerability.

ACTIVITY FEED

Recent activity

2 sources tracked across advisories and community write-ups. News coverage will land here when it surfaces.

2 SOURCESView more in app

No news coverage yet. Advisories and community discussion only.

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Exposure mapping

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Threat actor evidence

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Associated malware

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Detection signatures

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Vendor-by-vendor mapping

Cross-references every affected SKU, including bundled OEM variants.

Social activity2