CVE-2011-4096 in Squid
Summary
by MITRE
The idnsGrokReply function in Squid before 3.1.16 does not properly free memory, which allows remote attackers to cause a denial of service (daemon abort) via a DNS reply containing a CNAME record that references another CNAME record that contains an empty A record.
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Analysis
by VulDB Data Team • 08/07/2024
The vulnerability identified as CVE-2011-4096 represents a critical memory management flaw within the Squid caching proxy software that affects versions prior to 3.1.16. This issue resides in the idnsGrokReply function which processes DNS responses from authoritative servers. The flaw manifests when the DNS resolver encounters a specific chain of CNAME records where the final record contains an empty A record, creating a scenario that leads to improper memory deallocation within the Squid daemon.
The technical implementation of this vulnerability stems from insufficient memory cleanup procedures during DNS response parsing. When Squid receives a DNS reply containing nested CNAME records followed by an empty A record, the idnsGrokReply function fails to properly release allocated memory blocks. This memory leak occurs because the function does not adequately handle the reference counting or deallocation logic for DNS response structures when processing the particular chain of record references. The vulnerability specifically exploits the interaction between CNAME and A record processing within the DNS resolution subsystem, where the empty A record triggers a code path that bypasses normal memory deallocation routines.
The operational impact of this vulnerability is significant as it enables remote attackers to execute a denial of service attack against Squid proxy servers. An attacker can craft malicious DNS responses that contain the specific CNAME chain pattern, causing the Squid daemon to consume increasing amounts of memory until it eventually crashes or aborts. This results in complete service disruption for legitimate users who rely on the proxy server for internet connectivity and content caching. The attack requires only a single malicious DNS reply to be processed by the vulnerable Squid instance, making it particularly dangerous in environments where the proxy handles high volumes of DNS queries.
This vulnerability maps to CWE-401: Improper Release of Memory Before Removing Last Reference, which classifies memory management flaws that occur when software fails to properly release allocated memory resources. The attack pattern aligns with ATT&CK technique T1499.004: Endpoint Denial of Service, specifically targeting network services through memory exhaustion. The flaw demonstrates poor input validation and resource management practices within the DNS resolution component of Squid, where the software does not adequately sanitize or properly handle edge cases in DNS response structures. Organizations running vulnerable Squid versions face potential service interruptions that can affect thousands of concurrent users depending on the proxy infrastructure.
The recommended mitigation strategy involves upgrading to Squid version 3.1.16 or later, which contains the necessary patches to properly handle the memory deallocation for nested CNAME records. Administrators should also implement DNS response validation measures and consider deploying network monitoring tools to detect unusual DNS traffic patterns that might indicate exploitation attempts. Additionally, organizations should maintain regular patch management schedules and conduct vulnerability assessments to identify other potential memory management issues within their proxy infrastructure. The fix addresses the root cause by implementing proper reference counting and deallocation logic for DNS response structures, ensuring that all allocated memory is properly released regardless of the DNS record chain encountered during processing.