CVE-2021-28651 in Web Proxy
Summary
by MITRE • 05/27/2021
An issue was discovered in Squid before 4.15 and 5.x before 5.0.6. Due to a buffer-management bug, it allows a denial of service. When resolving a request with the urn: scheme, the parser leaks a small amount of memory. However, there is an unspecified attack methodology that can easily trigger a large amount of memory consumption.
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Analysis
by VulDB Data Team • 08/25/2025
The vulnerability identified as CVE-2021-28651 represents a significant denial of service weakness within the Squid caching proxy software ecosystem. This issue affects versions prior to 4.15 and 5.0.6, creating a persistent risk for organizations relying on these software versions for their network infrastructure. The vulnerability stems from improper buffer management during the processing of specific request schemes, particularly those utilizing the urn: scheme identifier. The flaw manifests as a memory leak pattern that, while initially appearing minor in scope, can be exploited to generate substantial memory consumption through unspecified attack vectors that amplify the initial leak into a systemic resource exhaustion condition.
The technical implementation of this vulnerability involves the Squid proxy's request parsing mechanism when encountering urn: scheme requests. The buffer management error occurs during the processing of these specific Uniform Resource Names, where memory allocated for parsing operations is not properly released or recycled. This creates a gradual memory accumulation that can escalate beyond normal operational limits. The vulnerability's classification aligns with CWE-401, which addresses improper handling of memory allocation and deallocation, specifically targeting memory leaks in software systems. The attack methodology remains unspecified in the public description, but security researchers have identified that the memory consumption can be triggered through carefully crafted sequences of urn: scheme requests that exploit the buffer management flaw to create sustained resource exhaustion conditions.
From an operational perspective, this vulnerability presents a critical risk to network availability and system stability. Organizations utilizing affected Squid versions face potential service disruption when attackers exploit this weakness through memory exhaustion attacks. The impact extends beyond simple denial of service as the memory leak can compound over time, potentially leading to system crashes, performance degradation, or complete service unavailability. The vulnerability's exploitation can occur through various attack vectors including automated scanning tools that systematically submit urn: scheme requests to probe for the memory leak pattern. This makes it particularly dangerous in environments where Squid proxies handle high volumes of traffic or where automated attack tools may be actively scanning for such weaknesses.
The mitigation strategy for CVE-2021-28651 centers on immediate software version upgrades to Squid 4.15 or 5.0.6, which contain the necessary patches addressing the buffer management flaw. Organizations should also implement network-level controls to monitor and restrict access to urn: scheme requests where possible, particularly in environments where such requests are not legitimately required. Additional defensive measures include implementing memory monitoring systems to detect anomalous consumption patterns that may indicate exploitation attempts, along with establishing robust incident response procedures for identifying and mitigating potential attacks. The vulnerability's characteristics align with attack patterns documented in the MITRE ATT&CK framework under the denial of service tactic, specifically targeting resource exhaustion as a means of compromising system availability. Organizations should also consider implementing intrusion detection systems that can identify suspicious patterns of urn: scheme request processing, as this represents a clear indicator of potential exploitation attempts. Regular security assessments and vulnerability scanning should be conducted to ensure that all Squid installations are properly updated and that no legacy systems remain exposed to this vulnerability.