CVE-2019-12520 in Web Proxy
Summary
by MITRE
An issue was discovered in Squid through 4.7 and 5. When receiving a request, Squid checks its cache to see if it can serve up a response. It does this by making a MD5 hash of the absolute URL of the request. If found, it servers the request. The absolute URL can include the decoded UserInfo (username and password) for certain protocols. This decoded info is prepended to the domain. This allows an attacker to provide a username that has special characters to delimit the domain, and treat the rest of the URL as a path or query string. An attacker could first make a request to their domain using an encoded username, then when a request for the target domain comes in that decodes to the exact URL, it will serve the attacker's HTML instead of the real HTML. On Squid servers that also act as reverse proxies, this allows an attacker to gain access to features that only reverse proxies can use, such as ESI.
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Analysis
by VulDB Data Team • 05/26/2024
The vulnerability described in CVE-2019-12520 represents a critical cache poisoning issue within Squid proxy servers affecting versions through 4.7 and 5.0. This flaw stems from how Squid processes and hashes absolute URLs during cache lookups, creating an avenue for attackers to manipulate cached content through carefully crafted user information components. The vulnerability specifically targets the handling of UserInfo elements within URLs, where Squid decodes username and password information and prepends it to the domain portion of the URL. This behavior creates a parsing inconsistency that attackers can exploit by crafting usernames containing special characters that serve as delimiters for domain boundaries. The technical implementation involves Squid's MD5 hashing mechanism which processes the complete absolute URL including decoded UserInfo, making the cache key susceptible to manipulation through strategic character insertion.
The operational impact of this vulnerability extends beyond simple content substitution to potentially enable advanced attack vectors within environments where Squid serves as a reverse proxy. When an attacker successfully manipulates the cache using this technique, they can serve malicious HTML content in place of legitimate responses, effectively performing cache poisoning attacks that can persist until the cache expires or is manually cleared. The vulnerability becomes particularly dangerous in reverse proxy configurations where the attacker can gain access to privileged features such as Edge Side Includes (ESI) processing capabilities that are normally restricted to legitimate reverse proxy operations. This allows attackers to potentially execute arbitrary code or access sensitive functionality that should be protected within the proxy infrastructure. The attack requires careful crafting of the initial request with encoded credentials that, when decoded, create the malicious URL structure, making the exploitation both subtle and potentially difficult to detect through normal monitoring.
The underlying technical flaw aligns with CWE-129 and CWE-134 categories, representing issues related to input validation and improper handling of user-provided data within security-critical components. The vulnerability demonstrates characteristics consistent with the ATT&CK technique T1190 - Proxying, where attackers leverage proxy servers to mask their activities while gaining access to restricted resources. The attack pattern follows a specific methodology where the attacker first establishes a cache entry with a crafted URL structure, then waits for legitimate requests that match their manipulated cache key. This vulnerability highlights the importance of proper URL parsing and normalization within proxy server implementations, particularly when dealing with encoded and decoded URL components. The issue also reflects broader concerns around cache integrity and the potential for attackers to manipulate caching mechanisms through carefully constructed inputs, making it a significant concern for organizations relying on Squid as a core infrastructure component. Organizations should implement immediate mitigations including updating to patched versions, implementing stricter URL validation, and monitoring for anomalous cache behavior that might indicate successful exploitation attempts.