CVE-2017-7443 in apt-cacher
Summary
by MITRE
apt-cacher before 1.7.15 and apt-cacher-ng before 3.4 allow HTTP response splitting via encoded newline characters, related to lack of blocking for the %0[ad] regular expression.
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Analysis
by VulDB Data Team • 08/26/2020
The vulnerability identified as CVE-2017-7443 affects apt-cacher and apt-cacher-ng proxy software versions prior to 1.7.15 and 3.4 respectively, representing a critical HTTP response splitting flaw that exploits the absence of proper input validation for encoded newline characters. This vulnerability resides in the proxy server's handling of HTTP requests and responses, where maliciously crafted URLs containing encoded newline sequences can manipulate the HTTP protocol flow and potentially lead to various security consequences including cache poisoning and cross-site scripting attacks.
The technical implementation of this vulnerability stems from the software's insufficient sanitization of user input, specifically failing to properly filter or reject URL parameters containing %0a or %0d sequences that represent newline characters in URL encoding. When these encoded sequences are processed by the proxy server, they can be interpreted as legitimate HTTP header terminators, allowing attackers to inject additional HTTP headers or manipulate response content. The flaw is categorized under CWE-113, which describes improper neutralization of CRLF (Carriage Return Line Feed) sequences in HTTP headers, making it particularly dangerous in proxy environments where HTTP responses are manipulated and cached.
The operational impact of this vulnerability extends beyond simple proxy functionality, as it can enable attackers to perform cache poisoning attacks by injecting malicious content into the proxy cache, potentially affecting all downstream users. Additionally, the vulnerability can be leveraged to conduct cross-site scripting attacks if the proxy is used in web application contexts, or to bypass security controls by injecting malicious headers that could be interpreted by web applications or browsers in unintended ways. The attack vector is particularly insidious because it can be executed through seemingly benign URL parameters that are commonly found in web applications and automated tools.
Organizations utilizing affected versions of apt-cacher or apt-cacher-ng should immediately implement mitigations including upgrading to patched versions, implementing strict input validation on all proxy requests, and deploying additional network monitoring to detect anomalous HTTP header patterns. The ATT&CK framework categorizes this vulnerability under T1071.004 for application layer protocol: HTTP, with potential connections to T1566 for credential access through manipulation of proxy configurations. Network administrators should also consider implementing web application firewalls or proxy security controls that can detect and block encoded newline sequences in HTTP requests, particularly in environments where these proxy services are used to cache or forward web content.