CVE-2021-36159 in libfetch
Summary
by MITRE • 08/03/2021
libfetch before 2021-07-26, as used in apk-tools, xbps, and other products, mishandles numeric strings for the FTP and HTTP protocols. The FTP passive mode implementation allows an out-of-bounds read because strtol is used to parse the relevant numbers into address bytes. It does not check if the line ends prematurely. If it does, the for-loop condition checks for the '\0' terminator one byte too late.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 08/07/2021
The vulnerability CVE-2021-36159 represents a critical out-of-bounds read flaw in libfetch versions prior to 2021-07-26, affecting widely used package management tools including apk-tools and xbps. This vulnerability manifests specifically within the FTP passive mode implementation where numeric strings are parsed using the standard library function strtol for converting address bytes. The flaw occurs due to inadequate validation of input string termination, creating a scenario where the parsing logic attempts to access memory beyond the intended buffer boundaries. The root cause stems from the absence of proper bounds checking when processing FTP server responses that contain IP address information in numeric format.
The technical exploitation of this vulnerability leverages the improper handling of string termination in the FTP protocol implementation. When libfetch processes FTP passive mode responses, it parses numeric values representing IP address components using strtol without ensuring the input string properly terminates before the expected boundary. This oversight creates a condition where a for-loop iterates beyond the valid string length, attempting to read memory locations that may contain arbitrary data or trigger segmentation faults. The vulnerability specifically affects the parsing of address bytes during FTP data connection establishment, where the application expects a specific numeric format but fails to validate the completeness of the input string before memory access operations.
The operational impact of CVE-2021-36159 extends across multiple security-critical applications that rely on libfetch for network communications. Package managers such as apk-tools and xbps are particularly vulnerable since they frequently engage in FTP and HTTP protocol operations during package retrieval and installation processes. Attackers could potentially exploit this vulnerability to execute arbitrary code, cause denial of service conditions, or extract sensitive information from memory through controlled input manipulation. The vulnerability aligns with CWE-129, which addresses improper validation of array index values, and demonstrates characteristics consistent with ATT&CK technique T1190 for exploitation of remote services through protocol manipulation.
Mitigation strategies for CVE-2021-36159 require immediate patching of libfetch to version 2021-07-26 or later, which implements proper bounds checking and termination validation for numeric string parsing. System administrators should prioritize updating all affected package management tools and verify that the patched version properly validates input strings before memory access operations. Additional defensive measures include implementing network segmentation to limit FTP access, monitoring for suspicious FTP protocol interactions, and conducting regular vulnerability assessments of package management infrastructure. The fix addresses the underlying issue by ensuring that string parsing operations verify proper termination conditions and implement appropriate bounds checking before accessing memory locations, thereby preventing the out-of-bounds read condition that could be exploited by malicious actors.