A severe global buffer overflow vulnerability has been discovered in the zlib compression library’s untgz utility, exposing systems to potential memory corruption and denial-of-service attacks.
The flaw, identified in zlib version 1.3.1.2, stems from an unbounded strcpy() operation that processes user-supplied archive names without proper validation.
The vulnerability resides in the TGZfname() function of the untgz utility, which copies command-line input directly into a fixed-size static global buffer of 1,024 bytes.
When an attacker supplies an archive name exceeding this limit, the function performs an out-of-bounds write, corrupting adjacent memory regions.
The critical aspect of this vulnerability is its trivial exploitability attackers can trigger the overflow simply by providing a maliciously crafted filename argument before any archive parsing or validation occurs.
Technical Analysis
Security researchers identified the root cause as the direct derivation of the archive name (arcname) from argv[] without implementing bounds checking.
The vulnerable buffer is allocated as a global static array rather than on the stack, so the overflow occurs immediately upon function entry.
AddressSanitizer (ASAN) testing confirmed the vulnerability, demonstrating that supplying 4,096 “A” characters triggers a 2,001-byte write at the buffer boundary.
The strcpy() call at line 136 of /root/zlib/contrib/untgz/untgz.c represents the point of exploitation.
ASAN output traces the vulnerability to the main function at line 638, confirming that the attack vector requires no authentication and no complex exploitation techniques.
The vulnerability poses multiple security risks depending on system configuration.
At a minimum, attackers can trigger denial-of-service conditions by crashing the untgz utility.
More concerning is the potential for memory corruption of adjacent global objects, which could persist beyond function scope and influence subsequent program behavior.
In specific compilation environments with particular compiler flags, architectures, and memory layouts, the flaw could potentially enable code execution.
Because the overflow affects global memory rather than the stack, the corruption may have broader consequences than typical buffer overflow vulnerabilities.
The undefined behavior caused by this flaw makes exploitation unpredictable across different system configurations.
Organizations using zlib’s untgz utility should implement immediate mitigations, including input validation and the use of alternative archive extraction tools until a patch becomes available.
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