CVE-2026-111458 in ssldump
Summary
by MITRE • 10/16/2026
The fanx_fingerprint function in ssl/sslxprint.c of ssldump allocates a heap buffer for formatting FAN/1 X.509 certificate fingerprints. The allocation size is computed as strlen(b64) + strlen(hex) + 31, but the format string "fan1:x509:server:passive:%s:sha256:%s" contains 33 fixed characters in addition to the two variable-length fields and a null terminator, requiring strlen(b64) + strlen(hex) + 34 bytes. This miscalculation results in a 3-byte heap-based buffer overflow on every invocation of the function with valid inputs. The overflow occurs because sprintf writes the full formatted string into the undersized heap buffer without bounds checking. An attacker who can supply a crafted pcap file containing a TLS X.509 certificate to a user running ssldump with FAN/1 fingerprinting enabled (for example with the -j -N options) can trigger the overflow. The resulting heap corruption may lead to denial of service through a crash or, in a worst-case scenario, arbitrary code execution in the context of the user running ssldump. The vulnerability is deterministic and requires no special privileges beyond the ability to provide input to the tool.
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Analysis
by VulDB Data Team • 10/16/2026
The sslxprint.c module within the ssldump utility contains a critical heap-based buffer overflow vulnerability located specifically within the fanx_fingerprint function. This component is responsible for formatting fingerprints of FAN/1 X.509 certificates, which are used in certain TLS implementations to identify server identities during secure connections. The root cause of this flaw lies in an incorrect calculation of the required memory allocation size before a sprintf operation is executed. Specifically, the developer computed the buffer size as the sum of the base64 encoded string length, the hexadecimal string length, and thirty-one bytes for fixed formatting characters. However, the actual format string used by sprintf includes thirty-three fixed characters plus two variable-length fields and a null terminator, necessitating an allocation of strlen(b64) + strlen(hex) + 34 bytes. This discrepancy results in a three-byte undersized heap buffer that is insufficient to hold the complete formatted output.
When ssldump processes network traffic captured in a pcap file using options such as -j and -N to enable FAN/1 fingerprinting, it invokes this function for each relevant TLS handshake encountered. The sprintf call writes the fully expanded string into the undersized heap buffer without performing any bounds checking. Because C standard library functions like sprintf do not validate destination buffer sizes, they will write past the allocated memory boundary if the input data exceeds the pre-calculated limit. This leads to a deterministic three-byte overflow that corrupts adjacent heap metadata or neighboring objects in the memory allocator's structures. The vulnerability is consistent and reproducible whenever valid inputs are processed under these specific configuration conditions, making it highly reliable for exploitation purposes.
The operational impact of this heap corruption can vary significantly depending on the memory layout at runtime and the capabilities of an attacker who controls the input data. In many cases, the immediate consequence will be a denial of service manifested as a segmentation fault or application crash due to corrupted pointers or invalid free operations triggered by subsequent memory management routines. However, in more sophisticated scenarios involving heap exploitation techniques such as fastbin attacks or metadata corruption, this overflow can potentially allow an attacker to achieve arbitrary code execution within the context of the user running ssldump. Since ssldump is typically used for network analysis and debugging, it may be run with elevated privileges or against sensitive traffic, thereby increasing the potential severity of a successful exploitation attempt.
This vulnerability aligns with CWE-120, which classifies buffer copies without checking size limits as a classic cause of memory corruption issues. Furthermore, from an offensive security perspective, this flaw can be leveraged through techniques documented in the MITRE ATT&CK framework under command and control or initial access vectors where crafted network packets are used to trigger remote code execution on analysis tools. To mitigate this risk, immediate patching is required to correct the allocation size calculation by adding three additional bytes to account for the missing characters in the format string length estimation. Additionally, developers should replace unsafe functions like sprintf with safer alternatives such as snprintf that enforce buffer boundaries and prevent overflow conditions regardless of input data characteristics. Until a patched version is available, users are advised to avoid using ssldump with FAN/1 fingerprinting enabled when processing untrusted or potentially malicious pcap files containing TLS certificates.