osslsigncode is a tool that implements Authenticode signing and timestamping. Prior to 2.13, an integer underflow vulnerability exists in osslsigncode version 2.12 and earlier in the PE page-hash computation code (pe_page_hash_calc()). When page hash processing is performed on a PE file, the function subtracts hdrsize from pagesize without first validating that pagesize >= hdrsize. If a malicious PE file sets SizeOfHeaders (hdrsize) larger than SectionAlignment (pagesize), the subtraction underflows and produces a very large unsigned length. The code allocates a zero-filled buffer of pagesize bytes and then attempts to hash pagesize - hdrsize bytes from that buffer. After the underflow, this results in an out-of-bounds read from the heap and can crash the process. The vulnerability can be triggered while signing a malicious PE file with page hashing enabled (-ph), or while verifying a malicious signed PE file that already contains page hashes. Verification of an already signed file does not require the verifier to pass -ph. This vulnerability is fixed in 2.13.