diff --git a/Common/Data/Encoding/Compression.cpp b/Common/Data/Encoding/Compression.cpp
index c348ed9313..0ac584243d 100644
--- a/Common/Data/Encoding/Compression.cpp
+++ b/Common/Data/Encoding/Compression.cpp
@@ -54,7 +54,7 @@ bool compress_string(const std::string& str, std::string *dest, int compressionl
 }
 
 /** Decompress an STL string using zlib and return the original data. */
-bool decompress_string(const std::string& str, std::string *dest) {
+bool decompress_string(const std::string& str, std::string *dest, size_t maxSize) {
 	if (!str.size())
 		return false;
 
@@ -85,7 +85,11 @@ bool decompress_string(const std::string& str, std::string *dest) {
 			outstring.append(outbuffer,
 				zs.total_out - outstring.size());
 		}
-
+		if (outstring.size() > maxSize) {
+			inflateEnd(&zs);
+			ERROR_LOG(Log::IO, "Decompressed data is larger than %d bytes, giving up", (int)maxSize);
+			return false;
+		}
 	} while (ret == Z_OK);
 
 	inflateEnd(&zs);
@@ -95,6 +99,6 @@ bool decompress_string(const std::string& str, std::string *dest) {
 		return false;
 	}
 
-	*dest = outstring;
+	*dest = std::move(outstring);
 	return true;
 }
diff --git a/Common/Data/Encoding/Compression.h b/Common/Data/Encoding/Compression.h
index 5323b8f549..b66db3e16c 100644
--- a/Common/Data/Encoding/Compression.h
+++ b/Common/Data/Encoding/Compression.h
@@ -4,4 +4,5 @@
 
 // inflate/deflate convenience wrapper. Uses zlib.
 bool compress_string(const std::string& str, std::string *dest, int compressionlevel = 9);
-bool decompress_string(const std::string& str, std::string *dest);
+// Fails if the output would be larger than maxSize (a small input can expand enormously).
+bool decompress_string(const std::string& str, std::string *dest, size_t maxSize = 256 * 1024 * 1024);
diff --git a/Common/File/VFS/DirectoryReader.cpp b/Common/File/VFS/DirectoryReader.cpp
index 386568d70b..1b5b78a06d 100644
--- a/Common/File/VFS/DirectoryReader.cpp
+++ b/Common/File/VFS/DirectoryReader.cpp
@@ -24,13 +24,17 @@ static std::string ToLowerASCII(std::string_view str) {
 	return lower;
 }
 
-void DirectoryReader::IndexDirectory(const Path &dir, const std::string &prefix) {
+void DirectoryReader::IndexDirectory(const Path &dir, const std::string &prefix, int depth) {
+	// A symlink loop would otherwise recurse until the stack runs out.
+	if (depth > 32) {
+		return;
+	}
 	std::vector<File::FileInfo> files;
 	File::GetFilesInDir(dir, &files);
 	for (const File::FileInfo &file : files) {
 		const std::string relative = prefix + file.name;
 		if (file.isDirectory) {
-			IndexDirectory(file.fullName, relative + "/");
+			IndexDirectory(file.fullName, relative + "/", depth + 1);
 		} else {
 			lowerIndex_[ToLowerASCII(relative)] = relative;
 		}
@@ -44,7 +48,7 @@ bool DirectoryReader::FindIgnoringCase(std::string_view path, std::string *actua
 	std::lock_guard<std::mutex> guard(indexLock_);
 	if (!indexBuilt_) {
 		// Only done on the first miss, so a directory where every name matches never pays for it.
-		IndexDirectory(path_, "");
+		IndexDirectory(path_, "", 0);
 		indexBuilt_ = true;
 		INFO_LOG(Log::IO, "Indexed %d files in %s to find names that differ in case", (int)lowerIndex_.size(), path_.c_str());
 	}
@@ -134,13 +138,22 @@ void DirectoryReader::ReleaseFile(VFSFileReference *vfsReference) {
 
 VFSOpenFile *DirectoryReader::OpenFileForRead(VFSFileReference *vfsReference, size_t *size) {
 	DirectoryReaderFileReference *reference = (DirectoryReaderFileReference *)vfsReference;
+	// On some platforms, directories can be opened as files. Their size is meaningless.
+	if (File::IsDirectory(reference->path)) {
+		return nullptr;
+	}
 	FILE *file = File::OpenCFile(reference->path, "rb");
 	if (!file) {
 		return nullptr;
 	}
 	File::Fseek(file, 0, SEEK_END);
-	*size = File::Ftell(file);
+	const int64_t fileSize = File::Ftell(file);
 	File::Fseek(file, 0, SEEK_SET);
+	if (fileSize < 0) {
+		fclose(file);
+		return nullptr;
+	}
+	*size = (size_t)fileSize;
 	DirectoryReaderOpenFile *openFile = new DirectoryReaderOpenFile();
 	openFile->file = file;
 	return openFile;
diff --git a/Common/File/VFS/DirectoryReader.h b/Common/File/VFS/DirectoryReader.h
index b291e7610b..8e3e81c1ea 100644
--- a/Common/File/VFS/DirectoryReader.h
+++ b/Common/File/VFS/DirectoryReader.h
@@ -36,7 +36,7 @@ public:
 private:
 	// Returns the path (relative to path_) of a file whose name matches path except for case.
 	bool FindIgnoringCase(std::string_view path, std::string *actualPath);
-	void IndexDirectory(const Path &dir, const std::string &prefix);
+	void IndexDirectory(const Path &dir, const std::string &prefix, int depth);
 
 	Path path_;
 	bool ignoreCase_ = false;
diff --git a/Common/File/VFS/ZipFileReader.cpp b/Common/File/VFS/ZipFileReader.cpp
index 595083f0f3..7deade30d5 100644
--- a/Common/File/VFS/ZipFileReader.cpp
+++ b/Common/File/VFS/ZipFileReader.cpp
@@ -16,8 +16,10 @@
 #include "Common/StringUtils.h"
 
 // Sanity cap on a single zip entry's declared uncompressed size, so a corrupt or
-// malicious zip can't drive an implausible (or, near UINT64_MAX, wrapping) allocation.
-static constexpr uint64_t MAX_ZIP_ENTRY_SIZE = 1ULL << 32;  // 4GB, generous for any real asset.
+// malicious zip can't drive an implausible allocation (which also has to fit in a
+// 32-bit size_t). Entries are read whole into memory; the largest real asset, an
+// uncompressed 8192x8192 replacement texture with mips, is under 512MB.
+static constexpr uint64_t MAX_ZIP_ENTRY_SIZE = 512ULL * 1024 * 1024;
 
 ZipContainer::ZipContainer() noexcept : sourceData_(nullptr), zip_(nullptr) {}
 
@@ -186,8 +188,13 @@ uint8_t *ZipFileReader::ReadFile(std::string_view path, size_t *size) {
 		return 0;
 	}
 	uint8_t *contents = new uint8_t[zstat.size + 1];
-	zip_fread(file, contents, zstat.size);
+	const zip_int64_t bytesRead = zip_fread(file, contents, zstat.size);
 	zip_fclose(file);
+	if (bytesRead < 0 || (uint64_t)bytesRead != zstat.size) {
+		ERROR_LOG(Log::IO, "Error reading %s from ZIP (%lld of %llu bytes)", temp_path.c_str(), (long long)bytesRead, (unsigned long long)zstat.size);
+		delete[] contents;
+		return 0;
+	}
 	contents[zstat.size] = 0;
 
 	*size = zstat.size;
@@ -402,6 +409,13 @@ VFSOpenFile *ZipFileReader::OpenFileForRead(VFSFileReference *vfsReference, size
 		delete openFile;
 		return nullptr;
 	}
+	// Callers allocate the returned size.
+	if (zstat.size > MAX_ZIP_ENTRY_SIZE) {
+		ERROR_LOG(Log::IO, "Zip entry %d claims an implausible size (%llu), refusing to open", reference->zi, (unsigned long long)zstat.size);
+		lock_.unlock();
+		delete openFile;
+		return nullptr;
+	}
 
 	openFile->zf = zip_fopen_index(zip_file_, reference->zi, 0);
 	if (!openFile->zf) {
@@ -429,15 +443,20 @@ void ZipFileReader::Rewind(VFSOpenFile *vfsOpenFile) {
 size_t ZipFileReader::Read(VFSOpenFile *vfsOpenFile, void *buffer, size_t length) {
 	ZipFileReaderOpenFile *file = (ZipFileReaderOpenFile *)vfsOpenFile;
 	_assert_(file);
-	_dbg_assert_(file->zf != nullptr);
-	return zip_fread(file->zf, buffer, length);
+	if (!file->zf) {
+		return 0;
+	}
+	// -1 on errors, like a bad CRC or a corrupt stream.
+	const zip_int64_t bytesRead = zip_fread(file->zf, buffer, length);
+	return bytesRead < 0 ? 0 : (size_t)bytesRead;
 }
 
 void ZipFileReader::CloseFile(VFSOpenFile *vfsOpenFile) {
 	ZipFileReaderOpenFile *file = (ZipFileReaderOpenFile *)vfsOpenFile;
 	_assert_(file);
-	_dbg_assert_(file->zf != nullptr);
-	zip_fclose(file->zf);
+	if (file->zf) {
+		zip_fclose(file->zf);
+	}
 	file->zf = nullptr;
 	vfsOpenFile = nullptr;
 	lock_.unlock();
diff --git a/Core/FileLoaders/DiskCachingFileLoader.cpp b/Core/FileLoaders/DiskCachingFileLoader.cpp
index 2e81f191f6..c40910de9e 100644
--- a/Core/FileLoaders/DiskCachingFileLoader.cpp
+++ b/Core/FileLoaders/DiskCachingFileLoader.cpp
@@ -87,11 +87,15 @@ size_t DiskCachingFileLoader::ReadAt(s64 absolutePos, size_t bytes, void *data,
 	Prepare();
 	size_t readSize;
 
-	if (absolutePos >= filesize_) {
+	if (absolutePos < 0 || absolutePos >= filesize_) {
 		bytes = 0;
 	} else if (absolutePos + (s64)bytes >= filesize_) {
 		bytes = (size_t)(filesize_ - absolutePos);
 	}
+	// The cache computes the last block as pos + bytes - 1.
+	if (bytes == 0) {
+		return 0;
+	}
 
 	if (cache_ && cache_->IsValid() && (flags & Flags::HINT_UNCACHED) == 0) {
 		readSize = cache_->ReadFromCache(absolutePos, bytes, data);
@@ -357,6 +361,7 @@ bool DiskCachingFileLoaderCache::MakeCacheSpaceFor(size_t blocks) {
 
 	while (cacheSize_ > goal) {
 		u16 minGeneration = generation_;
+		bool evicted = false;
 
 		// We increment the iterator inside because we delete things inside.
 		for (size_t i = 0; i < blockIndexLookup_.size(); ++i) {
@@ -377,6 +382,7 @@ bool DiskCachingFileLoaderCache::MakeCacheSpaceFor(size_t blocks) {
 				info.generation = 0;
 				info.hits = 0;
 				--cacheSize_;
+				evicted = true;
 
 				// TODO: Doing this in chunks might be a lot better.
 				WriteIndexData(blockIndexLookup_[i], info);
@@ -389,6 +395,12 @@ bool DiskCachingFileLoaderCache::MakeCacheSpaceFor(size_t blocks) {
 			}
 		}
 
+		// If we found nothing to evict and the next pass would look for the same generation,
+		// it can't make progress either.
+		if (!evicted && minGeneration == oldestGeneration_) {
+			ERROR_LOG(Log::Loader, "Disk cache: Nothing left to evict, %d blocks counted", (int)cacheSize_);
+			break;
+		}
 		// If we didn't find any, update to the lowest we did find.
 		oldestGeneration_ = minGeneration;
 	}
@@ -548,6 +560,9 @@ bool DiskCachingFileLoaderCache::LoadCacheFile(const Path &path) {
 	} else if (header.maxBlocks < MAX_BLOCKS_LOWER_BOUND || header.maxBlocks > MAX_BLOCKS_UPPER_BOUND) {
 		// This means it's not in our safety bounds, reject.
 		valid = false;
+	} else if (header.blockSize != DEFAULT_BLOCK_SIZE) {
+		// We only ever create these, and everything below divides by and allocates the block size.
+		valid = false;
 	}
 
 	// If it's valid, retain the file pointer.
@@ -600,6 +615,12 @@ void DiskCachingFileLoaderCache::LoadCacheIndex() {
 		if (index_[i].block == INVALID_BLOCK) {
 			continue;
 		}
+		// Two entries can't share a data block. Keeping both would serve one of them the wrong
+		// data, and count a block that eviction can't find (which then never finishes).
+		if (blockIndexLookup_[index_[i].block] != INVALID_INDEX) {
+			index_[i].block = INVALID_BLOCK;
+			continue;
+		}
 
 		if (index_[i].generation < oldestGeneration_) {
 			oldestGeneration_ = index_[i].generation;
diff --git a/Core/FileLoaders/HTTPFileLoader.cpp b/Core/FileLoaders/HTTPFileLoader.cpp
index 8830994ee6..c41a9c80b6 100644
--- a/Core/FileLoaders/HTTPFileLoader.cpp
+++ b/Core/FileLoaders/HTTPFileLoader.cpp
@@ -89,6 +89,11 @@ void HTTPFileLoader::Prepare() {
 				}
 				if (size_pos != header.npos) {
 					filesize_ = atoll(&header[size_pos]);
+					// The caches size their indexes from this. Nothing we'd load is this big.
+					if (filesize_ < 0 || filesize_ > (64LL << 30)) {
+						ERROR_LOG(Log::Loader, "HTTP: Implausible Content-Length: %s", header.c_str());
+						filesize_ = 0;
+					}
 				}
 			}
 			if (startsWithNoCase(header, "Accept-Ranges:")) {
diff --git a/Core/FileLoaders/RamCachingFileLoader.cpp b/Core/FileLoaders/RamCachingFileLoader.cpp
index 2e402ff1d4..c50255e4c5 100644
--- a/Core/FileLoaders/RamCachingFileLoader.cpp
+++ b/Core/FileLoaders/RamCachingFileLoader.cpp
@@ -88,6 +88,11 @@ size_t RamCachingFileLoader::ReadAt(s64 absolutePos, size_t bytes, void *data, F
 
 void RamCachingFileLoader::InitCache() {
 	std::lock_guard<std::mutex> guard(blocksMutex_);
+	// The whole file has to fit in memory (and a size_t, which may be 32 bits).
+	if (filesize_ <= 0 || (u64)filesize_ > (u64)(SIZE_MAX - BLOCK_SIZE) || (u64)filesize_ > (8ULL << 30)) {
+		ERROR_LOG(Log::IO, "Can't cache a file of %lld bytes in RAM, will fall back to regular reads.", (long long)filesize_);
+		return;
+	}
 	u32 blockCount = (u32)((filesize_ + BLOCK_SIZE - 1) >> BLOCK_SHIFT);
 	// Overallocate for the last block.
 	cache_ = (u8 *)malloc((size_t)blockCount << BLOCK_SHIFT);
diff --git a/Core/FileLoaders/ZipFileLoader.cpp b/Core/FileLoaders/ZipFileLoader.cpp
index 57e12bcf77..8c6bf1899f 100644
--- a/Core/FileLoaders/ZipFileLoader.cpp
+++ b/Core/FileLoaders/ZipFileLoader.cpp
@@ -74,6 +74,12 @@ bool ZipFileLoader::Initialize(int fileIndex) {
 	}
 
 	_dbg_assert_(zstat.index == fileIndex);
+	// The whole file is decompressed into memory. No disc image is anywhere near 4GB, and the
+	// size has to fit in a (possibly 32-bit) size_t too.
+	if (!(zstat.valid & ZIP_STAT_SIZE) || zstat.size > (4ULL << 30) || zstat.size > (u64)SIZE_MAX) {
+		ERROR_LOG(Log::IO, "File index %d in ZIP has an implausible size (%llu)", fileIndex, (unsigned long long)zstat.size);
+		return false;
+	}
 	dataFileSize_ = zstat.size;
 	dataFile_ = zip_fopen_index(zipArchive_, zstat.index, ZIP_FL_UNCHANGED);
 	if (!dataFile_) {
diff --git a/Core/FileSystems/BlobFileSystem.cpp b/Core/FileSystems/BlobFileSystem.cpp
index 7139c2fcad..586fa87982 100644
--- a/Core/FileSystems/BlobFileSystem.cpp
+++ b/Core/FileSystems/BlobFileSystem.cpp
@@ -92,6 +92,10 @@ size_t BlobFileSystem::SeekFile(u32 handle, s32 position, FileMove type) {
 			entry->second = fileLoader_->FileSize() + position;
 			break;
 		}
+		// Reads take the position as a file offset.
+		if (entry->second < 0) {
+			entry->second = 0;
+		}
 		return (size_t)entry->second;
 	}
 	return 0;
diff --git a/Core/Loaders.cpp b/Core/Loaders.cpp
index ecf8ba6f5a..f76ea3c6cc 100644
--- a/Core/Loaders.cpp
+++ b/Core/Loaders.cpp
@@ -678,6 +678,10 @@ void DetectZipFileContents(zip_t *z, ZipFileInfo *info) {
 	// is quite convenient and makes it easy to add shared heuristics.
 	for (int i = 0; i < numFiles; i++) {
 		const char *fn = zip_get_name(z, i, 0);
+		if (!fn) {
+			// A corrupt central directory entry.
+			continue;
+		}
 
 		zip_stat_t stat{};
 		zip_stat_index(z, i, 0, &stat);
diff --git a/Core/Util/GameManager.cpp b/Core/Util/GameManager.cpp
index 3e7fbbe935..a29b1ddf70 100644
--- a/Core/Util/GameManager.cpp
+++ b/Core/Util/GameManager.cpp
@@ -912,7 +912,8 @@ bool GameManager::InstallZippedISO(struct zip *z, int isoFileIndex, const Path &
 		return false;
 	}
 	std::string fn = fnPtr;
-	size_t nameOffset = fn.rfind('/');
+	// Zips made on Windows can use either separator.
+	size_t nameOffset = fn.find_last_of("/\\");
 	if (nameOffset == std::string::npos) {
 		nameOffset = 0;
 	} else {
@@ -934,6 +935,13 @@ bool GameManager::InstallZippedISO(struct zip *z, int isoFileIndex, const Path &
 		name = name.substr(2);
 	}
 
+	// The name is used as a filename in the destination, so mustn't be able to point outside it.
+	if (!IsSafePathComponent(name)) {
+		ERROR_LOG(Log::IO, "Bad ISO filename in zip: '%s'", fn.c_str());
+		SetInstallError(I18NCat::INSTALLZIP, "Zip archive corrupt");
+		return false;
+	}
+
 	Path outputISOFilename = destDir;
 	if (outputISOFilename.empty()) {
 		outputISOFilename = Path(g_Config.currentDirectory);
@@ -952,8 +960,12 @@ bool GameManager::InstallZippedISO(struct zip *z, int isoFileIndex, const Path &
 
 	installProgress_ = 1.0f;
 	InstallDone();
-	ResetInstallError();
-	return true;
+	if (success) {
+		ResetInstallError();
+	} else {
+		SetInstallError(I18NCat::INSTALLZIP, "Installation failed");
+	}
+	return success;
 }
 
 bool GameManager::InstallZippedPS1Disc(struct zip *z, const std::vector<std::string> &files, const Path &destDir) {