diff --git a/include/linkdefs.hpp b/include/linkdefs.hpp
index 9ffc944a..67a32de1 100644
--- a/include/linkdefs.hpp
+++ b/include/linkdefs.hpp
@@ -120,7 +120,7 @@ enum SectionModifier { SECTION_NORMAL, SECTION_UNION, SECTION_FRAGMENT };
 
 extern char const * const sectionModNames[];
 
-enum ExportLevel { SYMTYPE_LOCAL, SYMTYPE_IMPORT, SYMTYPE_EXPORT };
+enum ExportLevel { SYMTYPE_LOCAL, SYMTYPE_IMPORT, SYMTYPE_EXPORT, SYMTYPE_INVALID };
 
 enum PatchType {
 	PATCHTYPE_BYTE,
diff --git a/src/link/assign.cpp b/src/link/assign.cpp
index 7ccadc67..6211a6b9 100644
--- a/src/link/assign.cpp
+++ b/src/link/assign.cpp
@@ -119,6 +119,15 @@ static std::optional<size_t> getPlacement(Section const &section, MemoryLocation
 	SectionTypeInfo const &typeInfo = sectionTypeInfo[section.type];
 
 	for (;;) {
+		if (location.bank < typeInfo.firstBank
+		    || location.bank >= memory[section.type].size() + typeInfo.firstBank) {
+			fatal(
+			    "Invalid bank for %s section: 0x%02x",
+			    sectionTypeInfo[section.type].name.c_str(),
+			    location.bank
+			);
+		}
+
 		// Switch to the beginning of the next bank
 		std::deque<FreeSpace> &bankMem = memory[section.type][location.bank - typeInfo.firstBank];
 		size_t spaceIdx = 0;
diff --git a/src/link/object.cpp b/src/link/object.cpp
index 859b2cb8..490b3e6a 100644
--- a/src/link/object.cpp
+++ b/src/link/object.cpp
@@ -72,7 +72,7 @@ static int64_t readLong(FILE *file) {
 	tryRead(readLong, int64_t, INT64_MAX, long, var, file, __VA_ARGS__)
 
 // Helper macro to read a byte from a file to a var, or error out if it fails to.
-#define tryGetc(type, var, file, ...) tryRead(getc, int, EOF, type, var, file, __VA_ARGS__)
+#define tryGetc(var, file, ...) tryRead(getc, int, EOF, uint8_t, var, file, __VA_ARGS__)
 
 // Helper macro to read a '\0'-terminated string from a file, or error out if it fails to.
 #define tryReadString(var, file, ...) \
@@ -100,27 +100,31 @@ static void readFileStackNode(
 	tryReadLong(
 	    parentID, file, "%s: Cannot read node #%" PRIu32 "'s parent ID: %s", fileName, nodeID
 	);
-	node.parent = parentID != UINT32_MAX ? &fileNodes[parentID] : nullptr;
+	if (parentID == UINT32_MAX) {
+		node.parent = nullptr;
+	} else if (parentID >= fileNodes.size()) {
+		fatal("%s: Node #%" PRIu32 " has invalid parent ID #%" PRIu32, fileName, nodeID, parentID);
+	} else {
+		node.parent = &fileNodes[parentID];
+	}
 
 	tryReadLong(
 	    node.lineNo, file, "%s: Cannot read node #%" PRIu32 "'s line number: %s", fileName, nodeID
 	);
 
 	uint8_t type;
-	tryGetc(uint8_t, type, file, "%s: Cannot read node #%" PRIu32 "'s type: %s", fileName, nodeID);
-	node.type = static_cast<FileStackNodeType>(type & ~(1 << FSTACKNODE_QUIET_BIT));
-	node.isQuiet = (type & (1 << FSTACKNODE_QUIET_BIT)) != 0;
-
-	switch (node.type) {
+	tryGetc(type, file, "%s: Cannot read node #%" PRIu32 "'s type: %s", fileName, nodeID);
+	switch (type & ~(1 << FSTACKNODE_QUIET_BIT)) {
 	case NODE_FILE:
 	case NODE_MACRO:
+		node.type = FileStackNodeType(type);
 		node.data = "";
 		tryReadString(
 		    node.name(), file, "%s: Cannot read node #%" PRIu32 "'s file name: %s", fileName, nodeID
 		);
 		break;
-
 	case NODE_REPT: {
+		node.type = NODE_REPT;
 		uint32_t depth;
 		tryReadLong(
 		    depth, file, "%s: Cannot read node #%" PRIu32 "'s REPT depth: %s", fileName, nodeID
@@ -143,8 +147,13 @@ static void readFileStackNode(
 			    nodeID
 			);
 		}
+		break;
 	}
+	default:
+		fatal("%s: Node #%" PRIu32 " has unknown type 0x%02x", fileName, nodeID, type);
 	}
+
+	node.isQuiet = (type & (1 << FSTACKNODE_QUIET_BIT)) != 0;
 }
 
 // Reads a symbol from a file.
@@ -152,20 +161,25 @@ static void readSymbol(
     FILE *file, Symbol &symbol, char const *fileName, std::vector<FileStackNode> const &fileNodes
 ) {
 	tryReadString(symbol.name, file, "%s: Cannot read symbol name: %s", fileName);
-	tryGetc(
-	    ExportLevel,
-	    symbol.type,
-	    file,
-	    "%s: Cannot read `%s`'s type: %s",
-	    fileName,
-	    symbol.name.c_str()
-	);
+
+	uint8_t type;
+	tryGetc(type, file, "%s: Cannot read `%s`'s type: %s", fileName, symbol.name.c_str());
+	if (type >= SYMTYPE_INVALID) {
+		fatal("%s: `%s` has unknown type 0x%02x", fileName, symbol.name.c_str(), type);
+	} else {
+		symbol.type = ExportLevel(type);
+	}
+
 	// If the symbol is defined in this file, read its definition
 	if (symbol.type != SYMTYPE_IMPORT) {
 		uint32_t nodeID;
 		tryReadLong(
 		    nodeID, file, "%s: Cannot read `%s`'s node ID: %s", fileName, symbol.name.c_str()
 		);
+		if (nodeID >= fileNodes.size()) {
+			fatal("%s: `%s` has invalid node ID #%" PRIu32, fileName, symbol.name.c_str(), nodeID);
+		}
+
 		symbol.src = &fileNodes[nodeID];
 		tryReadLong(
 		    symbol.lineNo,
@@ -212,6 +226,15 @@ static void readPatch(
 	    sectName.c_str(),
 	    patchID
 	);
+	if (nodeID >= fileNodes.size()) {
+		fatal(
+		    "%s: \"%s\"'s patch #%" PRIu32 " has invalid node ID #%" PRIu32,
+		    fileName,
+		    sectName.c_str(),
+		    patchID,
+		    nodeID
+		);
+	}
 	patch.src = &fileNodes[nodeID];
 
 	tryReadLong(
@@ -247,9 +270,8 @@ static void readPatch(
 	    patchID
 	);
 
-	PatchType type;
+	uint8_t type;
 	tryGetc(
-	    PatchType,
 	    type,
 	    file,
 	    "%s: Cannot read \"%s\"'s patch #%" PRIu32 "'s type: %s",
@@ -257,7 +279,17 @@ static void readPatch(
 	    sectName.c_str(),
 	    patchID
 	);
-	patch.type = type;
+	if (type >= PATCHTYPE_INVALID) {
+		fatal(
+		    "%s: \"%s\"'s patch #%" PRIu32 " has unknown type 0x%02x",
+		    fileName,
+		    sectName.c_str(),
+		    patchID,
+		    type
+		);
+	} else {
+		patch.type = PatchType(type);
+	}
 
 	uint32_t rpnSize;
 	tryReadLong(
@@ -281,13 +313,6 @@ static void readPatch(
 	}
 }
 
-// Sets a patch's `pcSection` from its `pcSectionID`.
-static void
-    linkPatchToPCSect(Patch &patch, std::vector<std::unique_ptr<Section>> const &fileSections) {
-	patch.pcSection =
-	    patch.pcSectionID != UINT32_MAX ? fileSections[patch.pcSectionID].get() : nullptr;
-}
-
 // Reads a section from a file.
 static void readSection(
     FILE *file, Section &section, char const *fileName, std::vector<FileStackNode> const &fileNodes
@@ -296,11 +321,16 @@ static void readSection(
 	uint8_t byte;
 
 	tryReadString(section.name, file, "%s: Cannot read section name: %s", fileName);
+
 	uint32_t nodeID;
 	tryReadLong(
 	    nodeID, file, "%s: Cannot read \"%s\"'s node ID: %s", fileName, section.name.c_str()
 	);
+	if (nodeID >= fileNodes.size()) {
+		fatal("%s: \"%s\" has invalid node ID #%" PRIu32, fileName, section.name.c_str(), nodeID);
+	}
 	section.src = &fileNodes[nodeID];
+
 	tryReadLong(
 	    section.lineNo,
 	    file,
@@ -310,18 +340,23 @@ static void readSection(
 	);
 	tryReadLong(tmp, file, "%s: Cannot read \"%s\"'s' size: %s", fileName, section.name.c_str());
 	if (tmp < 0 || tmp > UINT16_MAX) {
-		fatal("\"%s\"'s section size ($%" PRIx32 ") is invalid", section.name.c_str(), tmp);
+		fatal(
+		    "%s: \"%s\"'s section size ($%" PRIx32 ") is invalid",
+		    fileName,
+		    section.name.c_str(),
+		    tmp
+		);
 	}
 	section.size = tmp;
 	section.offset = 0;
-	tryGetc(
-	    uint8_t, byte, file, "%s: Cannot read \"%s\"'s type: %s", fileName, section.name.c_str()
-	);
+
+	tryGetc(byte, file, "%s: Cannot read \"%s\"'s type: %s", fileName, section.name.c_str());
 	if (uint8_t type = byte & SECTTYPE_TYPE_MASK; type >= SECTTYPE_INVALID) {
-		fatal("\"%s\" has unknown section type 0x%02x", section.name.c_str(), type);
+		fatal("%s: \"%s\" has unknown section type 0x%02x", fileName, section.name.c_str(), type);
 	} else {
 		section.type = SectionType(type);
 	}
+
 	if (byte & (1 << SECTTYPE_UNION_BIT)) {
 		section.modifier = SECTION_UNION;
 	} else if (byte & (1 << SECTTYPE_FRAGMENT_BIT)) {
@@ -339,14 +374,7 @@ static void readSection(
 	tryReadLong(tmp, file, "%s: Cannot read \"%s\"'s bank: %s", fileName, section.name.c_str());
 	section.isBankFixed = tmp >= 0;
 	section.bank = tmp;
-	tryGetc(
-	    uint8_t,
-	    byte,
-	    file,
-	    "%s: Cannot read \"%s\"'s alignment: %s",
-	    fileName,
-	    section.name.c_str()
-	);
+	tryGetc(byte, file, "%s: Cannot read \"%s\"'s alignment: %s", fileName, section.name.c_str());
 	if (byte > 16) {
 		byte = 16;
 	}
@@ -499,7 +527,16 @@ void obj_ReadFile(std::string const &filePath, size_t fileID) {
 		readSymbol(file, sym, fileName, nodes[fileID]);
 		sym_AddSymbol(sym);
 		if (std::holds_alternative<Label>(sym.data)) {
-			++nbSymPerSect[std::get<Label>(sym.data).sectionID];
+			int32_t sectionID = std::get<Label>(sym.data).sectionID;
+			if (sectionID < 0 || static_cast<size_t>(sectionID) >= nbSymPerSect.size()) {
+				fatal(
+				    "%s: `%s` has invalid section ID #%" PRId32,
+				    fileName,
+				    sym.name.c_str(),
+				    sectionID
+				);
+			}
+			++nbSymPerSect[sectionID];
 		}
 	}
 
@@ -522,15 +559,41 @@ void obj_ReadFile(std::string const &filePath, size_t fileID) {
 		Assertion &assertion = patch_AddAssertion();
 
 		readAssertion(file, assertion, fileName, i, nodes[fileID]);
-		linkPatchToPCSect(assertion.patch, fileSections);
+
+		if (assertion.patch.pcSectionID == UINT32_MAX) {
+			assertion.patch.pcSection = nullptr;
+		} else if (assertion.patch.pcSectionID >= fileSections.size()) {
+			fatal(
+			    "%s: Assertion #%" PRIu32 "'s patch has invalid section ID #%" PRIu32,
+			    fileName,
+			    i,
+			    assertion.patch.pcSectionID
+			);
+		} else {
+			assertion.patch.pcSection = fileSections[assertion.patch.pcSectionID].get();
+		}
+
 		assertion.fileSymbols = &fileSymbols;
 	}
 
 	// Give patches' PC section pointers to their sections
 	for (std::unique_ptr<Section> const &sect : fileSections) {
-		if (sectTypeHasData(sect->type)) {
-			for (Patch &patch : sect->patches) {
-				linkPatchToPCSect(patch, fileSections);
+		if (!sectTypeHasData(sect->type)) {
+			continue;
+		}
+		for (size_t i = 0; i < sect->patches.size(); ++i) {
+			if (Patch &patch = sect->patches[i]; patch.pcSectionID == UINT32_MAX) {
+				patch.pcSection = nullptr;
+			} else if (patch.pcSectionID >= fileSections.size()) {
+				fatal(
+				    "%s: \"%s\"'s patch #%zu has invalid section ID #%" PRIu32,
+				    fileName,
+				    sect->name.c_str(),
+				    i,
+				    patch.pcSectionID
+				);
+			} else {
+				patch.pcSection = fileSections[patch.pcSectionID].get();
 			}
 		}
 	}
diff --git a/src/link/patch.cpp b/src/link/patch.cpp
index 36c839ca..97ba3107 100644
--- a/src/link/patch.cpp
+++ b/src/link/patch.cpp
@@ -78,7 +78,8 @@ static uint32_t getRPNByte(uint8_t const *&expression, int32_t &size, Patch cons
 }
 
 static Symbol const *getSymbol(std::vector<Symbol> const &symbolList, uint32_t index) {
-	assume(index != UINT32_MAX); // PC needs to be handled specially, not here
+	assume(index != UINT32_MAX);       // PC needs to be handled specially, not here
+	assume(index < symbolList.size()); // This needs to be checked before calling
 	Symbol const &symbol = symbolList[index];
 
 	// If the symbol is defined elsewhere...
@@ -270,17 +271,19 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 			value = op_shift_right_unsigned(popRPN(patch), value);
 			break;
 
-		case RPN_BANK_SYM:
-			value = 0;
+		case RPN_BANK_SYM: {
+			uint32_t symID = 0;
 			for (uint8_t shift = 0; shift < 32; shift += 8) {
-				value |= getRPNByte(expression, size, patch) << shift;
+				symID |= getRPNByte(expression, size, patch) << shift;
 			}
 
-			if (Symbol const *symbol = getSymbol(fileSymbols, value); !symbol) {
+			if (symID >= fileSymbols.size()) {
+				fatalAt(patch, "Requested `BANK()` of invalid symbol ID #%" PRIu32, symID);
+			} else if (Symbol const *symbol = getSymbol(fileSymbols, symID); !symbol) {
 				errorAt(
 				    patch,
 				    "Requested `BANK()` of undefined symbol `%s`",
-				    fileSymbols[value].name.c_str()
+				    fileSymbols[symID].name.c_str()
 				);
 				isError = true;
 				value = 1;
@@ -290,12 +293,13 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 				errorAt(
 				    patch,
 				    "Requested `BANK()` of non-label symbol `%s`",
-				    fileSymbols[value].name.c_str()
+				    fileSymbols[symID].name.c_str()
 				);
 				isError = true;
 				value = 1;
 			}
 			break;
+		}
 
 		case RPN_BANK_SECT: {
 			// `expression` is not guaranteed to be '\0'-terminated. If it is not,
@@ -420,13 +424,13 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 			}
 			break;
 
-		case RPN_SYM:
-			value = 0;
+		case RPN_SYM: {
+			uint32_t symID = 0;
 			for (uint8_t shift = 0; shift < 32; shift += 8) {
-				value |= getRPNByte(expression, size, patch) << shift;
+				symID |= getRPNByte(expression, size, patch) << shift;
 			}
 
-			if (value == -1) { // PC
+			if (symID == UINT32_MAX) { // PC
 				if (patch.pcSection) {
 					value = patch.pcOffset + patch.pcSection->org;
 				} else {
@@ -434,9 +438,11 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 					value = 0;
 					isError = true;
 				}
-			} else if (Symbol const *symbol = getSymbol(fileSymbols, value); !symbol) {
-				errorAt(patch, "Undefined symbol `%s`", fileSymbols[value].name.c_str());
-				sym_TraceLocalAliasedSymbols(fileSymbols[value].name);
+			} else if (symID >= fileSymbols.size()) {
+				fatalAt(patch, "Invalid symbol ID #%" PRIu32, symID);
+			} else if (Symbol const *symbol = getSymbol(fileSymbols, symID); !symbol) {
+				errorAt(patch, "Undefined symbol `%s`", fileSymbols[symID].name.c_str());
+				sym_TraceLocalAliasedSymbols(fileSymbols[symID].name);
 				value = 0;
 				isError = true;
 			} else if (std::holds_alternative<Label>(symbol->data)) {
@@ -447,6 +453,7 @@ static int32_t computeRPNExpr(Patch const &patch, std::vector<Symbol> const &fil
 			}
 			break;
 		}
+		}
 
 		pushRPN(value, isError);
 	}