[Bug] Heap-buffer-overflow (Underflow) in SQFuncState::PopTarget accessing index -1

[oneafter]

Description

We discovered a Heap-buffer-overflow vulnerability in the Squirrel compiler. The crash occurs in SQFuncState::PopTarget() when parsing complex expressions involving binary or logical operations inside statements (e.g., for loops).

The ASAN report indicates a READ violation of 8 bytes located 8 bytes before the allocated heap region. This strongly suggests the compiler is attempting to access a vector/stack at index -1 (Buffer Underflow), likely due to popping from an empty target stack.

Environment

• OS: Linux x86_64
• Complier: Clang
• Build Configuration: Release mode with ASan enabled.

Vulnerability Details

• Target: Squirrel (squirrel-lang)
• Vulnerability Type: CWE-125: Out-of-bounds Read (Underflow)
• Function: SQFuncState::PopTarget()
• Location: src/squirrel/squirrel/sqfuncstate.cpp:290
• Root Cause Analysis: The function PopTarget retrieves the last element from _target_stack.

// Example logic in sqfuncstate.cpp
long long PopTarget() {
    // If _target_stack is empty, size() is 0.
    // Accessing index [0 - 1] leads to a heap underflow of 8 bytes (sizeof long long).
    return _target_stack.back(); 
}

The ASAN report shows the crash happened after a deep recursion of Expression -> LogicalAndExp -> BitwiseOrExp -> BIN_EXP. It appears the compiler's internal state tracking of expression targets gets desynchronized, causing PopTarget to be called one time too many, resulting in an access to _target_stack[-1].

Reproduce

1. Build squirrel with Release optimization and ASAN enabled.
2. Run with the crashing file:

./build/bin/sq repro

ASAN report

==57882==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x503000002378 at pc 0x7f3958ffda7c bp 0x7fff9612d060 sp 0x7fff9612d058
READ of size 8 at 0x503000002378 thread T0
    #0 0x7f3958ffda7b in SQFuncState::PopTarget() /src/squirrel/squirrel/sqfuncstate.cpp:290:41
    #1 0x7f3958fe666c in void SQCompiler::BIN_EXP<void (SQCompiler::*)()>(SQOpcode, void (SQCompiler::*)(), long long) /src/squirrel/squirrel/sqcompiler.cpp:469:63
    #2 0x7f3958fe666c in SQCompiler::BitwiseOrExp() /src/squirrel/squirrel/sqcompiler.cpp:520:10
    #3 0x7f3958fe666c in SQCompiler::LogicalAndExp() /src/squirrel/squirrel/sqcompiler.cpp:494:9
    #4 0x7f3958fe547d in SQCompiler::LogicalOrExp() /src/squirrel/squirrel/sqcompiler.cpp:475:9
    #5 0x7f3958fe455c in SQCompiler::Expression() /src/squirrel/squirrel/sqcompiler.cpp:377:9
    #6 0x7f3958fdf9dc in SQCompiler::CommaExpr() /src/squirrel/squirrel/sqcompiler.cpp:369:13
    #7 0x7f3958fd5edb in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:311:13
    #8 0x7f3958fd5d22 in SQCompiler::Statements() /src/squirrel/squirrel/sqcompiler.cpp:205:13
    #9 0x7f3958fd5d22 in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:279:17
    #10 0x7f3958fdb1d3 in SQCompiler::ForStatement() /src/squirrel/squirrel/sqcompiler.cpp:1190:9
    #11 0x7f3958fd653d in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:217:29
    #12 0x7f3958fd5d22 in SQCompiler::Statements() /src/squirrel/squirrel/sqcompiler.cpp:205:13
    #13 0x7f3958fd5d22 in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:279:17
    #14 0x7f3958ff5752 in SQCompiler::CreateFunction(tagSQObject&, long long, bool) /src/squirrel/squirrel/sqcompiler.cpp:1580:13
    #15 0x7f3958fe01f4 in SQCompiler::FunctionStatement() /src/squirrel/squirrel/sqcompiler.cpp:1318:9
    #16 0x7f3958fd641c in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:268:13
    #17 0x7f3958fd5d22 in SQCompiler::Statements() /src/squirrel/squirrel/sqcompiler.cpp:205:13
    #18 0x7f3958fd5d22 in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:279:17
    #19 0x7f3958fdb1d3 in SQCompiler::ForStatement() /src/squirrel/squirrel/sqcompiler.cpp:1190:9
    #20 0x7f3958fd653d in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:217:29
    #21 0x7f3958fd5d22 in SQCompiler::Statements() /src/squirrel/squirrel/sqcompiler.cpp:205:13
    #22 0x7f3958fd5d22 in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:279:17
    #23 0x7f3958ff5752 in SQCompiler::CreateFunction(tagSQObject&, long long, bool) /src/squirrel/squirrel/sqcompiler.cpp:1580:13
    #24 0x7f3958fe01f4 in SQCompiler::FunctionStatement() /src/squirrel/squirrel/sqcompiler.cpp:1318:9
    #25 0x7f3958fd641c in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:268:13
    #26 0x7f3958fd5d22 in SQCompiler::Statements() /src/squirrel/squirrel/sqcompiler.cpp:205:13
    #27 0x7f3958fd5d22 in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:279:17
    #28 0x7f3958ff5752 in SQCompiler::CreateFunction(tagSQObject&, long long, bool) /src/squirrel/squirrel/sqcompiler.cpp:1580:13
    #29 0x7f3958fe01f4 in SQCompiler::FunctionStatement() /src/squirrel/squirrel/sqcompiler.cpp:1318:9
    #30 0x7f3958fd641c in SQCompiler::Statement(bool) /src/squirrel/squirrel/sqcompiler.cpp:268:13
    #31 0x7f3958fd48b0 in SQCompiler::Compile(SQObjectPtr&) /src/squirrel/squirrel/sqcompiler.cpp:180:17
    #32 0x7f3958fd38f6 in Compile(SQVM*, long long (*)(void*), void*, char const*, SQObjectPtr&, bool, bool) /src/squirrel/squirrel/sqcompiler.cpp:1633:14
    #33 0x7f3958f7f358 in sq_compile /src/squirrel/squirrel/sqapi.cpp:131:8
    #34 0x7f39590f0b92 in sqstd_loadfile /src/squirrel/sqstdlib/sqstdio.cpp:389:16
    #35 0x556bb0e12411 in getargs /src/squirrel/sq/sq.c
    #36 0x556bb0e13cb7 in main /src/squirrel/sq/sq.c:330:12
    #37 0x7f3958c311c9  (/lib/x86_64-linux-gnu/libc.so.6+0x2a1c9) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    #38 0x7f3958c3128a in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x2a28a) (BuildId: 274eec488d230825a136fa9c4d85370fed7a0a5e)
    #39 0x556bb0d30634 in _start (/src/squirrel/build/bin/sq+0x2d634) (BuildId: e84909622778a60c07bbbd32568cd4522dcfe97d)

0x503000002378 is located 8 bytes before 32-byte region [0x503000002380,0x5030000023a0)
allocated by thread T0 here:
    #0 0x556bb0dd0880 in realloc (/src/squirrel/build/bin/sq+0xcd880) (BuildId: e84909622778a60c07bbbd32568cd4522dcfe97d)
    #1 0x7f3958ffd56e in sqvector<long long>::_realloc(unsigned long long) /src/squirrel/squirrel/squtils.h:110:21
    #2 0x7f3958ffd56e in sqvector<long long>::push_back(long long const&) /src/squirrel/squirrel/squtils.h:79:13
    #3 0x7f3958ffd56e in SQFuncState::PushTarget(long long) /src/squirrel/squirrel/sqfuncstate.cpp

SUMMARY: AddressSanitizer: heap-buffer-overflow /src/squirrel/squirrel/sqfuncstate.cpp:290:41 in SQFuncState::PopTarget()
Shadow bytes around the buggy address:
  0x503000002080: 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00 00 00
  0x503000002100: fa fa 00 00 00 00 fa fa fd fd fd fd fa fa 00 00
  0x503000002180: 00 00 fa fa 00 00 00 00 fa fa fd fd fd fd fa fa
  0x503000002200: 00 00 00 00 fa fa fd fd fd fd fa fa fd fd fd fd
  0x503000002280: fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa 00 00
=>0x503000002300: 00 00 fa fa fd fd fd fd fa fa 00 00 00 00 fa[fa]
  0x503000002380: 00 00 00 00 fa fa fd fd fd fd fa fa 00 00 00 00
  0x503000002400: fa fa 00 00 00 00 fa fa 00 00 00 00 fa fa fa fa
  0x503000002480: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x503000002500: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x503000002580: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
  Addressable:           00
  Partially addressable: 01 02 03 04 05 06 07 
  Heap left redzone:       fa
  Freed heap region:       fd
  Stack left redzone:      f1
  Stack mid redzone:       f2
  Stack right redzone:     f3
  Stack after return:      f5
  Stack use after scope:   f8
  Global redzone:          f9
  Global init order:       f6
  Poisoned by user:        f7
  Container overflow:      fc
  Array cookie:            ac
  Intra object redzone:    bb
  ASan internal:           fe
  Left alloca redzone:     ca
  Right alloca redzone:    cb
==57882==ABORTING

[carnil]

CVE-2026-2659 appears to be associated with this issue.