Pointer re-assignment, heap use after free

I'm trying to write a simple function that swaps the nodes of a linked list (in which the values cannot be changed, just the nodes). Running into memory usage after free errors, and I can't pin it. I'm fairly certain the answer is quite straightforward and easy, I just can't see it right now.

void swapNodes(ListNode* a, ListNode *b){ //does not consider the root case
    auto * a1 = a->next;
    auto * a2 = a1->next;
    
    auto * b1 = b->next;
    auto * b2 = b1->next; // may be nullptr
    
    auto newA = std::make_shared<ListNode>(ListNode(b1->val, a2));
    auto newB = std::make_shared<ListNode>(ListNode(a1->val, b2));
    
    a->next = newA.get();
    b->next = newB.get();
}

I know it's missing a lot of checks, ignore that for now. Function receives two pointers (ideally references, but I'm restructuring still) to the nodes which need to be swapped.

Getting:

=================================================================
==32==ERROR: AddressSanitizer: heap-use-after-free on address 0x603000000028 at pc 0x000000383abe bp 0x7ffe9bd980f0 sp 0x7ffe9bd980e8
READ of size 8 at 0x603000000028 thread T0
    #3 0x7f2499b8c82f  (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
0x603000000028 is located 24 bytes inside of 32-byte region [0x603000000010,0x603000000030)
freed by thread T0 here:
    #7 0x7f2499b8c82f  (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
previously allocated by thread T0 here:
    #10 0x7f2499b8c82f  (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
Shadow bytes around the buggy address:
  0x0c067fff7fb0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fc0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fd0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7fe0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c067fff7ff0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c067fff8000: fa fa fd fd fd[fd]fa fa fd fd fd fd fa fa fa fa
  0x0c067fff8010: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8020: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8030: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8040: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c067fff8050: 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
  Shadow gap:              cc
==32==ABORTING