Off-by-One Heap Overflow Exploitation

This skill guides you through exploiting off-by-one (OBO) heap overflow vulnerabilities in glibc-based systems. An OBO vulnerability allows writing exactly one byte past a buffer's boundary, which can corrupt the

size

field of the next heap chunk.

Understanding the Vulnerability

What is an Off-by-One Overflow?

An off-by-one overflow occurs when a program writes one byte beyond the intended buffer boundary. This single byte can corrupt the

size

field of the adjacent heap chunk, enabling:

• Chunk size manipulation: Modify how much memory a chunk claims to occupy
• Overlapping chunks: Make one chunk contain another, enabling arbitrary writes
• Tcache poisoning: Redirect allocations to attacker-controlled addresses
• Double-free scenarios: Create conditions for memory reuse attacks

Two Types of OBO Vulnerabilities

1. Arbitrary byte OBO: Can overwrite the boundary byte with any value (0x00-0xFF)
2. Null byte OBO (off-by-null): Can only write 0x00 to the boundary
   • Common in

     strlen

     /

     strcpy

     mismatches
   • Can be exploited with House of Einherjar technique
   • With tcache, can create double-free conditions

General Attack Methodology

Step 1: Setup the Heap Layout

Allocate chunks in a specific arrangement to enable the attack:

[ Chunk A (0x20) ] [ Chunk B (0x20) ] [ Chunk C (0x20) ] [ Chunk D (0x20) ]

• Allocate three chunks (A, B, C) of the same size (e.g., 0x20)
• Allocate a fourth chunk (D) to prevent consolidation with the top chunk
• This creates a controlled heap layout for exploitation

Step 2: Create the Overflow Condition

1. Free chunk C: This inserts it into the tcache free-list (for 0x20 chunks)
2. Overflow from A into B: Use the OBO vulnerability to write one byte past A's boundary
3. Corrupt B's size field: Change B's size from 0x21 to 0x41 (or similar)

Step 3: Create Overlapping Chunks

After corrupting B's size:

[ Chunk A ] [ Chunk B (now claims 0x40, contains C) ] [ Chunk D ]

• Chunk B now "contains" the freed chunk C
• C is still in the tcache free-list
• This creates the overlapping condition needed for exploitation

Step 4: Trigger the Overlap

1. Free chunk B: This consolidates B with its "contained" chunk C
2. Allocate a 0x40 chunk: This allocation will reuse the B+C space
3. Modify C's fd pointer: Since C is still in tcache, you can poison it

Step 5: Tcache Poisoning

With C's fd pointer corrupted:

1. Free the overlapping chunk: This adds it to tcache
2. Poison C's fd: Point it to an address you control (e.g.,

   __free_hook

   )
3. Trigger allocation: Next malloc will place your fake chunk at the poisoned address
4. Achieve arbitrary write: Write shellcode or one_gadget to the target

Modern glibc Hardening (>= 2.32)

Safe-Linking Protection

Modern glibc uses safe-linking to protect tcache pointers:

fd = ptr ^ (chunk_addr >> 12)

This XOR encoding means:

• You need a heap leak to compute the XOR mask
• Single-byte size corruption alone is insufficient
• You must re-encode pointers before writing them

Bypass Strategy

1. Grow victim chunk: Make it fully cover a freed chunk you control
2. Leak a heap pointer: Use stdout, UAF, or partially controlled struct
3. Derive the key:

   heap_base >> 12

4. Re-encode pointers: Use the protect/reveal helper (see scripts)
5. Stage encoded values: Place them in user data, memcpy later
6. Combine with tcache attacks: Redirect to

   __free_hook

   or tcache entries

Double-Protect Technique

For leakless exploitation:

1. Encode a controlled pointer: Use

   PROTECT_PTR

   on a pointer you already own
2. Reuse the gadget: Encode your forged pointer with the same XOR key
3. Alignment check passes: No new address revelation needed

Off-by-Null Specific Attack

When you can only write 0x00:

1. Allocate three chunks: a, b, c in sequence
2. Free the middle chunk (b)
3. Overflow from a with 0x00: This reduces b's apparent size
4. Allocate two smaller chunks in b: b1 and b2
5. Free b1 and c: They consolidate, but b2 remains inside
6. Allocate new chunk: It contains b2, giving you control over its content

Practical Exploitation Workflow

1. Identify the Vulnerability

Look for:

• Buffer size calculations that are off by one

• strlen

  /

  strcpy

  mismatches
• Format string vulnerabilities with INT_MAX truncation
• Off-by-one in array bounds checking

2. Map the Heap Layout

Use

pwndbg

or

gef

:

heap bins
heap chunks
vmmap

Identify:

• Chunk sizes and positions
• Which chunks are in tcache
• Adjacent chunk relationships

3. Craft the Exploit

Use the helper scripts:

• scripts/protect_reveal.py

  for safe-linking encoding

• scripts/exploit_template.py

  for exploit scaffolding

4. Test and Iterate

• Start with a simple heap leak
• Progress to size corruption
• Add tcache poisoning
• Finalize with arbitrary write

Real-World Example: CVE-2023-6779

Vulnerability Details

• Location:

  __vsyslog_internal()

  in glibc 2.37-2.39
• Trigger:

  syslog()

  format strings exceeding

  INT_MAX

• Effect: Terminating

  \0

  corrupts next chunk's size byte
• Impact: Root via

  __free_hook

  overwrite

Exploitation Pipeline

1. Craft overlong

   openlog()

   ident: Makes

   vasprintf

   return adjacent heap buffer
2. Call

   syslog()

   : Smashes neighbor chunk's

   size | prev_inuse

   byte
3. Free and consolidate: Creates overlapping chunk with attacker data
4. Corrupt tcache metadata: Aim next allocation at

   __free_hook

5. Overwrite with system/one_gadget: Achieve root

Reproduction Harness

# Fork with gigantic argv[0]
# Call openlog(NULL, LOG_PID, LOG_USER)
# Call syslog(LOG_INFO, "%s", payload)
# where payload = b"A" * 0x7fffffff
# Check with: pwndbg heap bins

CTF Writeup Patterns

Bon-nie-appetit (HTB Cyber Apocalypse 2022)

• Vulnerability:

  strlen

  considers next chunk's size field
• Technique: General OBO with tcache poisoning
• Result: Arbitrary write primitive

Asis CTF 2016 b00ks

• Vulnerability: OBO leaks heap address via 0x00 byte
• Technique: Fake struct with fake pointers
• Result: Arbitrary write to

  __free_hook

  with one_gadget

PlaidCTF 2015 plaiddb

• Vulnerability: NULL OBO in

  getline

  function
• Technique: Multi-chunk consolidation with fd poisoning
• Result:

  __malloc_hook

  overwrite with one_gadget

Common Pitfalls

1. Tcache interference: Small chunks (<= 0x40) go to tcache, not fastbins
2. Safe-linking: Modern glibc requires XOR encoding
3. Alignment: Chunk addresses must be properly aligned
4. Consolidation: Adjacent free chunks may merge unexpectedly
5. Top chunk: Don't let your chunks consolidate with the top chunk

Debugging Tips

With pwndbg

# View heap layout
heap chunks
heap bins

# View specific chunk
heap chunk <address>

# Watch for changes
watch *(size_t*)<chunk_address>

With GDB + gef

heap-analysis
heap-context

When to Use This Skill

Use this skill when:

• You encounter an off-by-one vulnerability in a CTF or real target
• You need to corrupt heap metadata with a single-byte write
• You're working with glibc heap exploitation
• You need to bypass safe-linking in modern glibc
• You're analyzing CVE-2023-6779 or similar syslog vulnerabilities
• You need tcache poisoning techniques
• You're dealing with chunk size corruption attacks

Helper Scripts

Use the bundled scripts for:

• scripts/protect_reveal.py

  : Safe-linking encode/decode

• scripts/exploit_template.py

  : Exploit scaffolding

Run them with:

python scripts/protect_reveal.py --help
python scripts/exploit_template.py --help

References

• heap-exploitation.dhavalkapil.com
• Qualys CVE-2023-6246/6779/6780 Advisory
• Ubuntu CVE-2023-6779
• Breaking Safe-Linking in Modern Glibc