Logic Vulnerabilities

Exploits targeting program logic rather than memory corruption.

1. File Descriptor (FD) Abuse

Concept: Manipulate which file a hardcoded FD refers to.

Signals:

Program uses constant FD (e.g., read(3, buf, n))
FD assignment can be influenced before target code runs

FD basics:

0 = stdin, 1 = stdout, 2 = stderr
3+ = assigned in order by open()

Approach A - Claim target FD:

1. Close lower FDs to influence assignment
2. Open desired file (gets lowest available FD)
3. dup2() to target FD number
4. Execute vulnerable program

Approach B - FD exhaustion:

1. Open many files to consume FD numbers
2. Next open() returns predictable FD

Approach C - Symlink redirection:

1. Program opens user-controlled path
2. Create symlink pointing to sensitive file

2. TOCTOU (Time-of-Check to Time-of-Use)

Concept: Change what a path refers to between validation and use.

Signals:

Check: stat(), access(), lstat()
Gap/delay
Use: open(), read(), exec()
Path-based (not FD-based) operations

The race window:

Victim:    [CHECK path] ----delay---- [USE path]
Attacker:              [SWAP target]

Approach A - Symlink race:

Thread 1: Rapidly flip symlink between allowed and sensitive targets
Thread 2: Repeatedly trigger victim until race succeeds

Approach B - Rename race:

Rapidly rename/swap files at target path

Improving success rate:

Multiple racing processes
Tune timing based on victim's check-use gap
Use inotify to detect access and trigger swap

Detection Patterns

FD abuse indicators:

// Hardcoded FD without validation
read(3, buf, size);

// Sequential FD assumption
int fd = open(path, O_RDONLY);
// Assumes fd == 3

TOCTOU indicators:

// Vulnerable: path-based check then use
if (access(path, R_OK) == 0) {
    fd = open(path, O_RDONLY);  // Race window!
}

// Safer: FD-based throughout
fd = open(path, O_RDONLY);
fstat(fd, &st);  // Uses FD, not path

Debugging

bash

# Trace FD operations
strace -e trace=open,openat,read,write,dup2 <program>

# Watch file access for timing
inotifywait -m <path> -e access

Pitfalls

Issue	Cause	Solution
O_NOFOLLOW	Symlinks not followed	Use rename instead
openat()	FD-relative paths	Target the directory FD
Atomic operations	No race window	Different approach needed
Low success rate	Narrow window	More processes, better timing

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