Nabil Kara — CTF Writeups

A ZAP CSTI active scan rule

Tue, 09 Jun 2026 00:00:00 +0000

Introduction

I started to work on this project as an intern at Djezzy Optimum Télécom Algérie SPA, under the supervision of Mr. Beraoud Abdelkhalek who was behind the idea and whose invaluable guidance, transformative advice, and remarkable patience supported the work throughout its development.

The active scan rule described here was built as an alpha rule for ZAP. You can find the code here.

Implementation Basis: The CSTI-Alert Paper

The methodology comes from the paper: {{alert('CSTI')}}: Large-Scale Detection of Client-Side Template Injection. The part we are concerned with for this active scan rule is not the large-scale crawling infrastructure itself, but the detection methodology:

Facts - Hack The Box machine

Wed, 03 Jun 2026 00:00:00 +0000

Difficulty: Easy
OS: Linux
Author: LazyTitan333

An initial nmap scan reveals two open ports : 80 (HTTP) and 23 (SSH)

We start by exploring the web page :

If we click Start Exploring we see a couple of facts , a search bar and a comments section. I browsed the facts and found nothing useful , except that the comments profiles are potential SSH usernames. We opt for directory enumeration ,for instance, we can use ffuf :

Multi ﾊﾟﾜｰ RSA Full Week Engineering CTF 2025

Tue, 02 Sep 2025 00:00:00 +0000

Challenge Description

Source code

Chall

from sage.all import *
from Crypto.Util.number import *
import gmpy2
import random
from sympy import nextprime

FLAG = b'fwectf{REDACTED_REDACTED_REDACTED}'
m = bytes_to_long(FLAG)
r = random.randint(5, 30)

p = getPrime(256)
q = getPrime(256)
if p < q:
 p, q = q, p
N = pow(p, r) * q
phi = pow(p, r - 1) * (p - 1) * (q - 1)
e = 65537

c = pow(m, e, N)
print(f'c = {c}')
print(f'e = {e}')
print(f'N = {N}')

d1 = getPrime(2000)
d2 = nextprime(d1 + getPrime(1000))
e1 = gmpy2.invert(d1, phi)
e2 = gmpy2.invert(d2, phi)
print(f'e1 = {e1}')
print(f'e2 = {e2}')

Output.txt

c = 150585797027649865489287786588283519376444271481829451206461043013638461851456443295639137583076309442211685140040715015672365662062445178352437695738282641956247465007835614325215320828570574978027697247110122408959329602493024795984675855072829968568063483443564884590062240635887431125362354037916695863432044462478632010854829416594987121310115447040797978156754180611417998692235678830822995195693412798239945582015522540243621848514340825137784617038883681762243778481350214509050715611993272169019595988822536735533532328109537622265977681155970778571399007023021359255926523887958105866659589793221135086136634321526143644058657686559455390271432068714110879919249854512706311908215475720874421301327375628026151554425480889625476443996963226739224141229051215116294621373720061550447360540466603611877916643506521674077637662746353412174577577517713229695736915310128634392507257018082205528441763251409689312355800240394928915067047489667210410718244921919018887791400482889355718347742440819608756821762567978878895290820251733297461964155473495597853211579186825851137994290776971434620034797987966563429453923909529541320143776397669267613997300604747811315937419659086723279726826646032780534551617458772078374711989147497752615265133572176138301527850632921359443303144256863024531079818868821758707284519856822245327421770110054277128452078605253184196998977317191651032204260679733274112559665428206212555513334541342029165221468784610288416686450162503789244926494714898635616996486140153083238632042007797793700563323218257129418795358543031202313561185588455593386153322727840291176790523913519542549216392301233455213647649027484109309089206606335520666724332245092640179790951481672640009520232615798668179089441370616468610644268539377884248767999193537928526046999022147158284677984579677312334741055670888654273143019369722889958102904924117413980953194821337321878279459444456391516721152204499246038472658108777670283734288457011883626451484055732662043231790974669769949105126075064121602546376952003206428494498829916276757539596132583838525916520286113969608190165961798389572158151878875500492540747108296198243292566008234833
e = 65537
N = 186468293083788781245499353512563129672312798029703904462936796596026120571080426862373632093518207360391666952453308555988614378903855020162320651304963730462668109416515599034788196838330392323014227902075936472408096760922362616639831532722796773044161215603161573173693396763848532916596270542508246600865336824042340247413113147406841036910189001075097670228541035790259078800598326610948022398352585923649531112759445985740346135428877322273556322542215020775992731058099305658619025609383729517239811673596072211108962991986901295709256060275331345666057130911453857078541419835856196753136664547254360862900351337626911283810534959854452952272035002742600178127259539214115953749657961050280108421405551206427544090699506875821129502984234774688377256310602196702176007042636404063776926466585683290760236194258669323601484216988981113466416405284959939812524546732701485183931884838692814562595191552224757937077488587067020793692692549938889591330072594717978441992795102999379303433207854356430437846345643266409739062915660544094498466397963646286252575273661019638807620280311533417717322466737567313846948246565804965079064097678541329903335139345823245801485851723772625368820657081414997271906611200756811167771754039199663511386284954358869993716532869728924931127279616476581118702229755968849120582675066084548348641091550633395996908836876499056736123107365637770245726218450365257927541692911169875941639963606762719472741281838412109552457222110661735681401203267804449363273688233013042050750830535778904458171623334164553955827238989238553731488016772701812203067882783550639709170454593937643386446256536119770989304864520889045556071269357480411824619830061505470986036143575914439137724990710287384145813135368196679314377112131761811940172598300544938844479133061210634426452190259178180635554381170513818865646521472300868854934387822644776013828828533387245770709879233807065640965155395836324727727247566385145338896343366468455003789992017616034872629717115885307603763663604867417148970196011250152501225642510419108907592141672752218085088428240372007994449852792767351434018043118658591114512322934390623465185608109909287
e1 = 41580201693720582480693330964943873953272761954440070167783695980849492986662882513949168242004211132670119356752908751569637864785219861514846064298150109789186060860480552700920272463488413243406347872787482976412247245447832054401920594283666788061207365372191406054243898945396093580724391257502862264542139892002113479274778815312220710067519175895385029728136011707878570503892737327311559220318356052978117942035853947664507847769494960999568852350903791440660149716513527902855811711244727464905711093905648316805999328017344077741109433923704200400909675649273690871091313094751105766025879326677544581537926603326267474079786157380975934855898690386484742475696802305967739089815897725317069013189207569982900053713459672998534905787251877552189174587775147542089885096510488094105071666800247101089674649502031584763839755525727965498486527987626650988652717016765563711049277022756500157982361607140778095658226557996921026130637701014127285732701887164908135560640420831336153612762215604492226275755304784484586433397754740117463505077153516054839486722481181167234229522839072562070104349846627036424367454820389259772812583287412268551592814838917235531904330269843311050451607079925393154706621947637894340627920099890436967089957279051662270031234850834868788996666439029630296451502714368722270484850158064364294201493953114172135159291698520360808866584806672889034144771436797735399804868018466668980453787245180038654345402748074831992415906097556738039279373205673388915847115097595687636844845281031267671590617631079664730977219356874731914430598632093802245785346918434058687175986926298508052091652105059279150133460800817344425839682126964963172665944796661520065067011564066759081570107543463219913557175490516674841445611024874076012796831045476624324336824144536686982822389532337716389606704464995869146017139606884400942152132650479156746875064775899199421858957623079678439410930007225084885887507916565931484847922437795803010981718194290457886234411319205609157105187496215942426263799468200530172714083518151428613800222887516185125589229285415586754359693123597271154802410446280090888946329676448819766565178085178413
e2 = 85039848098040198629810228227981333547571544556783337097791600722534400132636079696903303346248897364881224585886454806975154545966948446220731675563154086955028649334868794080881245005339585924134379510941267320612901825195331762039504494208113074000658589388417531708151776552001429290730079651850438354450134417941536114627588684047285304149442482520534706112915132300363334355877266504867840336875699380810203774027956821177301695145239460742602239474808009495484157876332743288399678455954261820744636650959513617108267284843582028632761292881550278978932484825724119492458352905716592760686190086451077741620824644825883637432247606542138083779285819850363414635029032877436967954280148470447811352539940377678100502742624065767900310440814168233290500834855710005707058159197563629326782049935503972263918251175766238816831480903704391181267515124468008074136167514887505816881460820657424954418139484799844694782547580350564364558817948882864222013820625021944986053685725745314774681293740590727701113298280323591219898133230776271582267346789673381184226867314871136250586292185846670779312885676712427749758340163573785341699633900956786017292343325059891658652266756606090187161788473402586895334792711643881433133624951181775727523533052371714916897497811035395454066372050989114435626744825554699077429987277061832895800546415012022003883511707438045038605628208211909421854504776635296074426208716798470239164727186799413913395180313824277747343105859814610954674052243548895700598370536537947368616622939415516698306638823827111837566750309493160547631614984324373110960297446703268022461052398627980535338158153120866719678508677569334532530529647580833323049593479237592627920937370843841953419429014918761884065528591917331618164304459336833062437365299224341702429186447625010865403846061689858277518844804864401686840751180716835568865211637737929233885813901317995949748419432175395134689356497511677848916131101942879850703634693483037253067856351005939346103168898195695634993164654145535921173796299479632825317904540020964320393080050237770370124344704355059640345255127574387631628640364876804621760107827456260120265498390309061

Solution :

1- Explanation :

The challenge implements a variant of RSA, where the modulus is of the form

Leaky RSA - ASCIS CTF 2022

Mon, 18 Aug 2025 00:00:00 +0000

Intro

This is a challenge from ASCIS CTF 2022 I upsolved using cuso when I was learning the coppersmith method.

Source code

from secret import flag

Kbits1 = 120
Kbits2 = 493

p = random_prime(2^512-1, false, 2^511)
q = random_prime(2^512-1, false, 2^511)
N = p*q
e = 0x10001

# Wait this is illegal
Pdigits = p.digits(2)
leaky_p = [0]*19 + Pdigits[19:Kbits1] + [0]*19 + Pdigits[Kbits1+19:Kbits2]
# leaks 19 -> kbits1 lsbs , then kbits1+19 -> kbits2 msbs
# f(x,y,z) = x + (digists from 19 to kbits1)* 2 ^ 19 + y ^ 2^kbit s1 + (sum of the digits from kbits1+19 to kbits2) * 2 ^ (kbits1 + 19) + z * 2 ^ (kbits2)

c = pow(flag, e, N)
print(f"(c, e, N) = {(c, e, N)}")
print(f"leaky = {leaky_p}")

# (c, e, N) = (26332525917536404445261335188023824835582728456010807789427648382546117992477286201354477933620634042162778383500347554403856479653121560047163571802966352911016989944196656213695292273900862199180497043773236377160608017101154863030724519304212930989627167943539496731959903689581119612196554873637719776156, 65537, 107953240319236322637058433940161528510672490103418517617520324178241611238072198345853818249768509790971608169523056348162590719063088182552103689228188347112141257514135750575118448199789749158885349374251834100136760987321248910344201579746268207678856824451563937881565576119683013793260110227648499602781)
# leaky = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]

Solution

from Crypto.Util.number import *
import cuso
from sage.all import *
bits1 = 120
Kbits2 = 493

(c, e, N) = (26332525917536404445261335188023824835582728456010807789427648382546117992477286201354477933620634042162778383500347554403856479653121560047163571802966352911016989944196656213695292273900862199180497043773236377160608017101154863030724519304212930989627167943539496731959903689581119612196554873637719776156, 65537, 107953240319236322637058433940161528510672490103418517617520324178241611238072198345853818249768509790971608169523056348162590719063088182552103689228188347112141257514135750575118448199789749158885349374251834100136760987321248910344201579746268207678856824451563937881565576119683013793260110227648499602781)
leaky = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 1, 1, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 0, 1, 0, 1, 0, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 1, 1, 0, 1, 0, 0, 0, 1, 1, 1, 1, 1, 0, 0, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 1, 0, 1, 0, 0, 1, 0, 1, 1, 0, 1, 0, 0, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 1, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 0, 1, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 0, 0, 1, 0, 0, 0, 1, 1, 1, 0, 1, 0, 0, 1, 1, 1, 0, 0, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 0, 0, 0, 0, 1, 1, 0, 1, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 1, 1, 0, 1, 1, 0, 1, 1, 1, 0, 1, 1, 0, 1, 1, 0, 0, 1, 1, 0, 1, 0, 1, 1, 1, 1, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 1, 0, 1, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 1]

Kbits1 = 120
Kbits2 = 493


leaky_p = leaky

x,y,z = var('x' , 'y' , 'z')

f = x +\
 (2**19)*sum([2**i * leaky_p[19+i] for i in range(Kbits1-19)]) +\
 (2**Kbits1)*y +\
 (2**(Kbits1+19))*sum([2**i * leaky_p[19+Kbits1+i] for i in range(Kbits2-(Kbits1+19))]) + \
 (2**(Kbits2))*z


relations = [f]
bounds = {
 x: (0, 2 ** 19),
 y: (0, 2 ** 19),
 z: (0, 2 ** (512 - Kbits2))
}

roots = cuso.find_small_roots(relations, bounds, modulus = "p",
 modulus_multiple = N,
 modulus_lower_bound = 2**511,
 modulus_upper_bound = 2**512-1
)

print(f"roots = {roots}")

p = roots[0]["p"]
q = N // p
flag = pow(c, inverse_mod(e, (p-1)*(q-1)), N)
print(f"flag = {long_to_bytes(flag)}")

Flag : ASCIS{C0nGratulation_s0_Y0u_D0_kNow_about_H3rm4nN_M4y_730EF6498A3B0441B43400367B788817413F5A65A3900E2976D071232A2FC827}

SSSS sekaiCTF 2025

Mon, 18 Aug 2025 00:00:00 +0000

Challenge Description

Shamir SendS the Secret to everyone
Author : Utaha
Points : 100 pts

Source code

import random, os


p = 2 ** 256 - 189

FLAG = os.getenv("FLAG", "SEKAI{}")


def challenge(secret):

 t = int(input())

 assert 20 <= t <= 50, "Number of parties not in range"


 f = gen(t, secret)


 for i in range(t):

 x = int(input())

 assert 0 < x < p, "Bad input"

 print(poly_eval(f, x))


 if int(input()) == secret:

 print(FLAG)

 exit(0)

 else:

 print(":<")


def gen(degree, secret):

 poly = [random.randrange(0, p) for _ in range(degree + 1)]

 index = random.randint(0, degree)


 poly[index] = secret

 return poly


def poly_eval(f, x):

 return sum(c * pow(x, i, p) for i, c in enumerate(f)) % p


if __name__ == "__main__":

 secret = random.randrange(0, p)

 for _ in range(2):

 challenge(secret)

Let’s say S is the secret that we wish to share.

About Me

Tue, 08 Jul 2025 17:27:40 +0200

Hi! I’m Nabil Kara, a cybersecurity student passionate about pentesting, red teaming, cryptography, OSINT, AI security, Active Directory, and web exploitation. This blog is where I share my CTF writeups, tooling, and learning journey.

Nabil Kara — CTF Writeups

A ZAP CSTI active scan rule

Introduction

Implementation Basis: The CSTI-Alert Paper

Facts - Hack The Box machine

Multi ﾊﾟﾜｰ RSA Full Week Engineering CTF 2025

Challenge Description

Source code

Chall

Output.txt

Solution :

1- Explanation :

Leaky RSA - ASCIS CTF 2022

Intro

Source code

Solution

SSSS sekaiCTF 2025

Challenge Description

Source code

How Shamir’s secret sharing scheme (SSS) works ?

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